What Gap Between Decking Boards? | Wern-Wood

Quick answer: For timber decking, leave a consistent gap of 5–8mm between adjacent boards. The Timber Decking and Cladding Association (TDCA) recommends this range to allow drainage, ventilation and normal timber movement. In practice, getting the boards into a reasonably even condition before fitting and keeping the gap consistent across the deck matters more than trying to adjust the spacing by 1mm for the season.

TDCA timber decking guidance is the technical baseline. The advice below is how we apply that guidance to real treated softwood decking.

Should I use a 5mm, 6mm, 7mm or 8mm decking gap?

Any consistent gap within the recommended 5–8mm range can work. We would rather see a deck installed neatly at 6mm throughout than one that wanders from 5mm to 7mm, back to 5mm and then out to 8mm.

The exact gap can also help the board layout land cleanly across the finished deck. Before fixing the first board, measure the usable width of the deck, the actual finished board width and the number of boards you expect to use. If the resulting consistent gap sits within the 5–8mm range, use that spacing rather than blindly starting with a 5mm spacer and dealing with an awkward final board.

My decking boards arrived damp. Can I still fit them?

Treated decking is outdoor timber, so it is normal for boards to pick up moisture in storage and transport. If the boards are simply damp and water is not physically seeping out of them, that does not automatically make them unsuitable to install.

What matters more is how even the moisture level is across the pack. If one board is soaking wet and the next is much drier, the boards can be slightly different widths. That makes it much harder to produce even finished gaps because the wetter boards will shrink more as they dry.

If the pack is very wet or noticeably uneven, we recommend letting the boards dry more evenly before installation.

How should I store damp decking boards before fitting?

Stack the boards neatly on bearers so they are held off the ground.
Use small spacers between layers so air can circulate through the stack.
Support the boards evenly along their length to reduce the risk of bends and twists.
Keep the stack evenly weighted so boards remain as flat as possible while drying.
Cover the timber loosely with a tarp to protect it from heavy rain and direct sun while still allowing airflow.
Let the timber dry gradually. Do not try to force it dry quickly with direct heat.

The aim is not to make every board as dry as possible. The aim is to get the pack into a reasonably even condition before fitting.

Should I leave a smaller gap if the decking boards are wet?

We do not recommend deliberately closing the gap simply because the boards feel wet.

Wet timber shrinks as it dries, so a 5mm gap fitted while the boards are soaking wet can become noticeably larger later. But unless you are actually measuring the moisture content, you do not know how much moisture is in each board or how much dimensional change is still to come.

The better approach is to let an uneven or very wet pack settle and dry more consistently, then install the boards with an even gap within the recommended range.

Do decking gaps need to change in summer and winter?

Not by much. If the boards have been stored properly and are in a reasonably even condition, normal differences in seasonal humidity are less important than consistent installation.

If someone chooses a 6mm gap in July and a 5mm gap in February, that is not something we would worry about. The visible result is usually affected far more by inconsistent gaps from board to board than by a 1mm difference across the whole deck.

How do I keep the gap between decking boards consistent?

There are specialist decking installation tools designed to create consistent spacing. For a simple method, we normally recommend horseshoe packers used as spacers.

Work methodically from one end of the board to the other. Do not fix one end, assume the rest is straight and then discover the board has wandered by the time you reach the opposite side.

Check the gap along the length of each board before fully fixing it. A small amount of time spent keeping each row straight is much easier than trying to correct accumulated errors several boards later.

Plan the deck around the boards before you start fixing

A common mistake is to build the complete frame first and only then work out how the decking boards are going to land.

Where possible, plan the frame around the board layout, not the board layout around the finished frame. Measure the actual finished board width, add your intended gap and work out the complete run before fixing the first board.

Our treated timber decking boards finish at approximately 120mm wide. For example, if you have a 3,000mm usable board run after any required edge clearances and plan to use 24 boards:

24 boards × 120mm = 2,880mm of timber.
3,000mm − 2,880mm = 120mm remaining for the internal gaps.
24 boards create 23 internal gaps.
120mm ÷ 23 = approximately 5.2mm per gap.

That lands inside the recommended 5–8mm range, so a consistent gap of roughly 5.2mm would allow the board run to finish neatly without creating a very narrow final strip.

The useful formula is:

Gap = (usable board run − total board width) ÷ number of internal gaps

Remember to remove any required clearances at buildings, posts or finished edges from the usable run before doing the calculation.

What gap should I use when replacing decking boards?

If you are replacing a small number of boards in an existing deck, match the existing spacing where that spacing is already sensible.

For example, if the surrounding deck has consistent 7mm gaps, installing a group of replacement boards at 5mm may be visually obvious even though both dimensions sit within the normal range.

Do not copy an existing gap that is below the recommended minimum just for appearance. If the old deck has been fitted too tightly, treat that as a problem to investigate rather than a template to repeat.

My old decking has no gaps and water is pooling. What should I do?

If the boards have been installed hard against one another and water is pooling, the proper fix is usually to remove the boards and relay them with the correct spacing rather than trying to cut narrow gaps into the finished deck.

Lifting the boards also gives you the chance to inspect the subframe, drainage and support underneath. Tight board spacing can be a sign of an inexperienced installation, so it is worth checking that the problem is not limited to the surface boards.

Correct board gaps help surface water drain, but the deck structure should also have suitable drainage and should not create areas where water is trapped underneath.