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Deck planning tool
Composite decking expansion gap guide
Every composite board grows and shrinks with temperature, and the gap you leave at the ends is what decides whether that movement is invisible or ruinous. Set the board length and the temperature you will be working in, and this returns the end gap to leave, with the expansion arithmetic shown so you can check it. Tarimatec's coefficient is 2.81 × 10⁻⁵ per °C, measured to UNE 53126.
Tarimatec's end gap rule
The manufacturer's rule is short, and it is banded by board length rather than calculated per job, because longer boards move further for the same temperature change.
| Board length | Minimum gap at each end |
|---|---|
| Up to and including 1.5 m | 5 mm |
| Over 1.5 m, up to and including 2.5 m | 6 mm |
| Over 2.5 m | 10 mm |
These are minimums at each end, not a total to be shared between two ends, and they apply wherever a board end meets anything solid: a wall, a kerb, a post, a threshold, a planter, or another board in a butt joint.
Where the numbers come from
Thermal expansion is one equation. The change in length equals the coefficient, multiplied by the original length, multiplied by the temperature change:
Movement in mm = 0.0000281 × board length in mm × temperature change in °C
A 2,500 mm board installed at 20 °C and later baking at a 60 °C surface temperature has a 40 °C swing to absorb. That is 0.0000281 × 2500 × 40, or 2.8 mm. Tarimatec's 6 mm minimum for that board length covers it comfortably, which is the point: the banded rule already carries margin for a hot dark surface and a cool install, so a compliant gap is a safe gap.
Surface temperature is not air temperature. A dark board in direct sun sits far above ambient, which is why the tool below asks you to pick a design maximum surface temperature rather than a weather forecast. Pale colours in shade run cooler; dark colours in full sun run hottest. Tarimatec's solar reflectance index is measured above 80 on a white sample, per ASTM E1980 and Tecnalia report 082062-002, so a pale board genuinely does stay cooler than a dark one, but the index is measured on white and should not be read across the range.
Printable gap table
Movement figures below are calculated to a 60 °C design surface temperature. Print this page and take it to the site.
| Board length | Tarimatec minimum | Movement from a 20 °C install | Movement from a −5 °C install | Gap to set on site |
|---|---|---|---|---|
| 1.0 m | 5 mm | 1.1 mm | 1.8 mm | 5 mm |
| 1.5 m | 5 mm | 1.7 mm | 2.7 mm | 5 mm |
| 2.0 m | 6 mm | 2.2 mm | 3.7 mm | 6 mm |
| 2.5 m, the standard board | 6 mm | 2.8 mm | 4.6 mm | 6 mm |
| 3.0 m | 10 mm | 3.4 mm | 5.5 mm | 10 mm |
| 4.0 m, maximum special order | 10 mm | 4.5 mm | 7.3 mm | 10 mm |
Read the last two columns against the second: in every case the manufacturer's banded minimum is larger than the calculated movement, even from a cold install to a hot surface. Follow the band and you do not need to do the arithmetic on site. The arithmetic is here so you can satisfy yourself that the band is not arbitrary.
Side gaps, wall gaps and everything else
- Side gap between boards: 5 mm, and it is not a decision. The hidden H clip physically sets it, so every board in the field is spaced identically whether or not you thought about it.
- Wall and fixed-edge clearance: at least the end gap for that board length. Never butt a board hard against a wall, kerb, post or threshold.
- Butt joints between boards in a run: the same end gap at the joint, and both board ends must land on their own support, not share one.
- Fascia and trims: gapped as well. A fascia screwed tight into a corner has nowhere to go when it grows.
- Ventilation under the deck matters as much as the gaps. A sealed cavity runs hotter and holds moisture, which pushes the boards to the top of their movement range and keeps them there.
Installing in cold conditions
If you lay boards on a cold day, they are near the short end of their range and every gap you cut will look generous. It will not stay generous. The boards will grow toward the gap as the deck warms through spring and summer, and a gap that felt fussy in November is the thing that stops a buckle in July.
The reverse is the real danger. Boards installed hot, tight against a wall, have nowhere to go on the first cold night, and shrinkage pulls on the clips and the fixings instead. Whatever the weather, cut to the band and resist the urge to close things up because the gap looks wide.
Let boards acclimatise on site before cutting, ideally flat, out of direct sun and at the temperature they will be installed at. Cutting boards that came off a cold truck an hour earlier means you are measuring one length and installing another.
What going wrong looks like
Too tight, and you get the classic failures: boards bowing upward off the joist in mid-summer, ends riding over a kerb or wall, clips deforming, and a creak that arrives with the first hot week. Too loose, and it is cosmetic rather than structural, but it is permanent: gaps that swallow debris, an untidy perimeter, and a visible rhythm change where a run was set out differently from the rest.
Expansion damage is also the kind of thing that lands outside a warranty claim, because it is an installation variable rather than a product defect. Tarimatec's 25-year manufacturer warranty, administered in Canada by Zinodeck, assumes the specified substructure, fixings and gaps.
Related: the board, joist and clip calculator, edge and stair details for how the perimeter is finished, all planning tools, and the installation guide for the full sequence.
Getting a deck specified properly
Send us the deck dimensions, the board direction and where the fixed edges are, and we will quote the boards, the aluminium substructure, the clips and the fixings in Canadian dollars with the lead time stated. Boards are import to order: plan on 8 to 12 weeks from a confirmed order.
Get a project quote