Metal Roofing and Cladding Systems

Metal Roofing and Cladding

Chapter Introduction

Metal roofing and cladding systems represent a significant portion of roofing work in Canada, for commercial, industrial, and residential buildings alike. This chapter covers installation principles, slope calculations, thermal expansion rules, installation standards, and the requirements of the National Building Code (NBC) and CSA A123 (standards for membranes and coatings). You must master these concepts to pass the Red Seal exam, as questions cover both theory and practice.

Definitions and Essential Terminology

Metal roofing: a roofing system composed of metal panels or sheets (steel, aluminum, copper, zinc) assembled by interlocking, crimping, or screwing.

Metal cladding: exterior vertical or inclined wall covering, using the same materials as roofing.

Structural panel: a load-bearing panel that supports loads without continuous support (e.g., ribbed steel panel).

Architectural panel: a non-load-bearing panel, installed over continuous support (e.g., plywood, OSB).

Thermal expansion: dimensional variation of a material under the effect of temperature, expressed in mm/m/°C.

Ridge: the upper horizontal edge of a roof, where two slopes meet.

Valley: the re-entrant angle formed by the intersection of two roof slopes.

Flashing: a metal piece or membrane used to ensure watertightness at junctions (chimney, wall, penetration).

Crimping: mechanical fastening of panels together using a crimping tool (standing seam).

Standing seam: panel profile with raised vertical ribs, interlocked and crimped.

Physical Principles and Material Behaviour

Thermal Expansion

Metal expands and contracts with temperature variations. The coefficient of linear expansion (α) is expressed in mm/m/°C. For steel, α ≈ 0.012 mm/m/°C; for aluminum, α ≈ 0.024 mm/m/°C; for copper, α ≈ 0.017 mm/m/°C.

Expansion calculation: ΔL = α × L × ΔT, where ΔL is the change in length (mm), L the initial length (m), and ΔT the temperature difference (°C).

Example: a 6 m aluminum panel undergoes a ΔT of 50 °C. ΔL = 0.024 × 6 × 50 = 7.2 mm. This variation must be absorbed by the fasteners (slotted holes) and expansion joints.

Rule of thumb: provide an expansion joint every 12 to 15 m for steel, and every 6 to 8 m for aluminum.

Condensation and Ventilation

Condensation forms when water vapour meets a cold surface. In a metal roof, it is essential to install a vapour barrier on the interior side and adequate ventilation on the exterior side (air space ≥ 25 mm) to remove moisture. CSA A123.5 (vapour barriers) and CSA A123.4 (ventilation) provide the requirements.

Loads and Spans

Metal panels must resist snow, wind, and dead loads. The NBC, Division B, Part 4 (loads and calculations) specifies minimum values. For structural panels, the maximum span is determined by the manufacturer based on load testing. Never exceed the indicated spans.

Types of Metal Roofing Systems

Standing Seam Roofing

This system uses panels with raised vertical ribs, connected by crimping or interlocking. It is recommended for slopes of 3:12 (14°) and greater, but some certified systems allow minimum slopes of 1:12 (5°). Fasteners are concealed (clips), which reduces the risk of leaks.

Characteristic	Standing Seam	Ribbed Panels
Fasteners	Concealed clips	Exposed screws
Minimum slope	1:12 (with certification)	3:12
Watertightness	Excellent	Good
Use	Architectural, commercial	Industrial, agricultural

Ribbed Panel Roofing (Corrugated or Trapezoidal)

Panels are fastened with self-drilling screws with sealing washers. They are economical and quick to install, but exposed screws are potential weak points. Minimum slope of 3:12 (14°).

Batten Seam Roofing

Panels are fastened over wood or metal battens, then covered with caps. This is a traditional system, used for steep-slope roofs (≥ 4:12).

Metal Cladding

Cladding is installed vertically or horizontally. Panels are fastened to girts or over continuous support. The same rules for expansion and fastening apply.

Calculations and Sizing

Slope and Percentage

Roof slope is expressed as a ratio (e.g., 4:12) or as a percentage. To convert: percentage = (rise / run) × 100. To convert a slope to degrees: angle = arctan(rise / run).

Example: slope 4:12 → percentage = (4/12) × 100 = 33.3%; angle = arctan(0.333) ≈ 18.4°.

Developed Width of a Panel

For a roofing panel, the developed width is the actual width of the metal before roll-forming. It is greater than the coverage width (effective width) due to the ribs. The ratio is provided by the manufacturer.

Number of Panels and Overlap

For a roof slope of width W and panels with an effective width L, the number of panels = W / L (round up to the nearest whole number). The longitudinal overlap between panels must be at least 100 mm for slopes ≥ 3:12, and 150 mm for lower slopes.

Fastener Calculation

The number of screws per panel depends on wind load and span. As a general rule, for ribbed panels, install screws in every rib at each support. For standing seam clips, the maximum spacing is 300 mm to 600 mm depending on the manufacturer.

Installation Rules and Procedures

Substrate Preparation

The substrate must be dry, clean, and flat. For insulated roofs, install a vapour barrier (warm side), then insulation, then an air barrier (cold side). CSA A123.5 requires the vapour barrier to have a permeance ≤ 0.05 ng/(Pa·s·m²) for Canadian climates.

Panel Installation

49.Alignment: check the squareness of the roof slope before installing the first panel. Use a chalk line.

50.Fastening: start at the bottom of the slope (eave) and work up toward the ridge. Screws must be driven perpendicular to the surface, without over-tightening (risk of washer deformation).

51.Interlocking: for ribbed panels, engage the male rib into the female rib before screwing.

52.Crimping: for standing seams, use a manual or electric crimping tool. The crimp must be made at the base of the rib, without crushing the metal.

Flashings and Accessories

Eave flashing: installed around the roof perimeter, fastened to the wall or panel.

Ridge flashing: covers the upper junction of two slopes. Must allow ventilation (ventilated ridge).

Penetration flashing: for chimneys, vents, etc. Use pre-formed or lead flashings, with a minimum overlap of 100 mm.

Valley: fabricate a metal valley (galvanized steel or copper) with a minimum width of 300 mm, with a centre crimp to direct water.

Recommended Installation Sequence

59.Install eave and starter flashings.

60.Install panels from left to right (or right to left depending on the direction of the ribs).

61.Install valley and ridge flashings.

62.Install penetration flashings.

63.Final inspection and touch-up of screws.

Applicable Codes and Standards

National Building Code (NBC)

Division B, Part 4: design loads (snow, wind, dead load).

Division B, Part 5: separation of elements (moisture protection, condensation).

Division B, Part 9: requirements for small buildings (minimum slopes, overlaps).

CSA Standards

CSA A123.5: vapour barriers for roofing.

CSA A123.4: roof ventilation.

CSA G20: galvanized steel (zinc coating).

CSA G164: pre-painted steel (continuously coated steel sheet).

Other References

CAN/CGSB-1.181: metal primer.

CAN/CGSB-37.54: sealing compound.

Reference Tables

Thermal Expansion Coefficients

Material	α (mm/m/°C)
Steel (mild)	0.012
Aluminum	0.024
Copper	0.017
Zinc	0.022
Stainless steel	0.010

Recommended Minimum Slopes

Roof Type	Minimum Slope
Standing seam (certified)	1:12 (5°)
Standing seam (standard)	3:12 (14°)
Ribbed panels	3:12 (14°)
Batten seam	4:12 (18°)
Cladding (vertical)	0:12 (0°)

Minimum Longitudinal Overlaps

Slope	Minimum Overlap
≥ 3:12	100 mm
1:12 to 3:12	150 mm
< 1:12	200 mm (with sealant)

Common Pitfalls to Avoid

Forgetting expansion: never fasten a metal panel rigidly at both ends. Use slotted holes or sliding clips.

Over-tightening screws: a crushed washer loses its seal. Tighten until contact, then a quarter turn.

Ignoring the minimum slope: installing ribbed panels on a 1:12 slope without certification leads to leaks.

Confusing vapour barrier and air barrier: the vapour barrier goes on the warm side (interior), the air barrier on the cold side (exterior). Their permeance differs.

Neglecting ventilation: insufficient air space causes condensation and premature corrosion.

Using screws of incorrect length: screws must penetrate through the panel and into the substrate by at least 25 mm. A screw that is too short will not hold.

Not checking squareness: an out-of-square roof slope results in panels cut at an angle and leaks at the edges.

Installing flashings after the panels: eave and starter flashings must be installed before the panels to ensure proper overlap.

Exam Tips

Memorize the expansion coefficients: steel 0.012, aluminum 0.024, copper 0.017. Calculation questions are common.

Know the minimum slopes: 3:12 for ribbed panels, 1:12 for certified standing seam.

Understand the difference between structural and architectural panels: the former carries loads, the latter rests on a support.

Know how to read a roof plan: arrows indicate the direction of panel installation, hatching indicates flashings.

Review the CSA standards: questions often focus on permeance requirements (A123.5) and ventilation (A123.4).

Summary

Thermal expansion is the key factor in metal roofing design. Always provide sliding fasteners and expansion joints.

Minimum slopes vary by panel type: 3:12 for ribbed panels, 1:12 for certified standing seam.

Longitudinal overlaps must be at least 100 mm (slope ≥ 3:12) and increase as the slope decreases.

Ventilation (air space ≥ 25 mm) and vapour barrier (permeance ≤ 0.05 ng/(Pa·s·m²)) are mandatory to prevent condensation.

Flashings must be installed before the panels at the eaves and starter, and after for penetrations.

Reference standards are the NBC (Parts 4, 5, 9) and CSA standards A123.4, A123.5, G20, G164.

Fasteners must be spaced according to wind loads and spans, with screws penetrating the substrate by at least 25 mm.

The installation sequence is: eave flashings, panels, ridge and valley flashings, penetration flashings.

Self-Assessment Questions

109.Calculate the expansion of an 8 m steel panel for a ΔT of 60 °C.

110.What is the minimum slope for a standard ribbed panel?

111.What is the maximum permeance of a vapour barrier according to CSA A123.5?

112.What is the maximum spacing of clips for a standing seam roof?

113.Why should screws not be over-tightened?

Answers: 1) ΔL = 0.012 × 8 × 60 = 5.76 mm. 2) 3:12 (14°). 3) 0.05 ng/(Pa·s·m²). 4) 300 to 600 mm depending on the manufacturer. 5) A crushed washer loses its seal and can deform the metal.