The coefficient of thermal expansion (CTE, α) scales length change to temperature change: movement equals the coefficient times the element length times the temperature swing, ΔL = α·L·ΔT. Materials differ by an order of magnitude — steel around 11.7×10⁻⁶/K, aluminum 23.6, rigid PVC 60–70, HDPE 120–200 — and two footnotes matter as much as the numbers: wood’s along-grain α of 3–5 is nearly irrelevant because its movement is dominated by moisture rather than temperature, and PTFE’s expansion is so temperature-dependent that its α is only indicative.
Piping shows both faces of the coefficient. A 50 m carbon-steel run (α = 12×10⁻⁶/K) heated 80 °C grows 48 mm if free — but if fully anchored it grows nowhere and instead develops an axial stress σ = E·α·ΔT = 200,000 × 12×10⁻⁶ × 80 = 192 MPa, independent of length, thrusting about 405 kN into the anchors. The escape is flexibility: a guided-cantilever expansion loop with leg length L = √(3·E·D·ΔL/Sa) — 5.93 m for this run — absorbs the growth within the allowable stress range, the analysis ASME B31.1 and B31.3 require.
Facades fight the same physics at the joint line: a 3,000 mm aluminum panel over a 50 K solar-corrected temperature range moves 3.45 mm, and dividing by the sealant’s movement class with a safety factor sizes the joint — 17.3 mm for a ±25% sealant at a 1.25 factor, per the ASTM C1193 and ISO 11600 framework.