Do decking boards need gaps? Should you fit them with a gap?

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Decking can be a great addition to the outside of any home. It provides an area that can be used for sitting out, barbeque, entertaining guests, etc. It also adds an aesthetically pleasing feature to your garden.

However, there is some debate on the correct way of installing your decking.

In this article, we will be discussing the controversial subject, of whether decking boards need gaps when installed. Due to moisture levels and temperature, boards will expand and contract over time. Therefore, a gap is inevitable after the boards are fitted. But should they be installed with a gap?

do decking boards need gaps

Surprisingly, this is a topic of great debate. Some people like to install the deck boards tight, and others leave a gap. But who is right?

Any timber you buy, whether it’s for internal, or external use, will have some level of moisture in the wood. A common moisture content is between 12-18%.

When it comes to deck boards, they are generally quite a damp timber, as they are not kiln dried after treatment. This also means, the moisture content can vary slightly. However, it is needless to say, that shrinkage in the board is expected once exposed to drier warmer weather.

Deck boards can also expand slightly in some conditions, but it will be to a lesser extent. This is because the moisture content of the boards, isn’t likely to end up much higher than the time they are bought.

Due to this fact, it is always recommended to add a gap during installation. A small 2-3mm gap will allow for any small amount of expansion that might occur. It also avoids a massive gap when boards inevitably contract in the summer.

Reason you shouldn’t fit deck boards tight

The main reason you shouldn’t fit deck boards tight, is you can’t be 100% sure, that they are at their maximum moisture content when fitted.

Wood is a porous material, and the British weather is known to be particularly wet, especially during the winter months. Therefore, if your wood takes on more water, it will expand.

If your boards do expand and become tight, this can cause issues. Many people assume that this will cause the boards to buckle. However, this is very unlikely, I have never heard of this happening, even when deck boards have been fitted tightly.

The main issue is a lack of drainage and an increased risk of rot. Whilst the pressure treatment is designed to protect from rot, it is not immune from it. The pressure treatment simply extends the life of the timber, eventually it will still deteriorate.

The advantage of a gap, is it enables much better drainage, and it avoids water from pooling on the deck.

When fitting decking with a small gap, I have personally never seen the gap entirely close up again. However, in my own personal decking at home, the gap does close up considerably in winter. This is often back to around 3mm. Whilst in the summer this can open up as far as 6-7mm.

When is your decking being installed?

Another thing to consider, is when you are fitting your decking and how long have the boards been stacked and stored.

This can also affect the moisture content. Let’s assume you buy decking boards, and they are stacked outdoors over a period of time. Perhaps you need to wait until you have some spare time. You may also have the frame and supports to build before you start adding decking boards.

Let’s also assume it’s the middle of summer and the weather is very hot and dry. In these conditions, you would expect your stacked decking boards to reduce slightly in moisture content.

If they have reduced in moisture content, it is quite safe to assume, that they could expand if wet conditions cause them to take on water.

The same would be true if you are fitting in cooler, wetter conditions. At this point, it is quite safe to assume, that the boards are unlikely to ever expand very much. This means that a 2-3 mm gap will be more than adequate.


Whether you should install decking boards with a gap or not, is a debate that has been going for years, and I don’t expect people will stop arguing about it any time soon. However, I think I have proved that a small expansion gap is a wise choice.

It makes very little difference to the wider gap in summer, and I have never seen a 2mm gap completely close due to excess moisture in the boards.

By having a small gap on installation of between 2-3mm, this ensures your decking boards maintain a reasonable sized gap, all year round. This will look aesthetically pleasing throughout the seasons. Plus, it will protect your decking from premature rotting, over the longer term.

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