The Hidden Half of a Deck: Joists, Spacing and Keeping Water Moving
Photo: deck substructure joists bearers construction UK garden timber frame, via resource-wangsu.helplook.net
The boards are what everyone sees. They are also the part most people spend their time choosing. But a deck is really a roof over a crawl space, and the frame underneath decides whether it stays solid or turns into a wobbly, soggy problem. If the joists are undersized, the bearers sit in soil or the underside cannot breathe, no amount of expensive decking will save it.
Follow the load path
A deck works by passing load downwards: boards to joists, joists to bearers or beams, bearers to posts or pads, and pads to the ground. Every part of that path needs to be solid. Boards screwed to thin joists at wide centres will flex. Joists resting directly on soil will rot. Bearers supported only at the ends will sag in the middle.
Do not put timber straight on the ground. Use concrete pads or proprietary post bases, and keep the timber clear of soil. Where timber must be in ground contact, it needs the right treatment and the right detailing. A bearer sitting in wet soil is a bad detail even with treated timber.
Joist spacing and size
For a domestic deck, 400mm joist centres is a sensible default for common board thicknesses. Some thicker boards can span 600mm centres, but check the board supplier's figures rather than guessing. The joists themselves need to be sized for their span; a local timber supplier or span table will tell you what is needed. Do not copy a neighbour's deck without checking the spans.
Joist hangers make a neater job at the ledger and at any trimming. They also keep the joist ends out of trapped water. Noggins or blocking between joists help stop them twisting and make the frame feel more solid. If the deck is raised, brace the posts so it cannot wobble sideways. A deck that moves when you walk on it is not just annoying; movement opens gaps and lets water in.
Bearers, ventilation and height
Airflow is the part people forget. Leave a clear void under the deck so damp air can move. As a rough guide, aim for at least 150mm clear between the ground and the underside of the joists where you can. Do not block the sides with solid boards right down to the ground; use open slatted screens, gravel boards with gaps, or leave the sides open. Keep air bricks and vents clear if the deck sits near the house.
The deck should fall away from the building, typically around 1:80 to 1:100. That is a gentle slope, but it matters. Water should run off the boards and through the gaps, not sit in pools on the surface or drain towards the house.
Gaps, drainage and the boards
Decking boards need gaps. Around 5mm to 8mm between boards is a common working range, and it allows water to pass through and air to circulate. Too tight and the boards swell, trap debris and stay wet. Too wide and heels, keys and small toys find their way through. Run the gaps in line with the fall so water has an easy route out.
Keep the space under the deck clear of rubbish, leaves and soil. A permeable weed membrane over the ground can help stop weeds without turning the void into a plastic-lined puddle. Gravel can help drainage and discourage weeds, but do not fill the void up to the joists. Water needs somewhere to go.
Treated timber and fixings
Use treated timber that suits where it sits. Above ground, UC3 is the usual choice for decking and joists. For ground contact, you need UC4. Treated does not mean waterproof, and cut ends are a weak point. Seal cut ends with a suitable end-grain treatment. Use stainless steel or hot-dip galvanised fixings; plain screws will rust and stain the boards. Use decking screws rather than nails where you can, and leave the heads just below the surface without crushing the timber.
What goes wrong
A wobbly deck usually has one of three causes: posts that are not braced, joists that are too small or too far apart, or fixings that are too short. A rotten deck usually has water trapped somewhere: boards tight together, debris in the gaps, timber in soil, or a void with no airflow. Fix the cause, not just the board that looks bad.
Every spring, clear the gaps, check for soft spots, tighten any loose fixings and look at the underside if you can. A screwdriver pushed into a joist should meet resistance, not sink in. Replace boards early if they are failing; water gets into the frame through the smallest gap.
A straightforward spec
For a typical UK garden deck, think: treated joists at 400mm centres, bearers on concrete pads, a clear ventilated void, a fall away from the house and 5mm to 8mm board gaps. That is not a substitute for span tables or building regulations if the deck is raised or attached to the house, but it is a good starting point for a conversation with your supplier.
At Walsh Woodyard, we would rather sell you the right joist and a few galvanised hangers than see a lovely deck board sitting on a frame that will not last. The hidden half is the half that decides how long the whole thing lasts.
