Framing and Structural Systems

Chapter Introduction

This chapter covers the fundamental principles of light wood framing, bracing systems, structural loads, and the requirements of the National Building Code of Canada (NBC). You must master these concepts to pass the Red Seal exam, as they represent a significant portion of the questions on work organization and installation techniques. The focus is on understanding load paths, span calculations, connections, and fire and wind resistance requirements.

Loads and Load Paths

Types of Loads

Every structure must resist two main categories of loads: dead loads and live loads. Dead loads include the self-weight of materials: wood, drywall, cladding, roofing, etc. Live loads include occupants, furniture, snow, wind, and seismic forces.

The NBC requires that loads be calculated according to the articles in Part 4 (calculations) or Part 9 (housing and small buildings). For Part 9 buildings, minimum live load values are prescribed: 1.9 kPa for residential floors, 2.4 kPa for stair floors, and 4.8 kPa for assembly areas.

Load Path

The load path is the route a load follows from its point of application down to the ground. For a roof: shingles → roof sheathing → rafters or trusses → exterior wall or ridge beam → studs → foundation → soil. Each element must transfer its load to the element below without failure. A golden rule: no load should be supported by an element not designed to receive it (e.g., a floor supported by non-load-bearing partitions).

Snow Load Calculation

The ground snow load (Sₛ) is given by the NBC tables according to location. The roof snow load (Sᵣ) is calculated as follows:

Sᵣ = Sₛ × Cb × Cw × Cs × Ca

Where:

Cb = basic load factor (0.8 for ordinary roofs)

Cw = wind exposure factor (0.75 to 1.0)

Cs = slope factor (decreases as slope increases)

Ca = accumulation factor (for stepped roofs or local accumulations)

Exam trap: for a roof with a slope greater than 60°, the snow load can be considered zero, but wind load becomes predominant.

Platform Framing

General Principles

Platform framing is the dominant method in North America. Each floor is built as a platform on which the walls of the next floor are erected. The key elements are:

Bottom and top plates (double): distribute loads and provide nailing surfaces

Studs: spaced at 300 mm, 400 mm, or 600 mm depending on loads and spans

Headers: above openings, transfer loads to jack studs

Shims and blocking: prevent stud buckling and provide nailing surfaces

Stud Spacing

Maximum stud spacing depends on the supported load and lumber dimension:

Spacing	38×89 mm (2×4)	38×140 mm (2×6)
300 mm	Max span 3.0 m	Max span 4.0 m
400 mm	Max span 2.5 m	Max span 3.5 m
600 mm	Max span 2.0 m	Max span 3.0 m

These values are indicative; always consult the NBC tables (Part 9) for exact spans.

Jack Studs and Headers

Jack studs support the headers. The number required depends on the opening width and loads:

Opening Width	Jack Studs Required (2×4)
Up to 1.2 m	1
1.2 m to 2.4 m	2
2.4 m to 3.0 m	3

Headers must be sized according to the NBC span tables. A double 38×184 mm (2×8) header can support approximately 2.5 m for a standard floor load, but this varies with wood species and load.

Nailing Rules

Nailing is critical for structural strength. Nails must be spaced according to the NBC tables (Article 9.23.3.). Examples:

Stud to plate: 2 nails of 82 mm (3¼ in) per connection

Double top plate: 76 mm (3 in) nails spaced at 600 mm in a staggered pattern

Blocking between studs: 2 nails of 76 mm at each end

Rule of thumb: the nail length must be at least 2.5 times the thickness of the thinnest piece being fastened.

Bracing and Wind Resistance

Types of Bracing

Bracing prevents lateral deformation of the structure under wind or seismic forces. Acceptable methods according to the NBC (Article 9.23.10.):

44.Diagonal bracing: wood or metal members installed diagonally in walls, let into the studs

45.Structural sheathing: plywood or OSB (Oriented Strand Board) panels nailed to the exterior of walls

46.Shear wall bracing: interior walls specifically designed to resist lateral loads

Sheathing Panel Requirements

Structural panels must have a minimum thickness of 9.5 mm for stud spacing of 400 mm or less, and 12.5 mm for 600 mm spacing. Panel nailing: 64 mm (2½ in) nails spaced at 150 mm on panel edges and 300 mm on intermediate supports.

Shear Walls

A shear wall is a wall designed to resist lateral loads. NBC requirements:

Minimum length: 10% of the building length in each direction

Structural sheathing on at least one side

Anchorage to the foundation with 12.7 mm (½ in) anchor bolts spaced at a maximum of 2.4 m

Floors and Beams

Beams and Joists

Floor joists are sized according to the NBC span tables (Article 9.23.4.). Factors influencing span:

Joist dimension (38×184, 38×235, etc.)

Spacing (300, 400, 600 mm)

Wood species (MSR, #1/#2, etc.)

Live load (residential, commercial, etc.)

Dimension	Spacing	Max Span (wood #1/#2, 1.9 kPa load)
38×184 (2×8)	400 mm	3.2 m
38×235 (2×10)	400 mm	4.2 m
38×286 (2×12)	400 mm	5.0 m

Bearing and Clear Span

Clear span is the distance between supports. Joists must have a minimum bearing of 38 mm on walls or beams. The overhang (span beyond the support) must not exceed 3 times the joist depth.

Blocking and Bridging

Blocking between joists prevents overturning and distributes loads. It is required:

For spans greater than 2.1 m

At each support

At maximum intervals of 2.4 m for spans greater than 4.2 m

Blocking can be wood (same dimensions as the joists) or metal (steel bridging).

Roofs and Trusses

Rafters and Trusses

Two main systems for sloped roofs:

73.Rafters: installed individually, supported by exterior walls and the ridge

74.Trusses: prefabricated triangulated assemblies, supported only by exterior walls

Trusses are more efficient for spans greater than 6 m. They are designed by the manufacturer according to specified loads. The site must follow the manufacturer's installation plan.

Slope and Slope Factor

A roof slope is expressed as a vertical/horizontal ratio (e.g., 4/12 = 4 inches of rise per 12 inches of run). The slope factor (SF) is used to calculate the actual length of a rafter:

Actual length = Horizontal length × SF

For a 4/12 slope: SF = √(4² + 12²) / 12 = √(16 + 144) / 12 = √160 / 12 = 12.65 / 12 = 1.054

Truss Supports and Connections

Trusses must be supported by walls or beams, never by non-load-bearing interior partitions. The connection to the wall: anchorage with metal connectors or 82 mm nails, spaced according to the manufacturer's specifications. Temporary bracing is mandatory during installation to prevent lateral overturning.

Foundations and Anchorage

Types of Foundations

Foundations transmit loads to the soil. Common types:

Strip footings: under load-bearing walls, minimum width of 300 mm for Part 9 buildings

Isolated footings: under columns, sized according to the load

Slab-on-grade: with perimeter footings

Anchor Bolts

Walls must be anchored to the foundation with anchor bolts of 12.7 mm (½ in) diameter, embedded in concrete at least 100 mm. Maximum spacing is 2.4 m, with a bolt within 300 mm of each wall end. The washer must be at least 25 mm in diameter.

Wood Treatment in Contact with Soil

Any wood in contact with soil or concrete must be pressure-treated (CCA, ACQ, etc.) or be a naturally durable species (cedar, larch). Untreated wood rots quickly in the presence of moisture.

Span and Load Calculations

Simplified Calculation Method

To verify a span, use the maximum deflection formula:

Δ_max = L / 360

Where L is the span in mm. For a 4 m floor: Δ_max = 4000 / 360 = 11.1 mm. The total deflection (dead load + live load) must not exceed this value.

Linear Load on a Beam

The linear load (w) on a beam supporting a floor:

w = (floor load in kPa) × (tributary width in m)

Example: floor with a load of 2.4 kPa, tributary width of 3 m: w = 2.4 × 3 = 7.2 kN/m.

The maximum bending moment for a simply supported beam:

M_max = w × L² / 8

For w = 7.2 kN/m and L = 4 m: M_max = 7.2 × 16 / 8 = 14.4 kN·m.

Stress Verification

The bending stress (σ) is calculated:

σ = M_max / S

Where S is the section modulus (in mm³). For a rectangular beam: S = b × h² / 6. For a 89×235 mm beam: S = 89 × 235² / 6 = 819,000 mm³.

σ = 14,400,000 N·mm / 819,000 mm³ = 17.6 MPa

This value must be less than the allowable stress of the wood (e.g., 12.5 MPa for SPF #1/#2). If it is higher, you must increase the section or reduce the spacing.

Connections and Hardware

Types of Connectors

Metal connectors (hangers, saddles, brackets) are used for critical connections. They are rated according to their load capacity (e.g., a joist hanger for 38×235 joists with a capacity of 10 kN). The manufacturer provides nailing specifications for each connector.

Nails and Screws

Nails are classified by their diameter (gauge) and length. Common nails:

Type	Length	Diameter	Use
Common nail 64 mm	64 mm (2½ in)	3.76 mm (9 gauge)	Sheathing, studs
Common nail 82 mm	82 mm (3¼ in)	4.88 mm (8 gauge)	Structural connections
Concrete nail	32 mm	3.05 mm	Fastening to concrete

Structural screws (e.g., #10 or #12 wood screws) have greater withdrawal capacity than nails, but lower shear capacity for the same diameter.

Final Nailing Rules

Minimum of 2 nails per connection for load-bearing elements

Nails must penetrate at least 38 mm into the receiving member

Staggered nailing is required for double top plates

No nail should be installed in the end of a member within 25 mm of the edge

Fire Resistance Requirements

Fire Separations

Fire walls must have a minimum fire-resistance rating (FRR) according to the NBC. For Part 9 buildings, walls separating dwelling units must have a 45-minute FRR. Common assemblies:

38×89 mm studs spaced at 400 mm, with 12.7 mm gypsum on each side: 45-minute FRR

Adding mineral wool insulation: increases the FRR

Fire Caulking

Penetrations through fire walls (pipes, ducts, cables) must be caulked with an intumescent product. Wood shims must not obstruct the spaces required for caulking expansion.

Protection of Structural Elements

Wood columns and beams must be protected with gypsum or fire-retardant coating if the FRR requires it. 15.9 mm (5/8 in) gypsum offers superior protection to 12.7 mm gypsum.

Quality Control and Inspection

Critical Inspection Points

Before closing in walls, verify:

134.Alignment: walls must be plumb (tolerance of 6 mm over 3 m)

135.Level: top plates must be level (tolerance of 6 mm over 3 m)

136.Nailing: compliance with spacing and lengths

137.Blocking: present where required

138.Anchorage: anchor bolts installed and tightened

Dimensional Tolerances

Element	Tolerance
Wall plumbness	6 mm over 3 m
Top plate level	6 mm over 3 m
Building squareness	10 mm on the diagonal
Stud spacing	± 6 mm

Pitfalls to Avoid

142.Confusing clear span and span between centers: clear span is the distance between the inside faces of supports, not between centers.

143.Forgetting the slope factor: for a sloped roof, the rafter length is always greater than the horizontal projection.

144.Neglecting wind loads on low-slope roofs: roofs with a slope less than 10° can experience significant uplift.

145.Using nails that are too short: the minimum 38 mm penetration into the receiving member is often forgotten.

146.Ignoring blocking requirements: blocking is required for spans greater than 2.1 m, even if the floor seems stable.

147.Supporting a truss with an interior partition: trusses must only be supported by exterior walls or beams designed for that purpose.

148.Forgetting wood treatment in contact with concrete: untreated wood in contact with concrete or soil is a common failure.

149.Confusing wood species: SPF (spruce-pine-fir) has lower allowable stresses than Douglas fir or MSR (machine stress-rated) lumber.

150.Not checking span tables: maximum spans vary with species, load, and spacing; do not memorize a single value.

151.Forgetting temporary bracing: installing trusses without temporary bracing is a major cause of accidents.

Summary

The load path must be continuous and uninterrupted: each element transfers its load to the element below.

Loads are divided into dead loads and live loads; snow and wind are live loads according to the NBC.

Platform framing uses studs spaced at 300, 400, or 600 mm, with headers sized according to openings.

Bracing is mandatory to resist lateral loads; structural panels and shear walls are the main methods.

Spans for joists, rafters, and beams are determined by the NBC tables (Part 9) or by calculation (Part 4).

Connections must comply with nailing rules: minimum 38 mm penetration, maximum 150 mm spacing on panel edges.

Foundations must be anchored with 12.7 mm bolts spaced at a maximum of 2.4 m.

Fire resistance requires separations with a minimum 45-minute FRR for walls between dwelling units.

Installation tolerances are 6 mm over 3 m for plumbness and level.

Deflection calculations use Δ_max = L / 360 for floors.

Master these concepts, and you will be ready for the framing and structural systems questions on the Red Seal exam. Practice with the NBC span tables and load calculations to develop your speed and accuracy.

Framing and Structural Systems