Roof Framing — Trusses & Rafters

The roof is where a church becomes a church. Strip away the stained glass, the steeple, the sign out front — it’s the roofline that people recognise from a kilometre away. That soaring gable, that sweeping scissor profile, those steep Gothic pitches reaching toward the sky — all of it starts with roof framing. And roof framing, whether engineered trusses or conventional stick-built rafters, is some of the most demanding structural work on any construction project.

This guide covers both systems in depth: engineered trusses (the dominant system for large-span church sanctuaries) and conventional rafter framing (used for porticos, additions, complex intersections, and cathedral ceilings). It also covers the general roof framing knowledge that applies to both — pitch, snow loads, ventilation, sheathing, fall protection, and the inspection discipline that separates professional work from guesswork.

Every section here is written for the field. Theory matters, but what matters more is knowing how to set a 20-metre scissor truss without killing anyone, how to cut a birdsmouth that actually bears on the plate, and how to brace a roof system so it’s still standing when the wind picks up at 3:00 in the afternoon.

1. Truss Types for Churches

Not all trusses are created equal, and church construction uses a wider variety of truss profiles than almost any other building type. A typical commercial warehouse gets a sea of identical common trusses. A church gets scissor trusses over the sanctuary, parallel chord trusses over the fellowship hall, mono trusses at the clerestory, hip sets at the entrance, and attic trusses over the classroom wing — all on the same project. Understanding the types and their applications is fundamental.

Scissor Trusses — The Church Standard

The scissor truss deserves special attention because it appears on virtually every church project with a traditional sanctuary. The sloping bottom chord creates the vaulted ceiling that congregations associate with sacred space — that upward sweep drawing the eye toward the ridge. But scissor trusses are structurally more complex than common trusses. The sloping bottom chord generates significant horizontal thrust at the bearing points (similar to a rafter without a tie), which means the bearing walls and connections must be designed to resist that thrust. The bottom chord also experiences both tension and compression depending on load combinations, making permanent bracing of the bottom chord absolutely critical.

Scissor truss bottom chord slope is typically one-half the top chord slope. A 6/12 top chord pitch yields a 3/12 bottom chord slope. This ratio can be adjusted by the truss engineer, but steeper bottom chords mean higher horizontal thrust and heavier connections.

Hip Sets and Girder Trusses

A hip roof on a church entrance or fellowship hall requires a girder truss — a heavy truss that runs perpendicular to the common trusses and carries the hip jack trusses on its top chord. Girder trusses are typically multi-ply (two or three trusses bolted together) and are the heaviest trusses on the project. They require specific bearing conditions and are almost always crane-set individually with dedicated rigging.

2. TPIC Bracing Requirements

The Truss Plate Institute of Canada (TPIC) publishes bracing requirements that are not suggestions, not guidelines, and not “nice to have.” They are mandatory. Inadequate bracing is the single most common cause of truss failures during construction, and the results are catastrophic — progressive collapse (the “domino effect”), destroyed materials, massive schedule delays, and injuries or deaths. Every person involved in truss setting must understand both temporary and permanent bracing.

Temporary Bracing

Temporary bracing keeps trusses stable during installation — from the moment the first truss is set until all permanent bracing, sheathing, and diaphragm connections are complete. The sequence matters:

1. Ground bracing the first truss: The first truss set is the most vulnerable. It must be braced back to a permanent structure (shear wall, concrete wall, steel frame) or ground-braced with diagonal 2×4 braces at 45° minimum, staked or weighted at the base. A minimum of three brace points: one at the peak and one at each quarter-point of the span. The first truss does not stand on its own — ever.
2. Lateral restraint across top chords: As subsequent trusses are set, continuous 2×4 lateral bracing is installed on edge along the top chord at maximum 2400 mm (8’) intervals measured down the slope from the peak. This prevents individual trusses from rolling sideways. Nail the lateral brace to each truss top chord with at least two 16d nails per connection.
3. Diagonal bracing: Diagonal braces from the top chord down to the bottom chord of an adjacent truss, installed every 6 m maximum along the building length. These triangulate the temporary bracing system and prevent the entire row of trusses from racking as a group.
4. Bottom chord lateral bracing: Continuous 2×4 lateral bracing along the bottom chord at the same 2400 mm intervals as the top chord. This is especially critical for scissor trusses, where the sloping bottom chord wants to roll under load.
5. Do not strip temporary bracing until all roof sheathing is complete, all permanent bracing is installed, and the structural diaphragm is fully connected. Removing temporary bracing prematurely is one of the most common causes of construction collapses.

Permanent Bracing

Permanent bracing stays in the roof system for the life of the building. It is specified on the truss engineering drawings and is not optional. Common permanent bracing elements include:

• Continuous lateral bracing (CLB): Continuous 2×4 members running perpendicular to the trusses, nailed to each truss chord or web at locations specified by the truss engineer. CLB prevents lateral buckling of long compression members.
• T-reinforcement: Where CLB crosses a truss member, a short 2×4 “T” block is nailed perpendicular to the CLB and to the truss member. This prevents the CLB from rolling off the member it’s bracing. Without the T-reinforcement, CLB is marginally effective at best.
• Web bracing for compression webs: Long compression webs (typically the diagonal webs near the supports on long-span trusses) require lateral bracing to prevent buckling. The truss engineer specifies which webs need bracing and the bracing method — typically a continuous 2×4 nailed to the side of each web at mid-height.
• Bottom chord bracing on scissor trusses: Because scissor truss bottom chords experience compression under certain load combinations, they require permanent lateral bracing at intervals specified by the engineer. This is in addition to the top chord bracing that all trusses receive from the roof sheathing diaphragm.

3. Crane Setting for Large-Span Trusses

Church sanctuary trusses — 15 to 22 metres and weighing 500 to 1,500 kg each — are always crane-set. This is one of the highest-risk operations on any church project. A truss in the air is a massive sail, a pendulum, and an unbraced structural member all at once. Success depends on planning, communication, and absolute discipline.

Pre-Lift Planning

• Truss weights: Obtain from the truss manufacturer’s shop drawings. Include the weight of any field-splice hardware. Add 10% for rigging and spreader bar weight.
• Crane position: Work with the crane operator to determine setup location, boom length, and swing radius. The crane must be able to reach the farthest truss location without exceeding its load chart for that radius. A crane chart is physics, not a suggestion.
• Spreader bar: For trusses over 12 m, a spreader bar is typically required to distribute the lifting forces and prevent the truss from buckling at the pick points. The spreader bar length and capacity must match the truss geometry.
• Laydown area: Trusses should be staged as close to the building as possible, oriented for efficient picking (tag end toward the crane, ridge end toward the building). The ground must support the crane’s outrigger loads — matting or cribbing on soft ground.
• Wind limits: Most truss setting operations impose a wind limit of approximately 35 km/h. Trusses are essentially flat sails — even moderate wind creates enormous lateral forces on a suspended truss. Monitor wind conditions continuously. If the limit is reached, set the truss down and wait. No schedule pressure justifies setting trusses in unsafe wind conditions.

Rigging and Pick Points

• Pick points: Located at the quarter-points of the top chord span (approximately 25% and 75% of the span length from each end). These locations minimise bending stresses in the truss during lifting. The truss engineer may specify exact pick points on the shop drawings — use those locations, not your best guess.
• Slings: Use nylon web slings or polyester round slings. Never use wire rope or chain directly on a wood truss — the concentrated point load will crush the chord. Choker hitches around the top chord at pick points, with shackles to the spreader bar.
• Tag lines: Minimum two tag lines — one at each end of the truss — controlled by workers on the ground or on elevated platforms (never on the truss itself during the lift). Tag lines control rotation and prevent the truss from spinning in the wind.

Setting Sequence

1. First truss is lifted, set on bearing, and immediately ground-braced as described in Section 2. The crane does not release until all ground braces are secure.
2. Each subsequent truss is set at the correct spacing (per shop drawings) and immediately braced laterally to the previous truss at the top chord and bottom chord with temporary 2×4 bracing. The crane hook is not released until the truss is braced.
3. Diagonal bracing is installed every 6 m or as the TPIC bracing plan requires.
4. A designated signal person maintains communication with the crane operator throughout. Only one person gives signals. Radio or hand signals per CSA Z150.
5. Workers position themselves on scaffolding, elevated work platforms, or secured aerial lifts to receive trusses — never on unbraced truss top chords.
6. Never allow more than one un-braced truss standing. If a truss is set, it gets braced before the next one is picked. No exceptions, no “we’ll brace them in batches.”

4. Truss Handling and Storage

Trusses arrive on site as finished engineered products. Every member is sized to specific loads, every connector plate is pressed to precise specifications. Damage a chord, crack a plate, or bow a web during handling and you don’t have a truss anymore — you have scrap lumber with some metal plates stuck to it.

• Storage orientation: Store trusses upright on continuous blocking at 3 m maximum intervals. Do not lay trusses flat on the ground — deep trusses will roll, and the bottom chord will bend or break under the self-weight of the truss above.
• Blocking: Use 4×4 or 6×6 blocking on firm, level ground. Blocking prevents ground moisture contact and allows air circulation. The first truss in a row gets braced to the blocking or to stakes to prevent toppling.
• Weather protection: Cover stored trusses with breathable tarps. Do not use polyethylene sheeting — it traps moisture and promotes mould growth on the lumber. Lumber that gets soaked and then enclosed in poly can develop staining that shows through paint finishes.
• No stacking materials: Do not stack lumber, sheathing, or any other materials on top of stored trusses. The connector plates and web members are designed for specific load patterns — a stack of plywood on a bottom chord panel point is not one of them.
• Delivery inspection: Inspect every truss at delivery before the truck leaves. Check for cracked or split members, popped or buckled connector plates, excessive bowing, and damage from transport. Document defects with photos and notify the truss manufacturer immediately. Damaged trusses get rejected on the spot — not “set and we’ll fix it later.”

5. Truss Connection Details

A truss is only as strong as its connections to the building. The most perfectly engineered truss in the world is useless if the bearing connection fails, the uplift tie breaks, or the hurricane clip pops off in a windstorm. Connection details matter — every nail, every strap, every bolt.

Truss-to-Plate Connections

• Toenailing: Standard connection for basic gravity loading: three 82 mm (3-1/4”) nails per side, toenailed through the truss bottom chord into the top plate. This handles vertical and minor lateral loads.
• Hurricane ties / truss clips: Required in addition to toenailing for uplift resistance. Simpson Strong-Tie H2.5A or equivalent, nailed per the manufacturer’s schedule (typically 8–10 nails per clip). These straps wrap over the top chord and nail into the plate, creating a positive connection that resists wind uplift forces.
• Engineered connectors: For heavy trusses on large-span church applications, the engineer may specify custom steel bearing connectors — welded seat angles, bolted base plates, or purpose-designed hangers. Install exactly per the sealed shop drawing.

Bearing Requirements

• Minimum bearing length on wood plates: 89 mm (3-1/2”) — equivalent to a standard 2×4 plate width.
• On steel beams or concrete walls, bearing pads or bearing plates distribute the load and prevent point-loading the bottom chord.
• The truss heel must be fully supported on the plate — no overhang beyond the plate edge, and no gap between the truss and the plate. A truss sitting on half the plate is bearing on half its designed area.

Uplift Resistance for Ontario Wind Loads

Ontario wind loads for church buildings (assembly occupancy, importance factor I_w = 1.15) generate significant uplift forces, particularly on steep-pitched sanctuary roofs. The truss-to-wall and wall-to-foundation load path must be continuous. Hurricane ties connect truss to plate, the plate is anchored to the wall framing with holddowns or anchor straps, and the wall framing is anchored to the foundation with anchor bolts or straps. A break anywhere in this chain means the roof can lift off the building. The engineer designs the full uplift load path — the framing crew installs every connector in that chain, no shortcuts.

6. Truss Modifications — What You Cannot Do

This section is short because the rule is simple: do not modify a truss in the field without sealed written approval from the truss engineer. No exceptions, no “just a little notch,” no “it’s just a small hole.”

Absolutely Prohibited (Without Engineer Approval)

• Cutting any chord or web member
• Notching any chord or web member
• Drilling holes in any chord or web member
• Removing any web member (even temporarily)
• Adding loads not accounted for in the design (HVAC units, suspended ceilings, storage)
• Altering any connector plate
• Relocating bearing points

Potentially Permissible (With Sealed Written Approval Only)

• Small holes (25 mm max) in the centre third of the bottom chord depth, at the centre third of the span — for pulling wires or small pipes. The truss engineer must approve the specific location, size, and number of holes.
• Reinforcement of damaged members using plywood gussets or sistered members, per the engineer’s repair detail.
• Addition of hanger points for suspended loads, with the engineer verifying the truss can handle the added load at the specified panel point.

7. Common Truss Failures

Understanding how trusses fail helps you prevent failures. These are the most common causes of truss problems on construction sites, in roughly descending order of frequency:

1. Missing permanent bracing (#1 cause): The truss engineer specifies permanent bracing on the sealed drawings. If CLB, T-reinforcement, or web bracing is shown on the drawings and not installed, the truss system is operating below its design capacity. Compression members buckle, bottom chords roll, and the failure may not happen for years — until a heavy snow load or wind event pushes the system past its reduced capacity. Permanent bracing is not optional framing “extra credit.”
2. Inadequate temporary bracing: This causes the dramatic domino-collapse failures during construction. A row of unbraced trusses subjected to wind or accidental lateral impact falls like a row of playing cards. Prevention: follow the TPIC bracing plan to the letter.
3. Overloaded bottom chords: Mechanical contractors hanging HVAC equipment, ductwork, and piping from bottom chords at locations or in quantities not designed for in the truss. Electrical and AV contractors adding conduit runs and equipment. All suspended loads must be identified during design and accounted for in the truss engineering. Anything added after the fact needs the truss engineer’s written approval.
4. Bearing problems: Trusses set on inadequate bearing (less than 89 mm), off the bearing point, or on materials that can’t support the reaction (drywall, insulation, unsupported sheathing). The truss engineer designs for specific bearing conditions — the field must deliver those conditions.
5. Site damage: Forklifts hitting truss members, trades cutting or notching chords and webs for ductwork or piping, improper storage causing bending, weather damage to chord members. Every instance of damage must be documented and assessed by the truss engineer.
6. Connector plate failures: Plates that were not fully embedded during manufacture, plates damaged during shipping, or plates that have pulled out of water-damaged wood. Connector plate integrity is checked during delivery inspection — any plate that is popped, buckled, or shows gaps between the plate and the wood surface requires engineering assessment.

8. Rafter Layout and Cutting

Conventional rafter framing is the craft side of roof building — every cut calculated, every rafter individually measured and fitted. It requires a working understanding of geometry that no truss engineer’s shop drawing will give you. If truss setting is about logistics and discipline, rafter framing is about skill and precision.

Rafter Cuts

• Plumb cut (ridge cut): The vertical cut at the top of the rafter where it meets the ridge board or ridge beam. The angle matches the roof pitch — a 6/12 pitch means the plumb cut is at the angle whose tangent is 6/12. On a framing square, set 6 on the tongue and 12 on the blade, and the tongue gives you the plumb cut line.
• Birdsmouth (seat cut + heel cut): The notch where the rafter sits on the top plate. The birdsmouth has two components: the seat cut (level, bearing on the plate) and the heel cut (plumb, at the inside edge of the plate). The seat cut depth must not exceed one-third of the rafter depth — deeper than that weakens the rafter at its most critical point. The seat cut width should match the plate width for full bearing.
• Tail cut: The cut at the lower end of the rafter tail that forms the fascia surface. Typically a plumb cut, but may be a square cut or compound cut depending on the fascia detail.

Pattern Rafter Procedure

1. Calculate the theoretical rafter length using the run (half the span) and the pitch. Rafter length = run × (unit rafter length per metre of run from rafter tables).
2. Select the straightest rafter in the lumber pile. Mark and cut the ridge plumb cut.
3. Measure the calculated length along the top edge (crown side) and mark the birdsmouth location.
4. Cut the birdsmouth. Verify that seat cut depth does not exceed one-third rafter depth.
5. Test-fit the pattern rafter in place. It should bear fully on the plate at the birdsmouth and meet the ridge board at the correct height with full contact on the plumb cut.
6. Adjust if necessary. Once the pattern is confirmed, use it to mark all remaining common rafters. Always crown up — the natural bow in the lumber faces the sky so gravity loads tend to straighten it rather than increase the bow.

9. Ridge Board vs. Ridge Beam

This is one of the most misunderstood distinctions in roof framing, and getting it wrong has structural consequences. They look similar. They sit at the ridge. But they do completely different things.

Ridge Board (Non-Structural)

• A ridge board is a non-structural member that serves as a nailing surface where opposing rafters meet. It does not carry vertical load.
• The ridge board works only when ceiling joists or rafter ties connect the opposing rafters at or near the plate level. The joists/ties resist the horizontal thrust that the rafters exert on the walls. Without them, the walls spread and the ridge drops.
• Minimum depth: equal to the cut end (plumb cut) of the rafter. A common rafter cut from a 2×10 at 6/12 pitch has a plumb cut depth of about 220 mm — so the ridge board must be at least a 2×10 (actual 235 mm). Using a 2×6 ridge board with 2×10 rafters is a code violation and a structural deficiency.
• Material: typically No. 2 SPF dimensional lumber (2×8, 2×10, 2×12) or LVL for long runs.

Ridge Beam (Structural)

• A ridge beam is a structural member that carries the vertical component of the rafter loads. It is required when there are no ceiling joists or rafter ties — i.e., for cathedral ceilings, open vaulted spaces, or any condition where the rafters are not tied together at the bottom.
• A ridge beam must be supported at each end (and at intermediate points if the span requires) by posts that carry the load to the foundation. The load path must be continuous. A ridge beam sitting on a non-bearing wall is not a ridge beam — it’s a liability.
• Material: typically LVL, PSL, or glulam, sized by the structural engineer for the tributary roof load and the beam span.
• Exposed glulam ridge beams in church sanctuaries: One of the most dramatic architectural features in church design. A massive glulam ridge beam spanning the full length of the sanctuary, exposed to view from below, with rafters framing into it. These beams can be 265×600 mm or larger, spanning 8–15 m between posts. They arrive on site as finish-grade products and must be handled, stored, and installed with the same care described in the structural timber section — nylon slings only, protective wrapping, no Sharpies.

10. Collar Ties, Rafter Ties, and Ceiling Joists

These three members are often confused, sometimes used interchangeably in conversation, and frequently installed in the wrong location. They are not the same thing. They serve different structural functions. Getting this wrong can mean a roof that spreads, a ridge that separates, or collar ties installed where rafter ties should be — which is structurally useless for the purpose intended.

The Three Members, Defined

11. Hip and Valley Rafter Framing

Hip and valley rafters are where conventional framing gets genuinely complex. These are the diagonal rafters at roof intersections — hips at outside corners, valleys at inside corners — and they carry loads from jack rafters while running at 45° to the common rafters. The geometry involves compound angles that make experienced carpenters reach for their calculators and apprentices reach for their phones to call in sick.

Hip Rafter Layout

• Run calculation: A hip rafter runs diagonally across the building corner. Its run is 1.414 times the run of a common rafter (the diagonal of a square with sides equal to the common rafter run). For a building with a 6 m common rafter run, the hip run is 6 × 1.414 = 8.485 m.
• Unit rise: The hip rafter has the same total rise as the common rafter but covers a longer run. Its pitch per metre of run is shallower. For a 6/12 common rafter (150 mm rise per 300 mm run), the hip rises 150 mm per 424 mm of hip run. On the framing square, use 6 on the tongue and 17 on the blade (17” being the diagonal of a 12” square).
• Backing vs. dropping: A hip rafter’s top corners project above the plane of the roof sheathing because of its diagonal orientation. To make the sheathing lie flat, the hip rafter must be either backed (bevelled on both top edges to match the roof planes) or dropped (the birdsmouth deepened to lower the hip until the top edges align with the roof planes). Dropping is more common in production framing; backing is the more precise traditional method.

Jack Rafters and Compound Cheek Cuts

• Jack rafters are common rafters cut short to meet the hip or valley rafter. They decrease in length at a constant increment (the “common difference”) based on the rafter spacing.
• The cut where a jack rafter meets a hip or valley is a compound cheek cut (also called a side cut or bevel cut). This cut has both a plumb angle and a bevel angle — the rafter is cut at a plumb angle for the pitch and bevelled sideways to fit tight against the hip or valley.
• Framing squares with rafter tables give the side cut angles. A Construction Master calculator makes the math much faster. Either way, cut a test piece first — compound angles are easy to get backward, and every carpenter has at least one story about cutting 16 jacks with the bevel facing the wrong way.

Valley Rafters

Valley rafters follow the same geometry as hip rafters but at inside corners where two roof planes intersect. The key differences: valley rafters typically use a supporting valley (a single long valley rafter running from plate to ridge) with a shortened valley meeting it where the secondary ridge intersects. Jack rafters frame into the valley rafter from both sides. The nailing of jack rafters to valley rafters must carry the full tributary roof load — face-nailing with three 82 mm nails per jack minimum, or as the engineer specifies.

12. Lookout and Rake Framing

The overhangs at the eaves and rakes are what give a church roofline its proportions. A church with no overhang looks like a cardboard box. A church with generous, well-framed overhangs looks like it was designed by someone who understood both architecture and weather protection. Overhangs also serve the critical function of keeping water away from the wall below.

Eave Overhangs

Eave overhangs are formed by the rafter tails extending beyond the wall plate. The tail length is determined by the architect — typically 300–600 mm for church buildings. The subfascia (and eventual fascia board) attaches to the ends of these tails, creating the straight eave line that frames the building.

Rake Overhangs

Rake overhangs (at the gable ends) are more complex because there’s no rafter tail to extend. Two methods:

• Lookout blocks (ladder framing): Short 2×4 blocks (lookouts) run perpendicular from the last inboard rafter (typically the second rafter from the gable end) out through the gable wall framing to support the fly rafter (the outermost rafter at the rake). The lookouts are spaced at 400–600 mm o.c. and nailed to both the inboard rafter and the fly rafter. This creates a ladder-like assembly that supports the rake overhang.
• Extended sheathing with blocking: For narrow rake overhangs (under 300 mm), the roof sheathing can be extended past the gable wall with blocking between the last two rafters at each sheathing edge to provide support. This method is simpler but limits overhang width.

The fly rafter at the rake is the most visible rafter on the building — it’s the one people see from the ground against the sky. It must be straight, properly supported, and accurately plumb-cut at both ends. A wavy fly rafter is visible from across the parking lot and cannot be fixed after the fascia is installed without tearing the whole assembly apart.

13. Pitch Terminology and Common Church Pitches

Roof pitch is expressed as rise-over-run: the number of units of rise per 12 units of horizontal run. A 6/12 pitch rises 6 inches for every 12 inches of horizontal run (or equivalently, 150 mm per 300 mm). Understanding pitch is fundamental — it determines rafter lengths, cut angles, material selection, snow shedding capability, and even the construction methodology (steep roofs require different fall protection than low-slope roofs).

Pitch-to-Angle Conversion Table

Church roofs span the full range. A classroom wing might have a modest 4/12 pitch, while the sanctuary gable soars at 12/12 or steeper. Gothic-inspired designs push to 14/12 or 16/12 — pitches where the roof is essentially a wall, and the framing, sheathing, and working conditions change dramatically. Any pitch above 8/12 requires roof jacks for worker footing and alters the fall protection approach.

Rafter Length Calculation

Rafter length is a function of the run (half the building span for a symmetrical gable) and the pitch. The formula is straightforward: Rafter Length = Run × Rafter Length Factor. The rafter length factor for each pitch is derived from the Pythagorean theorem — it equals the square root of (1 + (rise/12)²). For a 6/12 pitch, the factor is √(1 + 0.25) = 1.118, so a 5 m run produces a rafter line length of 5.59 m. Add the overhang length (tail) and subtract the ridge thickness deduction (half the ridge board or beam thickness) to get the cutting length.

14. Snow Loads for Ontario

Ontario gets snow. Sometimes it gets a lot of snow. And churches, as assembly-occupancy buildings with large roof areas and steep pitches that create drift zones, are particularly sensitive to snow loading. Every structural member in the roof — every truss, rafter, ridge beam, and connection — is designed for specific snow loads based on the project location. Understanding these loads helps framing crews appreciate why the engineer specified those heavy trusses and why the bracing details matter.

Ground Snow Loads by Ontario City (NBC Table C-2)

Importance Factor for Churches

Churches are classified as assembly occupancy (major occupancy Group A, Division 2). The importance factor for snow loads on assembly buildings is I_s = 1.15 per NBC 2020 Table 4.1.6.5. This means the design snow load on a church roof is 15% higher than on a standard commercial or residential building in the same location. The importance factor recognises that assembly buildings have higher consequences of failure due to the number of occupants.

Special Snow Loading Conditions on Church Roofs

• Unbalanced snow loading: Steep church roofs can have snow slide off one side (windward) while accumulating on the other (leeward). The NBC requires analysis of unbalanced load cases where one side of the roof has significantly more snow than the other. This asymmetric loading creates different forces in the truss than a uniform load and can be the controlling design case.
• Drift accumulation at intersecting roofs: Where a lower roof meets a higher wall (as at a clerestory or where a classroom wing meets the sanctuary), wind-driven snow accumulates against the wall. These drift loads can be 2–4 times the basic roof snow load and are the most common cause of localised roof failures on church buildings. The structural engineer must specifically design for drift zones.
• Sliding snow loads: Snow sliding off a steep upper roof onto a lower roof below creates impact loading. Common at church buildings where the sanctuary roof overhangs a lower narthex or entrance.

15. Roof Ventilation

A properly ventilated roof attic prevents ice damming, reduces summer heat gain, controls moisture accumulation, and extends the life of the roofing material. An improperly ventilated roof does none of those things and instead grows mould, rots the sheathing, creates ice dams that back water under shingles, and creates a maintenance headache that lasts the entire life of the building.

OBC Requirements (9.19.1)

• Ventilation ratio: Minimum 1/300 of the insulated ceiling area when ventilation is balanced (roughly equal intake and exhaust). If ventilation is unbalanced (more exhaust than intake, or vice versa), the ratio increases to 1/150.
• Balanced ventilation: At least 50% of the required ventilation area must be provided at the eave/soffit (intake), with the remainder at or near the ridge (exhaust). The physics is simple: cool air enters at the soffit, warms as it contacts the roof deck, rises, and exits at the ridge, carrying moisture with it.
• Soffit vents with baffles: Insulation baffles (Styrofoam or cardboard rafter bays) must be installed at each rafter bay at the eave to maintain a clear airflow path from the soffit vent to the attic space above the insulation. Without baffles, the blown-in insulation blocks the soffit vents and the ventilation system is dead.
• Ridge vents: Continuous ridge vents provide the most effective and aesthetically clean exhaust ventilation. The framing crew must leave a slot (typically 50–75 mm on each side of the ridge) in the sheathing to create the exhaust opening. Do not sheathe over the ridge and expect the ridge vent to work through solid plywood.

Cathedral Ceiling Ventilation Challenges

Cathedral ceilings — particularly with scissor trusses — present unique ventilation challenges. The reduced attic space means the airflow path from soffit to ridge is narrow and can be easily blocked by insulation. For scissor truss roofs:

• Maintain a minimum 63 mm (2-1/2”) clear air space above the insulation in each rafter bay, continuous from soffit to ridge.
• Use rigid insulation baffles (not just cardboard) to maintain the air channel under the roof sheathing.
• Consider whether the insulation depth required for energy code compliance can fit in the available depth while maintaining the ventilation channel. If not, the assembly may need to shift to an unvented (hot roof) design with spray foam insulation — a decision that must be made during design, not discovered during framing.

16. Fall Protection for Roof Framing

Roof framing is the most dangerous phase of construction for falls. You’re working at height, on sloped surfaces, with heavy materials, often in wind. Ontario regulation is clear, the consequences of non-compliance are severe, and the consequences of an actual fall are worse. This section is not optional reading.

Ontario Reg. 213/91, Section 26

• 3-metre trigger: Fall protection is required for any work at a height of 3 m (10 feet) or more above a floor, surface, or platform. On virtually every church roof framing operation, you are above 3 m. Fall protection is not a discussion — it is a legal requirement.
• Hierarchy of fall protection (in order of preference):
  1. Guardrails: The preferred method. Temporary guardrail systems at eave edges and gable ends, meeting the requirements of Section 26.1 (1070 mm top rail, mid-rail, toe board on open sides).
  2. Travel restraint: A system that prevents the worker from reaching the fall edge. The lanyard length is set so the worker cannot physically reach the point where a fall could occur.
  3. Fall arrest: A full-body harness, shock-absorbing lanyard, and anchor point rated for 22.2 kN (5,000 lbs). Fall arrest does not prevent a fall — it arrests one. The system must limit free-fall distance to 1.5 m maximum.
  4. Safety net: Rarely used in church construction but included in the hierarchy.

Anchor Points

• Anchor points must be rated for the fall arrest load (22.2 kN per worker attached).
• A single truss web or chord is NOT an acceptable anchor point unless specifically rated and approved by a P.Eng. for that purpose. A standard truss web in tension might handle 8–15 kN — well below the fall arrest anchor requirement. A compression web used as an anchor point can buckle under the dynamic load of a fall.
• Acceptable anchor points include: engineered roof anchors (installed per manufacturer specs), steel beams or columns rated for the load, and purpose-designed lifeline systems.
• A horizontal lifeline along the ridge, engineered by a P.Eng. and installed per CSA Z259.16, allows multiple workers to move along the roof while remaining tied off.

Written Plans

• A written fall protection plan is required before any work begins at height. The plan identifies the hazards, the fall protection methods to be used, the anchor points, and the training requirements.
• A rescue plan is mandatory wherever fall arrest is used. If a worker falls and is suspended in a harness, suspension trauma can cause serious injury or death within minutes. The rescue plan must specify how the worker will be rescued, by whom, with what equipment, and within what timeframe. “Call 911” is not a rescue plan.

Roof Pitch and Working Conditions

• Above 8/12 pitch (33.7°): Roof jacks (adjustable metal brackets nailed to the roof through the sheathing into rafters) with scaffold-grade planks are required to provide a working platform. Walking on sheathing above 8/12 is not feasible — you will slide.
• On Gothic-pitch church roofs (12/12 and above, 45°+), the “roof” is essentially a wall. Scaffolding on the exterior face of the roof, or swing-stage access, may be required for sheathing and roofing installation.

17. Roof Sheathing

Roof sheathing is the structural skin of the roof. It transfers wind and gravity loads to the trusses or rafters, provides the diaphragm action that braces the building laterally, and serves as the substrate for the roofing membrane. Getting sheathing right means getting the orientation, staggering, fastening, and gapping right. Getting it wrong means a roof that leaks, a diaphragm that doesn’t work, and an inspector who makes you tear it off and start over.

Panel Orientation and Staggering

• Orientation: OSB and plywood roof sheathing is installed with the long dimension (2440 mm / 8’) perpendicular to the rafters or trusses. This orientation spans three or more framing members and provides maximum strength and stiffness.
• Staggering: End joints must be staggered by at least one rafter/truss spacing (400 mm minimum). Aligning end joints on the same rafter creates a weak line across the roof.
• H-clips: Where panel edges are unsupported between framing members (the long edges of standard 1220 mm-wide panels at 600 mm o.c. framing), H-clips are inserted between adjacent panels to provide edge support and maintain the required expansion gap. H-clips also prevent differential deflection between adjacent panels, which shows up as ridges under the roofing membrane.
• Expansion gap: Leave a 3 mm (1/8”) gap between all panel edges and ends. This allows for moisture expansion and prevents buckling. A panel that expands with no gap will buckle — and buckled sheathing under shingles looks terrible and voids roofing warranties.

Nailing Schedule

• Standard schedule: 8d common nails (63 mm / 2-1/2”) or equivalent pneumatic nails. 150 mm (6”) o.c. along panel edges, 300 mm (12”) o.c. in the field (intermediate supports).
• Enhanced diaphragm nailing: In high-wind zones or where the engineer specifies enhanced diaphragm action, the edge nailing may be reduced to 100 mm (4”) or even 75 mm (3”) o.c. This is specified on the structural drawings and is not a field decision.
• Overdriven nails: A nail driven through the face of the sheathing (where the head breaks through the surface) does not count for structural purposes. It has zero withdrawal resistance and zero shear capacity at that location. Set your pneumatic nailer pressure so the nail head is flush with the panel surface or just barely below. One PSI adjustment at the compressor prevents thousands of defective fasteners.
• Minimum edge distance: 10 mm (3/8”) from panel edge. Closer and the nail tears out under shear. Mark a pencil line 10 mm in from the edge if you need to — it’s faster than re-nailing a panel after the inspector rejects it.

18. Fascia and Subfascia

The fascia is the visible face of the eave — the board (or metal) that runs along the ends of the rafter tails and defines the roofline. Behind it, the subfascia does the real structural work of straightening the eave line and providing a solid nailing surface for the fascia, soffit, and eavestrough.

Subfascia

• Function: The subfascia (typically 2×8 or 2×10 SPF) is nailed to the rafter tails and establishes the straight, level eave line. It compensates for variations in rafter tail length, lumber bow, and plate irregularities.
• Installation method: Snap a chalk line from the wall outward to the rafter tails at the design fascia height. Do not measure from the ridge down — ridge variations accumulate and create a wavy eave line. Establish your reference from the wall plate, which is your most controlled surface.
• Straightening: Use a string line along the length of the eave. Adjust individual rafter tails (trim or shim) to bring the subfascia into a straight line. Irregularities of more than 5 mm are visible after fascia installation and look terrible from ground level.
• Nailing: Three 82 mm (3-1/4”) nails per rafter tail, face-nailed through the subfascia into the rafter end.

Fascia

• Wood fascia: Cedar, pine, or composite boards nailed to the subfascia. Requires painting and periodic maintenance.
• Aluminum brake-formed fascia: Custom-formed aluminum cladding that wraps the subfascia and returns to the soffit. Maintenance-free, colour-matched, and the standard finish on most church projects. The aluminum fascia is installed by the siding or sheet metal contractor after the subfascia is straight and true. If your subfascia isn’t straight, the aluminum fascia will faithfully reproduce every wave and bump for the entire congregation to admire from the parking lot.

Frieze Board

The frieze board is a trim board installed at the junction where the wall meets the soffit — it fills the gap between the top of the wall cladding (siding, brick, stone) and the underside of the soffit. On church projects, the frieze board provides a clean visual termination of the wall and a sealed transition to the roof assembly. It is typically wood (cedar or primed pine) or aluminum brake-formed to match the fascia and soffit. The framing crew must ensure blocking is in place between rafters or trusses at the wall line to provide a solid nailing surface for the frieze board installation.

19. Common Quality Issues and Inspection Points

Quality in roof framing is verified by inspection — both self-inspection by the framing crew and formal inspection by the site supervisor and building official. The following checklists cover the most common deficiencies found during roof framing inspections. Walk these lists before you call for inspection and fix every deficiency yourself. An inspector who finds a clean, well-framed roof remembers your work the next time. An inspector who finds a mess remembers that too.

Truss Installation Inspection Checklist

Conventional Rafter Inspection Checklist

Sheathing Inspection Checklist

Rafter Span Reference

The following table provides approximate maximum spans for common rafter sizes at standard spacings, based on typical Ontario loading conditions (1.5 kPa snow load, No. 2 SPF). These are reference values only — always confirm spans with the structural engineer for specific project conditions, particularly for church assembly-occupancy buildings where the I_s = 1.15 importance factor applies.

20. Standards and References

Every section in this guide connects to specific codes, standards, and industry documents. These are the authoritative references for roof framing on church construction projects in Ontario. Know them, have access to them on site, and be prepared to cite them when your work is questioned.