The theoretical expansion and contraction movement of long run roofing and cladding sheets due to temperature change of the material can be calculated and then an appropriate fastening method specified. Only thermal movement along the sheet length need be considered, as thermal movement across the sheet is accommodated by the profile shape.
Design Guideline
(a) Determine The Thermal Expansion Coefficient
Select the appropriate thermal expansion coefficient from the table below.
Thermal Expansion Coefficients
| Sheet Material | Thermal Expansion Coefficient (mm/m°C) |
| Steel | 0.012 |
| Aluminium | 0.023 |
| Copper | 0.017 |
| GRP* | 0.029 |
*GRP refers to glass reinforced polyester material used to manufacture the Dimond Duraclad, Durolite and Maxilite products.
(b) Determine The Expected Temperature Range
The temperature extremes (from nighttime winter to daytime summer) that the sheet material is expected to attain need to be assessed.
Use the following table as a general rule to select the likely maximum and minimum temperatures.
These tabulated values may not be reached in less severe environments (e.g. North Island coastal) but may be exceeded
in special circumstances (e.g. sheltered valley Central Otago).
Guide To Surface Temperatures
| Roof Type | Surface Appearance | Un-insulated Roof Max* | Un-insulated Roof Min* | Insulated Roof Max* | Insulated Roof Min* |
| Steel or Aluminium | Unpainted | 50 | -10 | 60 | -15 |
| Steel or Aluminium | Light Colour | 50 | -10 | 60 | -15 |
| Steel or Aluminium | Dark Colour | 65 | -10 | 80 | -15 |
| Copper | Unpainted | 65 | -10 | 80 | -15 |
| Duraclad (GRP)** | Light Colour | 50 | -10 | 60 | -15 |
| Duraclad (GRP)** | Dark Colour | 60 | -10 | 70 | -15 |
| Natural Lighting (GRP)* | Clear or Tint | 45 | -10 | - | - |
*Guideline Temperature Extremes °C
**GRP refers to glass reinforced polyester material used to manufacture the Dimond Duraclad, Durolite and Maxilite products.
(c) Calculate The Theoretical Thermal Movement
Theoretical Thermal Movement (mm) = ∝ x ΔT x L
Where ∝ = thermal expansion coefficient, mm/mºC
ΔT = (max temp) – (min temp) = expected temperature range, ºC
L = roof sheet length, m
Example – Light colour, insulated, steel roof, sheet length 12m
Theoretical Thermal Movement = 0.012 x (60-(-15))x12 = 10.8mm
(d) Specify The Appropriate Fixing Method
The theoretical thermal movement must be accommodated by the method used to fix the roof sheeting to the roof structure. Low rib sheet profiles are less rigid and are therefore able to bow slightly between purlin lines and accommodate more movement than the more rigid, high rib profiles. Select the appropriate fixing method from the following table.
Fixing Methods To Accommodate Thermal Movement
| Fixing Method | Theoretical Thermal Movement (mm) - High Rib | Theoretical Thermal Movement (mm) - Low Rib | Theoretical Thermal Movement (mm) - Decking (DD400,DD300) | Theoretical Thermal Movement (mm) - Decking (DD630) |
| 1. Solid Fix -Screw fasteners without oversize holes, profile washers may not be necessary | UP to 10mm | Up to 13mm | - | - |
| 2. Oversize Holes, One End - Top 2/3 of sheet length: screw fasteners without oversize holes, profile washers may not be necessary. Bottom 1/3 of sheet length: screw fasteners with 10mm ø oversize holes, and profile washers with 36mm ø EPDM seals | 10 - 15mm | 13 - 20mm | ||
| 3. Oversize Holes, Both Ends - Top 1/4 of the sheet length: screw fasteners with 10mm ø holes and profile washers with 36mm ø EPDM seals. Middle 1/2 of sheet length: screw fasteners without oversize holes, profile washers may not be necessary. *Bottom 1/4 of the sheet length: screw fasteners with 10mm ø holes, and profile washers with 36mm ø EPDM seals. | 15 - 20mm | 20 - 26mm | ||
| 4. Clip Fastening of Decking | - | - | Up to 30mm | **Up to 80mm |
**Consideration must be given to sheet clearances between the building structure to achieve this max amount of movement. Any Natural Lighting sheets need to match this special design. Call 0800 ROOFSPEC to discuss.
*Note that Duraclad (GRP) sheets require a minimum pre-drilled hole diameter of 2mm greater than the screw diameter, and require washers.
(e) Extra Long Roof Runs
Proposed lengths of sheeting that give theoretical thermal movement outside the scope of the table - Fixing methods to accommodate thermal movement, will require the sheets to be in two or more separate lengths. The joining of these lengths must accommodate the thermal movement and therefore should be specified to the requirements for a Step Joint.
(f) Horizontal Cladding
Avoid end laps and use a butt joint with a top hat flashing joiner. Consideration for thermal expansion movement on wall cladding should be made on wall runs above 8m. This may involve the use of a butt joint top hat flashing.
