Most Decks Fail at the Joist, Not the Board

The board takes the blame; the joist does the damage

Every winter someone tells me their decking was rubbish. The boards have gone green, the surface is slick, maybe a few have lifted or gone soft. Sometimes the board is the problem. More often, the deck underneath has been holding water against it since October.

A timber deck in the UK is not fighting rain. It is fighting trapped moisture. Rain falls, runs off, and the deck dries when air moves underneath and between the boards. Take away the airflow and you have built a very expensive drying rack for algae.

The detail that matters most is the one nobody photographs: the gap between the deck and the ground, and the way the joists are supported. If the joists are sitting in soil, on damp slabs with no void, or boxed in with solid sides, the deck is already in trouble. The boards are just the visible symptom.

Clearance is not a luxury

A deck needs to breathe. In practice, that means keeping the joists clear of the ground and giving the underside a route for air. Bearers on paving slabs are fine if the slabs are level and the void is open. Joists laid straight onto soil are not.

As a working rule, aim for at least 150mm clear from the ground to the underside of the joists, and more where the garden is shaded or the ground stays wet. If the deck is low, do not solve it by blocking the sides with boards and hoping. Either raise the deck or accept that you need a different detail.

Use proper joist hangers and fixings rather than nailing through the sides of joists. A hanger keeps the top of the joist flush and spreads the load. It also stops the kind of movement that opens up board gaps or makes fixings squeak. Joist tape on the top edge is useful protection, but it is not a substitute for ventilation. It keeps water off the joist; it does not dry the deck.

The 5mm argument, and why it is not really an argument

Board spacing is where good intentions meet British weather. Timber boards move. They swell in the wet and shrink in the dry. If you butt them tight because it looks neater on the day, you are asking for cupping, squeaks and a deck that holds water in the seams.

For timber decking, a 5 to 8mm gap between boards is the sensible range. Go towards 8mm if the deck is exposed, shaded or in a wet part of the garden. Composite boards usually want a smaller gap, but follow the manufacturer’s figure — do not invent one. The gap is not a design feature. It is drainage and air movement.

Run boards with a fall away from the house, even if it is only a subtle one. A perfectly level deck can still shed water if the gaps are right, but a slight fall helps. Do not trap the ends against a solid wall with no gap. Water needs to leave the deck, not sit in the last board.

Fixings that survive a wet winter

Plain screws from a bargain bucket will rust. In a damp garden, that rust stains the boards and weakens the fixing. Use stainless steel or properly coated decking screws, and use the right length — long enough to bite into the joist, not so long that they poke through underneath.

For hidden fixings, read the instructions and do not mix systems. A clip system that relies on a specific gap and a specific board thickness will not forgive improvisation. If you are using nails, use the correct annular ring shank nails and check them as the deck seasons. A popped nail is easy to fix; a deck that has been nailed too tight is not.

End grain is the thirsty part of any board. Treat cut ends with a suitable preservative, especially on treated softwood. Do not rely on the factory treatment to cover a fresh cut. It does not.

Timber choices for a wet British garden

Treated softwood is the default for a reason. It is affordable, widely available and, if the detailing is right, it gives good service. Do not buy unseasoned boards and expect them to stay flat. Some movement is normal; violent cupping is usually a sign that the boards were too wet when they went down or that the joists are too far apart.

Durable hardwoods and naturally durable softwoods like oak, larch or cedar can work well, but they are not magic. Oak moves. Larch can be lively. Cedar weathers to grey and needs airflow just as much as pine. Composite decking avoids rot, but it still needs a properly ventilated subframe and the correct joist spacing. The material changes; the drainage rules do not.

For winter grip, choose a board with a textured or grooved surface and keep it clean. Green algae is a maintenance issue, not a reason to replace the deck. A stiff brush and a suitable cleaner in spring is worth more than a coat of sealer slapped on over damp timber in November.

What I would put on the order

The order is boring, and that is the point: treated joists sized for the span, proper hangers, stainless or coated screws, spacers for consistent gaps, and boards from a batch you have actually looked at. Add end-grain preservative and a brush. Leave the solid side panels off. Let the air in.

A deck does not fail because it is made of wood. It fails because someone built a lid over a puddle. Get the joists up, keep the void open, space the boards, fix them properly and let the British weather run off instead of settling in. The boards will last a lot longer when the detail underneath gives them a chance.