This chapter covers all the knowledge required for the Red Seal exam in carpentry concerning roofing systems. You must master not only installation techniques, but also engineering principles, slope calculations, loads, ventilation, and the requirements of the National Building Code of Canada (NBC). Roofing is a complex system where every component interacts; an error in waterproofing or ventilation can lead to major failures. This chapter is structured to follow the logic of the exam: first the fundamental principles, then the components, the calculations, and finally the specific traps.
2. Fundamental Principles of Roofing Systems
2.1 Functions and Performance Requirements
A roof must fulfill four critical functions:
8.Environmental separation: resist weather (rain, snow, wind, sun) and prevent water infiltration.
9.Structural support: transmit loads (dead, live, wind, snow) to the supporting structure (walls, beams, columns).
10.Thermal insulation: control heat transfer for energy efficiency and comfort.
11.Vapour and ventilation control: manage moisture to prevent condensation and deterioration.
Performance: The roof must maintain these functions throughout its useful service life, which varies by material (asphalt shingles: 20–25 years; metal roofing: 40–70 years; elastomeric membrane: 30–50 years).
2.2 Roof Types by Slope
Slope is the most determining factor in the choice of roofing system. It is expressed as a ratio (x/12) or as a percentage (%).
Roof Type
Slope (ratio)
Slope (%)
Typical Materials
**Flat roof**
0/12 to 2/12
0% to 16.7%
Bituminous membrane, EPDM, TPO, PVC
**Low-slope roof**
2/12 to 4/12
16.7% to 33.3%
Asphalt shingles (with underlayment), membrane, metal
**Medium-slope roof**
4/12 to 8/12
33.3% to 66.7%
Asphalt shingles, wood shingles, metal
**Steep-slope roof**
8/12 and up
66.7% and up
Wood shakes, metal, slate, tiles
Golden rule: The lower the slope, the more robust and continuous the waterproofing system must be. A flat roof does not shed water by gravity; it must be impermeable.
2.3 Essential Terminology
Ridge: The horizontal top edge formed by the meeting of two roof slopes.
Valley: The inward angle formed by the meeting of two roof slopes.
Rake (or gable edge): The inclined or horizontal edge of a roof slope.
Eave (or overhang): The part of the roof that extends beyond the exterior wall.
Flashing: A piece or material used to ensure waterproofing at the junction between the roof and a vertical element (wall, chimney).
Battens (or purlins): Strips of wood or metal fastened to the rafters to support the roofing material (often for metal or tiles).
Counter-battens: Battens fastened perpendicular to the rafters to create a ventilation space.
3. Roof Structure
3.1 Types of Framing
The carpenter must know the following structural systems:
30.Rafters: Inclined wood members (often 2x6, 2x8, 2x10) that support the lathing or sheathing. They transmit loads to the load-bearing walls.
31.Roof trusses: Prefabricated triangulated structures (wood or metal) that support the sheathing. They are spaced at 400 mm or 600 mm (16" or 24") and offer long spans without interior walls.
32.Purlins: Horizontal beams placed on walls or posts, supporting the rafters perpendicularly.
33.Sheathing (or roof deck): Panels of plywood (OSB or plywood) or laminated wood fastened to the rafters or trusses. It serves as the base for the roofing material and as a structural diaphragm to stiffen the building.
Sheathing thickness calculation: It is determined by the support spacing and the snow load. For example, for a 600 mm (24") spacing, 15.9 mm (5/8") OSB sheathing is often required for high snow loads. Always check the NBC tables or the manufacturer's specifications.
3.2 Load Calculations
The carpenter must understand loads to size or verify the structure.
Dead load (D): The self-weight of all materials (framing, sheathing, roofing, insulation).
Live load (L): The weight of occupants, furniture, snow (S), and rain (Pr).
Snow load (S): Calculated according to the NBC (Appendix C) and based on geographic location. It is expressed in kPa (kN/m²). Example: Montreal ~ 2.5 kPa; Quebec City ~ 3.0 kPa.
Wind load (W): The pressure or suction exerted by wind on the roof. It can be positive (push) or negative (uplift). Low-slope roofs are more prone to uplift.
Basic formula (NBC): The specified snow load is S = Is × Ss × Cb × Cw × Cs × Ca where:
Is = importance factor for the building
Ss = ground snow load (kPa)
Cb = basic snow load factor (often 0.8)
Cw = wind exposure factor
Cs = slope factor (decreases as slope increases)
Ca = accumulation factor (for valleys and stepped roofs)
Trap to avoid: Do not confuse snow load and rain load. Rain can accumulate on a flat roof if drains are blocked, creating an additional load (Pr).
4. Ventilation and Moisture Control
4.1 Ventilation Principles
Adequate ventilation of the roof space is crucial to:
Prevent water vapour condensation (which causes wood rot and metal corrosion).
Reduce attic temperature in summer (extends the service life of shingles).
Prevent ice damming in winter.
Ventilation rule (NBC): The ratio of net free ventilation area to ceiling area must be at least 1/300 for cold roofs (with ventilated attics). This area must be divided equally between air intakes (eaves) and air exhausts (ridge).
Formula: Total required ventilation area = (Attic floor area) ÷ 300.
Example: For a 100 m² attic, 0.33 m² of total net free ventilation is required (0.165 m² for intake and 0.165 m² for exhaust).
4.2 Types of Ventilation
Type
Location
Function
**Perforated soffits**
Eave
Air intake
**Ventilated soffits**
Eave
Air intake
**Ridge vents**
Ridge
Air exhaust
**Gable vents**
Gable wall
Exhaust or intake (depending on wind)
**Static roof vents**
Near the ridge
Air exhaust
**Turbine vents**
Near the ridge
Air exhaust (wind-driven)
Golden rule: Air must flow from the eave (intake) to the ridge (exhaust). An obstruction (insulation, dust) cancels the effectiveness of the system.
4.3 Vapour Barrier and Air Barrier
Vapour barrier: A material (polyethylene, membrane) installed on the warm side of the insulation to prevent water vapour diffusion into the cavity. It must be continuous and sealed at all penetrations.
Air barrier: A system that prevents air movement (and therefore vapour) through the building envelope. It may be the same material as the vapour barrier, but it must resist wind pressure.
Rule (NBC): The vapour barrier must be installed on the interior (warm) side of the insulation. In cold Canadian climates, it is mandatory for most buildings.
5. Roofing Materials and Installation Techniques
5.1 Asphalt Shingles
This is the most common material for sloped roofs. It comes in two types: strip shingles and architectural (dimensional) shingles.
Installation procedure (key steps):
71.Underlayment: Install a layer of asphalt-saturated felt (tar paper) or synthetic membrane over the sheathing. Overlaps must be at least 50 mm (2") horizontally and 100 mm (4") vertically.
72.Ice and water shield: In regions prone to ice damming, install a self-adhering membrane (ice and water shield) over the first 1.2 m (4 ft) from the eave and in the valleys.
73.Starter course: Install the first row of shingles, cutting them so that the joints do not align with the second row.
74.Application: Install subsequent rows, offsetting the joints by half a tab (or according to the pattern). Fasten with roofing nails (galvanized) of at least 30 mm (1-1/4") in length, driven 12 mm (1/2") from the top edge of the shingle.
75.Valleys: Use the woven method or the open cut method. The open cut method is more reliable: the shingles from each slope are cut diagonally along the valley, leaving a visible strip of membrane.
76.Ridge: Use ridge shingles (pre-cut or cut from standard shingles) installed overlapping in the direction opposite to the prevailing wind.
Number of nails: Minimum 4 nails per shingle (6 for high wind areas). Each nail must penetrate at least 19 mm (3/4") into the sheathing.
5.2 Metal Roofing
Types: Corrugated metal sheets (steel, aluminum), standing seam panels, metal shingles.
Advantages: Durability, light weight, fire resistance, low maintenance.
Installation:
Panels are fastened to battens or directly to the sheathing.
Fasteners (screws, nails) must be stainless steel or galvanized with EPDM sealing washers.
Standing seams are crimped to ensure watertightness.
Rake and ridge flashings are essential to prevent wind-driven water infiltration.
Trap to avoid: Never fasten metal panels directly to sheathing without an underlayment, as condensation can form under the metal.
5.3 Low-Slope Roofing (Membrane)
Bituminous membranes: Multi-layer systems (BUR - Built-Up Roofing) with layers of felt and hot or cold bitumen.
Elastomeric membranes: EPDM (rubber), TPO, PVC. They are installed in a single layer, adhered or ballasted.
Modified bitumen membranes: SBS or APP modified bitumen, applied with a torch or adhesive.
Critical points:
Flashings (parapets) must have a minimum height of 200 mm above the membrane level.
Metal flashings must be mechanically fastened and sealed.
Roof drains must be installed at the lowest point and must be accessible for maintenance.
The minimum slope for a membrane roof is 1% (1/8" per foot) to prevent ponding water.
5.4 Other Materials
Wood shakes: Cedar or pine, installed on battens. They require a minimum slope of 4/12.
Slate: Natural stone, very heavy (requires a reinforced structure), minimum slope of 6/12.
Concrete or clay tiles: Heavy, durable, minimum slope of 4/12.
6. Flashing and Waterproofing
6.1 Role of Flashing
Flashing are pieces of metal (aluminum, galvanized steel, copper, lead) or membrane that ensure watertightness at junctions. The critical areas are:
Valleys: Metal valley flashing or membrane.
Rakes: Drip edge flashing installed along the eaves and gables to direct water into the gutter.
Chimneys: Two-piece flashing (base and cap) with counter-flashing.
Adjacent walls: Step flashing for sloped roofs against a wall.
Overlap rule: All flashing must have an overlap of at least 100 mm (4") on the adjacent surface and be mechanically fastened and sealed with a compatible sealant.
6.2 Gutters and Downspouts
Sizing: The capacity of a gutter depends on the roof area drained and the rainfall intensity. A rule of thumb: a 100 mm (4") gutter can drain approximately 50 m² of roof area.
Slope: Gutters must have a minimum slope of 1/16" per foot (0.5%) towards the downspout.
Installation: Hangers (brackets) must be spaced a maximum of 600 mm (24") apart. Downspouts must be fastened to the wall and discharge at least 1.8 m (6 ft) from the foundation.
7. Roof Safety
Safety is an absolute priority. Falls from roofs are a major cause of serious injuries.
Protective equipment:
Guardrails: Mandatory for any work surface more than 1.8 m (6 ft) above the ground.
Lifeline: Horizontal or vertical anchorage system with a harness and energy absorber.
Scaffolding: Must be erected by a qualified person and inspected before use.
Ladders: Must extend at least 1 m (3 ft) above the landing point and be secured at the top and bottom.
Basic rules:
Never work on a wet, icy, or frost-covered roof.
Use footwear with slip-resistant soles.
Keep the work area clear (tools, materials).
Check the soundness of the sheathing before walking on it.
8. National Building Code (NBC) - Key Rules
The NBC is the reference document for the exam. Here are the most relevant sections for roofing:
Section 9.26: Roof coverings. This section covers requirements for materials, minimum slopes, underlayment, flashing, and ventilation.
Section 9.25: Control of condensation and air infiltration.
Minimum 1 layer of No. 15 felt for slope ≥ 4/12; 2 layers for slope < 4/12
**Ice dam protection membrane**
Required over 1.2 m (4 ft) from the eave edge in areas where the average January temperature is ≤ -5°C
**Attic ventilation**
Net free area ≥ 1/300 of the ceiling area
**Flashing**
Must be installed at all junctions and penetrations
Trap to avoid: The NBC specifies minimum requirements. Material manufacturers may have stricter requirements. In case of conflict, the stricter requirement prevails.
9. Practical Calculations for the Carpenter
9.1 Slope Calculation
Slope is the ratio of vertical rise to horizontal run.
Divide: 1,333 cm² for intake (eaves) and 1,333 cm² for exhaust (ridge).
9.4 Snow Load Calculation (Example)
For a residential building (Is = 0.8) in Quebec City (Ss = 3.0 kPa), with a 6/12 roof slope (Cs = 0.8), wind-exposed (Cw = 0.75), and no accumulation (Ca = 1.0):
S = 0.8 × 3.0 × 0.8 × 0.75 × 0.8 × 1.0 = 1.15 kPa
This load is used to verify the capacity of the rafters or trusses. The carpenter must know that this value is a specified load, not an actual load.
10. Inspection and Repair Procedures
10.1 Inspection of an Existing Roof
Before repairing or re-roofing, you must inspect:
164.Structure: Sagging, cracks, rot in rafters or sheathing.
168.Ventilation: Obstructions, insulation blocking the air intakes.
Trap to avoid: Never install a new layer of shingles over an existing roof without checking the condition of the sheathing. If the sheathing is damp or rotted, it must be replaced.
10.2 Repair of Damaged Areas
Individual shingles: Carefully lift them, remove the nails, slide in the new shingle, and fasten it with nails (driven under the shingle above).
Valley: If the membrane is damaged, it must be replaced over the entire length of the valley.
Flashing: Straighten it, clean it, and apply new sealant.
11. Traps to Avoid
Here are the most frequent errors on the exam and on the job site:
177.Confusing slope and angle: Slope is a ratio (x/12), not an angle in degrees. Do not use the tangent function without converting.
178.Forgetting the ice dam protection membrane: In cold climates, it is mandatory on the eaves. Forgetting it leads to ice damming and infiltration.
179.Insufficient ventilation: Calculate ventilation based on the attic floor area, not the roof area.
180.Wrong direction for shingle installation: Shingles must be installed from bottom to top, with offset joints. An aligned joint is a potential leak.
181.Improperly placed nails: Nails must be driven 12 mm from the top edge. Too low, they are exposed to water; too high, they do not hold the shingle.
182.Poorly overlapped underlayment: Felt overlaps must be at least 50 mm. Insufficient overlap lets water through.
183.Ignoring manufacturer's requirements: Warranties are voided if the installation does not meet the manufacturer's specifications (number of nails, minimum slope, etc.).
184.Neglecting flashing: Flashing is the leading cause of leaks. Improper flashing installation is a serious error.
185.Not checking the snow load: A carpenter must know that the structure must be designed for the local snow load. A change in slope or material may require a structural check.
186.Working without fall protection: This is a violation of the occupational health and safety code and a major cause of fatal accidents.
12. Summary
The roof is a complete system: structure, ventilation, waterproofing, covering material.
Slope determines the choice of material and the installation method.
Ventilation (1/300) is essential for roof durability and condensation prevention.
Flashing are the most vulnerable points; they must be installed carefully with adequate overlap.
The NBC (Section 9.26) is the normative reference for minimum requirements.
Calculations for slope, area, ventilation, and snow load are basic skills.
Safety (fall protection) is non-negotiable.
Always consult the manufacturer's specifications and the NBC before starting a project.
13. Exam Tips
Read the questions carefully: Several answer choices are designed to trap those who do not read the details (units, conditions, etc.).
Memorize key values: Minimum slopes, overlaps, nail spacing, ventilation ratios.
Practice the calculations: Slope, area, ventilation, snow load. Redo the examples in this chapter without looking at the solutions.
Visualize the procedures: For each material, imagine the installation steps in order.
Use the NBC: Familiarize yourself with the table of contents and key sections. You may not have the book at the exam, but the questions are based on its requirements.
Manage your time: Do not get stuck on a difficult question. Move on to the next one and come back later.
14. Normative References
National Building Code of Canada (NBC) 2020, Sections 9.23, 9.25, 9.26.
CSA O86: Engineering design in wood.
Manufacturer standards: Asphalt Roofing Manufacturers Association (ARMA), National Roofing Contractors Association (NRCA) - for installation details.
This chapter has provided you with the essential knowledge. Mastery comes with practice and repetition. Good luck with your Red Seal exam preparation!
