Polycarbonate Thermal Expansion: Expansion Gap Calculator and Fixing Design
Why Polycarbonate Expands — and Why It Matters
Polycarbonate has a coefficient of linear thermal expansion of about 0.065 mm/m/°C (65 × 10&sup6;/K) — roughly 6–7× steel and 2× aluminium. A 6 m panel in a climate that swings 60 °C between winter and summer moves about 23 mm. If the panel is fixed rigidly at both ends, that movement turns into buckling, stress whitening or fastener tear-out. Correct expansion-gap design prevents these failures before they start.
Expansion Calculation Formula
ΔL = L × α × ΔT
- ΔL = expected length change (mm)
- L = panel length (mm)
- α = 0.065 mm/m/°C (polycarbonate)
- ΔT = temperature range the panel experiences (°C)
Example: 6000 mm panel, ΔT = 60 °C → ΔL = 6000 × 0.000065 × 60 = 23.4 mm. The frame must allow this movement at one end (sliding) or both ends (centre-fixed).
Expansion Gap Quick Reference
| Panel length | ΔT = 40°C | ΔT = 60°C | ΔT = 80°C |
|---|---|---|---|
| 2 m | 5.2 mm | 7.8 mm | 10.4 mm |
| 4 m | 10.4 mm | 15.6 mm | 20.8 mm |
| 6 m | 15.6 mm | 23.4 mm | 31.2 mm |
| 8 m | 20.8 mm | 31.2 mm | 41.6 mm |
Divide the movement between both ends by centre-fixing, or allow full movement at one sliding end. Always add 3–5 mm minimum clearance beyond the calculated value for installation tolerance.
Fixing Design Rules
- Never fix both ends rigidly — always leave one end free to slide
- Use oversize fastener holes — hole diameter ≥ 8 mm for M6 fixings, positioned to allow movement
- Washered or pressure-plate fixing — distribute load and allow sliding without point stress
- Minimum edge distance — 2× hole diameter from panel edge
- Seal with EPDM, not rigid sealant — silicone/EPDM gaskets accommodate movement; rigid adhesives crack
- Centre-fix long panels — anchor the middle, let both ends move symmetrically
FAQ: Polycarbonate Thermal Expansion
How much does polycarbonate expand in the sun?
A 6 m panel exposed to a 40–60°C surface temperature swing moves 15–23 mm. Dark-coloured panels get hotter than clear ones — use the surface temperature, not air temperature, for ΔT.
What expansion gap should I leave for polycarbonate roofing?
Calculate with the formula, then add 3–5 mm installation tolerance. For a typical 3–6 m roof panel, 10–25 mm total allowance is normal depending on climate.
Can I glue polycarbonate panels together?
Solvent bonding works for non-structural joints but creates a rigid seam that cannot expand — never use it across the full panel width on large sheets. Use mechanical joints with EPDM gaskets for expansion.
Does dark polycarbonate expand more than clear?
Yes — darker colours absorb more solar heat, raising the panel surface temperature and increasing ΔT. For large dark panels, use a larger expansion allowance.
Thermal Expansion of Polycarbonate vs Other Materials
| Material | Expansion coefficient (mm/m/°C) | 6 m panel @ 60°C swing |
|---|---|---|
| Polycarbonate | 0.065 | 23.4 mm |
| Acrylic | 0.070 | 25.2 mm |
| Aluminium | 0.023 | 8.3 mm |
| Steel | 0.012 | 4.3 mm |
| Glass | 0.009 | 3.2 mm |
Polycarbonate and acrylic move 5–7× more than metal and 7× more than glass. This is why PC panels must never be fixed the way you fix glass — glass installation assumes near-zero movement, while PC installation must engineer for centimetres of movement on long runs.
Common Expansion Failures (and How to Avoid Them)
- Buckling — panel fixed rigidly at both ends bows in summer heat → always leave one end sliding
- Stress whitening — white hazing around fasteners from restrained expansion → oversize holes + washers
- Fastener pull-out — screws work themselves loose as the panel slides → use washered fixings or profile systems
- Sealant cracking — rigid sealants fail as the panel moves → EPDM gaskets or low-modulus sealant only
- Panel pop-out — frame too tight on a hot day → design for the highest surface temperature
Installation Checklist
- Cut panels 3–5 mm shorter than the frame opening (expansion + tolerance)
- Centre-fix panels over 3 m — anchor the middle, let both ends float
- Use EPDM edge gaskets — never rigid glue or mortar
- Oversize all fastener holes by 3–4 mm and use washered fixings
- Leave 2–3 mm gap between adjacent panels for expansion joints
- Seal ends after installation — end caps prevent debris from jamming the slide
Standards and References
- ISO 4892-2 — accelerated weathering for outdoor expansion behaviour data
- ASTM D696 — coefficient of linear thermal expansion test method (polycarbonate ≈ 65 × 10&sup6;/°C)
- ASTM E831 — TMA measurement of linear thermal expansion
- EN 1991-1-1 — structural load context for roof installations
- ISO 178 — flexural properties relevant to span/fixing design
Get Expansion-Safe Installation Advice
Send your panel length, colour, climate range and fixing plan. Bakway provides installation drawings with expansion gaps sized for your project — factory-confirmed before you drill a single hole.