Steep-Slope Specialty Roofing (Tile, Slate, Shake, and Synthetic)

Special Steep-Slope Roofing (Tile, Slate, Wood and Synthetic Shakes)

Chapter Introduction

This chapter covers steep-slope roofing systems other than conventional asphalt shingles. The journey-person roofer must master the installation specifics, ventilation requirements, installation standards, and load calculations for tile (clay, concrete), natural slate, wood shake and shingle, and synthetic roofing products. These materials demand increased precision, specific minimum slopes, and a thorough understanding of thermal movement and fastening methods.

The National Building Code of Canada (NBC) and CSA (Canadian Standards Association) standards govern this work. The candidate must know the requirements for support battens, double battens, minimum slopes, and underlayments required for each material.

2. Minimum Slopes and Deck Preparation

2.1 Regulatory Minimum Slopes

Each steep-slope roofing material has a minimum slope expressed as a vertical/horizontal ratio (e.g., 4:12) or in degrees (°) . The table below summarizes the common requirements.

Material	Minimum Slope (ratio)	Minimum Slope (degrees)	Note
Concrete tile	2.5:12	11.8°	With double underlayment and reinforced waterproofing details
Clay tile	3:12	14°	Standard minimum slope; 4:12 recommended for cold climates
Natural slate	4:12	18.4°	Absolute minimum slope; 6:12 recommended
Wood shake	3:12	14°	With Class A or B underlayment
Wood shingle	3:12	14°	May be installed at 2.5:12 with special underlayment
Synthetic (composite)	2:12	9.5°	Per manufacturer; always verify the technical data sheet

Golden rule: The lower the slope, the more performant the underlayment must be. At slopes below 3:12, a self-adhering underlayment (ice and water shield) is mandatory over the first 1.2 m (4 ft) of the eave and in the valleys.

2.2 Roof Deck and Battening

The deck can be a solid sheathing (OSB or plywood) or open battening (spaced wood battens). The choice depends on the material and ventilation requirements.

Solid deck: Required for slate and tile when the slope is less than 5:12. Minimum thickness of 12.5 mm (1/2 in) for 400 mm (16 in) joist spacing.

Open battening: Used for wood shakes and certain tiles. Battens must have a minimum cross-section of 25 mm × 50 mm (1 in × 2 in) for shakes, and 38 mm × 75 mm (1.5 in × 3 in) for tiles.

Calculating batten spacing:

The spacing (E) between battens is calculated using the formula:

E = (Exposed length of material) + (Required overlap)

For a 420 mm long concrete tile with an exposed length of 300 mm, the spacing is 300 mm. For a 600 mm (24 in) wood shake with an exposure of 190 mm (7.5 in), the spacing is 190 mm.

> Exam tip: Batten spacing is measured center-to-center, never edge-to-edge. A 5 mm error on each batten can create a cumulative offset of 50 mm over 10 rows.

2.3 Under-Roof Ventilation

Ventilation is essential to prevent condensation and premature deterioration of materials. The NBC requires a ratio of 1:300 (1 m² of ventilation for 300 m² of roof area) for attics, and 1:150 if the slope is less than 1:6.

Air intakes: Eaves, perforated soffits.

Air exhausts: Ventilated ridge caps, static vents, or turbines.

For slate and tile roofs, ridge ventilation is often combined with open battening to allow air circulation under the tiles. This circulation removes moisture and reduces ice formation in winter.

3. Tile Roofing (Concrete and Clay)

3.1 Material Characteristics

Property	Concrete Tile	Clay Tile
Weight	45–55 kg/m²	35–45 kg/m²
Durability	50+ years	75+ years
Frost resistance	Good (with treatment)	Excellent
Water absorption	8–12%	5–8%
Fire resistance	Class A	Class A

The weight of tiles is a critical factor. A concrete tile roof can add 50 kg/m² to the dead load. The candidate must verify the structural load-bearing capacity of the framing before any installation. The NBC (Appendix A, Table A-1) provides minimum design loads.

3.2 Tile Installation

Installation steps:

32.Underlayment: Install a #30 (bituminous felt) underlayment or self-adhering membrane over the entire deck.

33.Battening: Install treated wood (or metal) battens spaced according to the exposed length of the tile.

34.Tile placement: Each tile is attached to the batten with a hook or a nail made of stainless steel or copper. Ridge and rake tiles are mechanically fastened.

35.Valleys: Valleys must be constructed with a self-adhering membrane of minimum 900 mm width, then covered with tiles cut diagonally.

Fastening: Each tile must be secured with at least two attachment points (nails or hooks). In high wind regions (basic wind speed greater than 110 km/h), the number of fasteners must be increased by 50%.

Cutting tiles: Cutting is done with a diamond blade grinder or a wet saw. Cuts must be clean to prevent cracking. Tiles cut at an angle (for valleys and hips) must be set with cement mortar or a compatible adhesive.

3.3 Ridge and Hip Tiles

Ridge tiles are typically special half-cylinder shaped pieces. They are set on cement mortar or on a metal ridge support.

Mortar: Use a 1:3 mix (cement:sand) with a bonding agent.

Metal support: Preferred for seismic or high wind regions.

Ridge ventilation requirement: The ridge must allow an air outlet of at least 25 mm of continuous width under the ridge tiles.

4. Natural Slate Roofing

4.1 Selection and Classification

Slate is classified by grade (S1, S2, S3) according to its resistance to water absorption and frost. For the Canadian climate, only grade S1 (excellent) is recommended for permanent roofing.

Grade	Water Absorption	Frost Resistance	Use
S1	< 0.25%	Excellent	Permanent roofing
S2	0.25–0.45%	Good	Temporary roofing or mild climates
S3	> 0.45%	Poor	Not recommended for roofing

Common dimensions: Standard slate measures 400 mm × 250 mm (16 in × 10 in) with a thickness of 4.8 mm (3/16 in). The thickness determines the maximum exposed length.

4.2 Calculating Exposed Length

The exposed length (L) is calculated using the formula:

L = (Total slate length - Overlap) / 2

For a 400 mm slate with a 75 mm overlap:

L = (400 - 75) / 2 = 162.5 mm

Overlap rule: The minimum overlap is 65 mm for slopes of 6:12 and steeper. For slopes from 4:12 to 6:12, the overlap must be increased to 75 mm.

Number of slates per m²:

N = 1,000,000 / (L × Width)

For L = 162.5 mm and width = 250 mm:

N = 1,000,000 / (162.5 × 250) = 24.6 slates/m²

4.3 Slate Fastening

Each slate is secured with two nails made of copper or stainless steel, placed 25 mm from the top edge and 40 mm from the side edges. Nails must penetrate at least 20 mm into the deck.

Nails: Minimum length of 32 mm (1.25 in) for a 12.5 mm deck. Use large-head nails (6.5 mm diameter).

Laying in courses: Vertical joints must be offset by at least 75 mm between adjacent courses. This staggered arrangement prevents water infiltration.

4.4 Special Details

Valleys: Slate valleys are constructed using the closed weave method or as open valleys with a metal membrane of 600 mm width.

Rakes: Rake slates must be cut at right angles and secured with an additional nail.

Penetrations: Chimney and vent flashings must be metal (copper, lead, or stainless steel) and integrated under the adjacent slates.

5. Wood Shakes and Shingles

5.1 Types of Wood Shakes and Shingles

Type	Manufacturing	Length	Butt Thickness
Shake	Hand-split or machine-split	450–600 mm	10–20 mm
Shingle	Sawn	400–600 mm	5–10 mm

Western Red Cedar is the most commonly used species in Canada for its natural rot resistance. Shakes are thicker and offer better insulation, but they are heavier and more expensive.

5.2 Slope and Underlayment

Minimum slope: 3:12 for both types.

Underlayment: For shakes, a Class B underlayment (#30 felt) is required. For shingles, a Class D underlayment (#15 felt) is sufficient.

Spacing between shakes/shingles: A 6 mm gap must be left between each shake/shingle to allow for expansion when wet.

5.3 Installation and Exposure

Maximum exposure:

Type	Length	Maximum Exposure
Shake	450 mm (18 in)	190 mm (7.5 in)
Shake	600 mm (24 in)	250 mm (10 in)
Shingle	400 mm (16 in)	125 mm (5 in)
Shingle	600 mm (24 in)	190 mm (7.5 in)

Fastening: Each shake/shingle is secured with two nails placed 20 mm from the edge and 40 mm above the exposure line. Nails must be hot-dipped galvanized steel or aluminum.

Double battening: For shakes, double battening is often required: a first 25 × 50 mm batten installed horizontally, then a second 25 × 50 mm batten installed vertically. This technique improves ventilation and stability.

5.4 Treatment and Finishing

Wood shakes and shingles can be left natural (to weather to grey) or treated with a stain or sealant. Treatment must be applied before installation to protect all faces. Factory-treated shakes (autoclaved) offer better protection against fungi and insects.

6. Synthetic Roofing (Composite)

6.1 Materials and Advantages

Synthetic roofing imitates slate, wood shake, or tile, but is manufactured from polymers (polypropylene, PVC, EPDM rubber) or fiberglass and resins. Their weight is 5 to 15 kg/m², which is 3 to 10 times less than natural materials.

Property	Synthetic	Natural Slate	Concrete Tile
Weight (kg/m²)	5–15	25–35	45–55
Cost	Medium	High	Medium
Durability	30–50 years	75+ years	50+ years
Frost resistance	Excellent	Excellent	Good
Recyclability	Variable	No	Yes

6.2 Installing Synthetic Shingles

Installation generally follows the same principles as asphalt shingles, with important differences:

90.Underlayment: A self-adhering underlayment is required over the entire roof for slopes below 4:12.

91.Fastening: Synthetic shingles are secured with large-head nails (9.5 mm) and a calibrated pneumatic stapler. The number of nails is specified by the manufacturer (typically 4 to 6 per shingle).

92.Cutting: Cutting is done with a hook knife or circular saw. Cut edges must be covered with bituminous cement to prevent infiltration.

Ventilation: Synthetic shingles require attic ventilation compliant with the NBC (1:300). Some manufacturers require a 1:150 ratio to maintain the warranty.

6.3 Synthetic Tiles and Slates

These products are installed on battens or directly on the deck, depending on the system. Synthetic tiles often interlock with a mechanical locking system, eliminating the need for visible nails.

Expansion requirement: Synthetic materials have a higher coefficient of thermal expansion than natural materials. A gap of 3 to 5 mm must be provided between each tile to allow for expansion.

7. Applicable Standards and Codes

7.1 National Building Code of Canada (NBC)

The NBC 2020 (current edition) contains roofing requirements in Section 9.26 (Roofing). Key points:

Article 9.26.2.1: Minimum slope for each type of roofing.

Article 9.26.4.1: Underlayment requirements based on slope and material.

Article 9.26.5.1: Fastening of shingles and tiles.

Article 9.26.11.1: Attic ventilation.

7.2 CSA Standards

CSA A123.5: Bituminous underlayments for roofing.

CSA O118.1: Wood shakes and shingles (cedar).

CSA A82: Natural slate.

CSA A220: Concrete roof tiles.

7.3 Other References

Canadian Electrical Code, Part I, Chapter V: applicable for electrical roof de-icing systems installed on roofs (Rule 8-200 for roof circuits).

CSA B149.1: Natural gas and propane installation code (relevant for chimney vents penetrating the roof).

8. Practical Calculations for the Exam

8.1 Calculating Roof Area

For a rectangular two-slope roof:

Total area = 2 × (Length × Slope width)

If one slope measures 12 m × 8 m:

Area = 2 × (12 × 8) = 192 m²

Slope factor: To convert a floor area to a roof area, use the slope factor:

Factor = √(1 + (slope)²)

For a 6:12 slope (0.5):

Factor = √(1 + 0.25) = √1.25 = 1.118

Roof area = Floor area × 1.118

8.2 Material Calculations

Concrete tiles: Each tile covers approximately 0.09 m² (300 mm × 300 mm exposure). For 192 m²:

Number of tiles = 192 / 0.09 = 2,134 tiles

Add 5% for cuts and breakage: 2,134 × 1.05 = 2,241 tiles.

Slates: With an exposure of 162.5 mm × 250 mm (0.0406 m²):

Number = 192 / 0.0406 = 4,729 slates

Add 10% for cuts: 4,729 × 1.10 = 5,202 slates.

8.3 Calculating Total Weight

For a concrete tile roof at 50 kg/m²:

Total weight = 192 m² × 50 kg/m² = 9,600 kg = 9.6 tonnes

Verify that the framing can support this additional load (dead load) in addition to the snow load (specified roof load per the NBC, typically 1.5 to 3.5 kPa depending on the region).

9. Common Pitfalls to Avoid

138.Confusing minimum slope and recommended slope: The minimum slope is the absolute threshold, but the recommended slope provides a safety margin. A slate roof at 4:12 can work, but 6:12 is preferable for Canadian climates.

139.Forgetting the overlap in batten calculations: Batten spacing is based on the exposed length, not the total length of the material. A calculation error of 10 mm per row creates a 100 mm offset over 10 rows.

140.Neglecting ventilation under tiles: Concrete and clay tiles require air circulation under the surface to remove moisture. Solid battening (without gaps) can cause condensation and premature deterioration.

141.Using ordinary steel nails for slate: Nails must be copper or stainless steel. Galvanized steel can corrode when in contact with natural slate.

142.Ignoring snow load: The weight of tiles adds to the snow load. A roof designed for asphalt shingles cannot necessarily support concrete tiles without structural reinforcement.

143.Installing wood shakes without spacing: The 6 mm gap between shakes is essential for expansion. Without this gap, shakes will warp and lift.

144.Forgetting penetration flashings: Every penetration (chimney, vent, antenna) must have a metal flashing integrated under the roofing material. A poorly installed flashing is the leading cause of infiltration.

145.Confusing underlayment classes: Class A (self-adhering) is required for low slopes and valleys. Class B (#30 felt) is for shakes. Class D (#15 felt) is for shingles and steep slopes.

10. Summary

Minimum slopes: Tile 2.5:12 to 3:12, slate 4:12, wood shake 3:12, synthetic 2:12.

Underlayments: Self-adhering for low slopes and valleys; #30 felt for shakes; #15 felt for shingles.

Battening: Center-to-center spacing = exposed length. Double battening for shakes and tiles on solid decks.

Fastening: Copper or stainless steel nails for slate; hooks or nails for tiles; galvanized nails for wood shakes; large-head nails for synthetic.

Ventilation: Ratio 1:300 (attics) or 1:150 (slope < 1:6). Ridge ventilation mandatory for tiles and slate.

Calculations: Roof area = floor area × slope factor. Number of pieces = area / area covered per piece, with 5–10% waste.

Weight: Concrete tile 50 kg/m², slate 30 kg/m², wood shake 10 kg/m², synthetic 10 kg/m². Verify structural capacity.

Standards: NBC Section 9.26, CSA A123.5, CSA O118.1, CSA A82, CSA A220.

The candidate must be able to quickly calculate quantities, select the appropriate underlayment, and identify installation defects in photos or diagrams. Mastery of minimum slopes and fastening requirements is essential to pass the exam.

11. Self-Assessment Questions

159.What is the minimum slope for a natural slate roof according to the NBC?

160.Calculate the batten spacing for a 420 mm concrete tile with a 300 mm exposure.

161.What type of nails must be used to fasten natural slate?

162.What is the difference between a shake and a shingle in terms of thickness and exposure?

163.What ventilation ratio is required for an attic with a 2:12 roof slope?

164.How many 400 × 250 mm slates are needed to cover 150 m² with a 162.5 mm exposure?

165.What underlayment is mandatory in valleys for a tile roof?

166.Why is a 6 mm gap left between wood shakes?

167.What is the approximate weight of a concrete tile roof per m²?

168.Which CSA standard governs cedar wood shakes and shingles?

Answers: 1) 4:12; 2) 300 mm center-to-center; 3) Copper or stainless steel; 4) Shakes are thicker (10–20 mm) with a maximum exposure of 250 mm; shingles are thinner (5–10 mm) with an exposure of 190 mm; 5) 1:150; 6) 150 / 0.0406 = 3,695 slates + 10% = 4,065; 7) Self-adhering membrane of 900 mm; 8) To allow for expansion when wet; 9) 45–55 kg/m²; 10) CSA O118.1.