Sheet metal expands and contracts with temperature changes at a rate that architects must account for in specifications. Fail to provide for thermal movement and the trim will split sealant joints, buckle, or pull fasteners out of the parapet wall over time. This guide covers the physics, the numbers, and the specification language that keeps sheet metal trim performing through decades of thermal cycling.
Coefficients of Thermal Expansion by Material
The coefficient of thermal expansion (CTE) is the amount a material expands per unit length per degree of temperature change, expressed in inches per inch per °F (or mm/mm per °C).
| Material | CTE (in/in/°F) | CTE (mm/mm/°C) | Relative to Steel |
|---|---|---|---|
| Steel (carbon / Galvalume) | 6.5 × 10⁻⁶ | 11.7 × 10⁻⁶ | 1.0× (baseline) |
| Aluminum | 13.1 × 10⁻⁶ | 23.6 × 10⁻⁶ | 2.0× |
| Copper | 9.8 × 10⁻⁶ | 17.6 × 10⁻⁶ | 1.5× |
| Zinc | 17.3 × 10⁻⁶ | 31.1 × 10⁻⁶ | 2.7× |
| Stainless steel (304) | 9.9 × 10⁻⁶ | 17.8 × 10⁻⁶ | 1.5× |
The practical implication: aluminum coping caps move approximately twice as much as steel coping caps for the same temperature change and run length. This is why aluminum specifications require more frequent expansion provisions — and why substituting aluminum for steel in a spec written for steel without adjusting joint spacing is a design error.
Calculating Thermal Movement for Spec Writing
The formula for thermal movement in sheet metal is:
ΔL = L × CTE × ΔT
Where: L = section length (inches), CTE = coefficient (in/in/°F), ΔT = max temperature differential (°F)
Example: Aluminum Coping in Phoenix, AZ
A 10-foot (120-inch) aluminum coping cap with a ΔT of 150°F (from -20°F winter low to 180°F summer surface temperature on a dark-colored cap):
ΔL = 120 in × 13.1 × 10⁻⁶ × 150°F = 0.236 inches ≈ nearly 1/4 inch
This is the movement per 10-foot section. The slip joint (minimum 3-inch lap) must be able to absorb this movement without opening to water infiltration. A 3-inch lap at maximum extension leaves 2.75 inches of engagement — adequate, but only if the joint detail is correctly executed.
Example: Steel Coping in Chicago, IL
A 10-foot (120-inch) steel coping cap with a ΔT of 120°F (standard Chicago temperature differential for light-colored steel):
ΔL = 120 in × 6.5 × 10⁻⁶ × 120°F = 0.094 inches ≈ 3/32 inch
Much more manageable — which is why steel coping can span 12 feet between joints vs. 10 feet for aluminum.
SMACNA Maximum Joint Spacing Recommendations
| Material | Max. Exposed Joint Spacing | Min. Lap at Joint | Notes |
|---|---|---|---|
| Steel (Galvalume / Kynar-coated) | 12 feet (3.66m) | 3 inches | Reduce to 10' in hot climates or dark colors |
| Aluminum | 10 feet (3.05m) | 3 inches | Reduce to 8' in very hot climates or dark colors |
| Copper | 10 feet (3.05m) | 3 inches | Standard for architectural copper roofing |
| Zinc | 8 feet (2.44m) | 3 inches | Highest CTE; requires most frequent joints |
Climate modifier: SMACNA joint spacing recommendations are for temperate climates with roughly 100°F annual temperature differential. In desert Southwest climates (Phoenix, Las Vegas, Tucson) where dark coping can hit 180°F on the surface, reduce aluminum joint spacing to 8 feet. In northern climates where temperatures range from -30°F to +90°F, steel joint spacing at 10 feet is more conservative than the 12-foot SMACNA maximum.
Dark Colors and Solar Heat Gain
Color significantly affects surface temperature — and therefore thermal movement — in exposed sheet metal. Solar reflectance is measured on a 0–1 scale; lower values mean more heat absorption.
| Color Type | Typical Solar Reflectance | Estimated Peak Surface Temp (90°F day) | Effective ΔT vs. Light Color |
|---|---|---|---|
| White / light gray | 0.60–0.75 | ≈115–125°F | Baseline |
| Medium gray / tan | 0.35–0.55 | ≈130–145°F | +15–25°F |
| Dark brown / dark blue | 0.15–0.30 | ≈150–165°F | +30–45°F |
| Black | 0.05–0.10 | ≈165–180°F | +45–60°F |
For a black aluminum coping cap in a hot climate, peak-to-trough temperature differential could be 200°F or more (180°F summer peak to -20°F winter low in a northern U.S. city). Run this through the movement calculation:
ΔL = 96 in (8 ft) × 13.1 × 10⁻⁶ × 200°F = 0.252 inches per 8-foot section
At this level of movement, reduce joint spacing to 6 feet for black aluminum in northern climates with significant temperature differentials. Or choose a lighter color that reduces peak surface temperature by 40–50°F and reduces movement back within the 8-foot SMACNA standard spacing.
Sealant Selection for Expansion Joints
The sealant at coping cap slip joints must be able to accommodate the calculated movement without failing adhesively or cohesively. Two sealant types are appropriate:
Neutral-Cure Silicone
Silicone sealants conforming to ASTM C920, Type S, Grade NS, Class 50, Use M. The "Class 50" designation means the sealant can accommodate ±50% movement of the joint width. A 1/4-inch sealant bead accommodates ±1/8 inch of movement — which is sufficient for most steel coping applications. For aluminum, use a 1/2-inch bead to accommodate larger movements.
Caution: Acetoxy-cure silicone (the type that smells like vinegar) can cause discoloration of some PVDF coatings and can accelerate corrosion in concealed metal applications. Always specify neutral-cure silicone for metal-to-metal or metal-to-PVDF applications.
Polyurethane Sealant
Polyurethane sealants conforming to ASTM C920, Class 25, Use M are also appropriate for coping joints and are preferred by some contractors for their paintability and lower cost. They are not as UV-stable as silicone over long periods, which matters if the sealant bead is exposed to direct sunlight.
Specification Language for Thermal Movement
3.2 THERMAL MOVEMENT PROVISIONS
A. Provide slip joints at each coping section joint. Lap dimension shall be minimum 3 inches. Apply continuous sealant bead in lap 1 inch back from leading edge, before assembling sections.
B. Maximum joint spacing: 12 feet for steel coping; 10 feet for aluminum coping. Reduce maximum spacing by 2 feet for dark-colored coping (solar reflectance <0.25) or where annual temperature differential exceeds 150°F.
C. Do not fasten coping cap sections through lap — lap must remain free to slide to accommodate thermal movement.
D. Sealant at slip joints: Neutral-cure silicone conforming to ASTM C920, Type S, Grade NS, Class 50, Use M. Acetoxy-cure silicone not acceptable. Confirm sealant compatibility with PVDF coating before application.
E. Movement capacity: verify that minimum 3-inch lap provides adequate engagement at full thermal contraction for the specified section length, material, and design temperature range.
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Frequently Asked Questions
How much does aluminum sheet metal expand compared to steel?
Aluminum has a coefficient of thermal expansion of approximately 13.1 × 10⁻⁶ inch/inch/°F, compared to 6.5 × 10⁻⁶ for steel. This means aluminum expands and contracts roughly twice as much as steel for the same temperature change. A 10-foot run of aluminum coping experiencing a 100°F temperature differential moves approximately 0.16 inches — nearly 3/16 inch — which is significant enough to split a rigid sealant joint without proper slip joint details.
What is the maximum joint spacing for aluminum coping caps?
SMACNA recommends slip joints at maximum 10-foot intervals for exposed aluminum coping caps. In climates with large temperature differentials or for dark-colored aluminum that experiences elevated surface temperatures, reducing joint spacing to 8 feet is prudent. For steel coping caps, SMACNA allows up to 12-foot maximum joint spacing.
How does dark color affect thermal expansion in sheet metal?
Dark-colored sheet metal absorbs more solar radiation. In full sun, a dark PVDF-coated coping cap can reach surface temperatures of 160–180°F on a 90°F day, while a light-colored cap might reach only 120–130°F. This 40–50°F additional temperature differential amplifies thermal movement by 30–40% compared to a light-colored cap, requiring reduced joint spacing or careful sealant joint sizing.
What sealant is compatible with PVDF-coated sheet metal at expansion joints?
Neutral-cure silicone (ASTM C920, Type S, Grade NS, Class 50, Use M) is the preferred sealant at expansion joints on PVDF-coated sheet metal. Acetoxy-cure silicone is not acceptable — it can stain PVDF coatings. Polyurethane sealant (ASTM C920, Class 25, Use M) is also acceptable. Always confirm sealant-to-coating compatibility before specifying.
Does SMACNA specify thermal movement provisions for all trim types?
SMACNA addresses thermal expansion provisions for coping caps (Chapter 2), gravel stops (Chapter 3), and gutters (Chapter 4). The manual requires slip joints at regular intervals for all exposed metal trim runs. For drip edge and concealed flashings, thermal movement is less critical because the pieces are shorter and movement is absorbed by lapping.