In brief: Recycled plastic decking boards move with temperature – according to the Ecoplank installation guide, a ±30°C temperature swing changes a board’s length by about 5 mm per metre. That is why they are never installed tight. The guide specifies three gaps: 10 mm at board end joints, 10 mm from a wall or other fixed element, and a side gap between boards (1 mm for tongue-and-groove boards). Installation is best carried out at +15°C to +25°C. The colder it is during installation, the wider the end gap.
Contents
- Why decking boards move
- Three places where you leave an expansion gap
- Table: end gap vs installation temperature
- What happens when boards are laid tight
- Gaps are not everything – joists and fixing
- Frequently asked questions
1. Why decking boards move
Every material changes its dimensions with temperature. Wood moves mainly because of moisture. A board made of recycled polyolefins does not absorb moisture – instead, it gets longer in the heat and shorter in frost.
The scale is specific. The Ecoplank installation guide states that with a temperature change of ±30°C, a board shrinks or expands by about 5 mm per running metre. For a 3 m board, that is already about 15 mm. On a deck in full sun, the surface heats up more than the air, and in winter the same deck stands in frost.
This is not a flaw of the material. Every plastic profile behaves the same way – including WPC boards, although there the wood flour limits the movement somewhat (more in our comparison WPC or recycled plastic). The difference is that this movement has to be planned at the installation stage rather than fixed after the first summer.
2. Three places where you leave an expansion gap
| Where | Gap according to the Ecoplank guide | Why |
|---|---|---|
| Board end joints (end to end) | 10 mm (adjust to temperature – see the table below) | The board moves mainly along its length |
| Wall, low wall, post or other fixed element | 10 mm – applies to boards and joists | The board must not press against something it cannot move |
| Between boards side by side | 1 mm for tongue-and-groove boards | Movement across the width is small, but not zero |
End joints. This is the most important gap, because the board moves mainly along its length. The guide is explicit: the longitudinal expansion gap must always be kept.
Walls and fixed elements. Leave a 10 mm gap not only at the boards, but also between a joist and the wall and between joist ends. Joists move too.
Between boards. Movement across the width of a board is much smaller than along its length. With a tongue-and-groove system, 1 mm is enough.
3. Table: end gap vs installation temperature
A board laid on a cool day is “contracted” – it will lengthen in summer, so it needs more room. A board laid in hot weather is already “stretched” – the gap can be smaller.
End-joint gap in mm by board length (Ecoplank tongue-and-groove board installation guide):
| Installation temperature | 1 m board | 2 m board | 3 m board |
|---|---|---|---|
| −10°C | 10 | 15 | 18 |
| 0°C | 8 | 11 | 13 |
| +10°C | 6 | 9 | 11 |
| +15°C | 5 | 7 | 9 |
| +20°C | 4 | 5 | 7 |
| +25°C | 3 | 3 | 5 |
| +30°C | 2 | 2 | 3 |
| +35°C | 1 | 1 | 2 |
Installation outside the +15°C to +25°C range is possible, but the guide does not recommend it. What matters is the temperature of the material, not just the air – boards that have been lying in the sun are warmer than the thermometer shows.
4. What happens when boards are laid tight
A board that has nowhere to expand in summer starts pressing against its neighbour or the wall. The stress has to go somewhere. Most often it shows like this:
- the board bulges between joists because it has no room to lengthen;
- screws work under load and can loosen or widen their holes;
- joints open unevenly in winter, because some boards got jammed and others did not;
- the deck looks worse, even though it looked perfect on day one.
It is also worth knowing that the board warranty does not cover dimensional changes caused by thermal expansion. In other words: plan the expansion gaps, because they are part of the installation, not of a complaint.
5. Gaps are not everything – joists and fixing
A gap only works if the board can actually slide into it. That is why expansion gaps go hand in hand with fixing. The key rules from the installation guide:
- Joist spacing – maximum 50 cm. Each board must have at least 3 support points.
- Board end – no further than 5 cm from a joist.
- Screws – A2 stainless steel, TORX, partially threaded (5×80), at least 2 screws on every joist.
- Pre-drilling – boards up to 1.5 m: Ø5 mm holes. Longer boards: two centre points Ø5 mm (these hold the board in place), end points Ø8 mm – the larger hole gives the screw play when the board moves.
- Slope – joists laid with a fall of at least 1% so that water does not stand on the deck.
- Material allowance – about 10% for offcuts.
We describe installation step by step in the article Board installation is more than fixing boards. Full installation guides are available in the Documents section, ready to download.
6. Frequently asked questions
How big should the expansion gap between decking boards be?
According to the Ecoplank guide: 10 mm at end joints (end to end), 10 mm from a wall or other fixed element, and 1 mm between tongue-and-groove boards lying side by side. The end-joint gap is adjusted to the installation temperature.
How much does a recycled plastic board expand?
With a temperature swing of ±30°C – by about 5 mm per metre of length. So a 3 m board can change its length by about 15 mm.
At what temperature should recycled plastic decking be installed?
Ideally between +15°C and +25°C. At lower temperatures, end gaps must be wider – at 0°C a 3 m board needs 13 mm, compared with 7 mm at +20°C.
Is an expansion gap needed if the board is short?
Yes. A shorter board moves less, but it still moves – even a 1 m board installed at +20°C needs 4 mm at the end joint.
Is a wall gap also needed for joists?
Yes. The guide requires 10 mm between a joist and the wall and between joist ends.
Why are larger holes drilled at the ends of long boards?
Because the board moves relative to the screws. A Ø8 mm hole at the end points gives it that play, while the Ø5 mm centre points hold the board in place.
Does the warranty cover problems caused by missing expansion gaps?
No. The warranty does not cover dimensional changes caused by thermal expansion – which is why expansion gaps must be planned during installation.
Planning a deck?
Tell us what kind of deck you are building – dimensions, board lengths, what it will adjoin – and we will help you choose the profile, joist spacing and quantity of material. Contact us or order free samples to see the material for yourself before you decide.
Sources and documents
- Ecoplank installation guide – deck, tongue-and-groove board: expansion gaps, gap table by temperature, joist spacing, fixing.
- Ecoplank board care and maintenance: wall gaps, installation temperature, warranty scope.
- Technical data sheet – tongue-and-groove board TG32×140: ecoplank.com.pl/dokumenty.