Joists are the ribs under the boards. On this ground they are also the part that rots first if someone beds them in the soil. This page is how we avoid that.
Why clay and timber should not touch
Middlesbrough clay holds water. After rain it stays soft at the top. A joist laid on that surface wicks moisture along the grain. The end grain, where we cut, drinks fastest. Paint on the outside does not stop it.
The clay also moves a little between wet and dry spells. A frame that rests on it will rise and drop with that movement. Boards open up. Fixings work loose. The deck feels springy for the wrong reason.
We want the joists in the air. The load goes down posts into pads. The soil can swell or shrink without taking the frame with it. That is the whole idea of a proper footing, and it is written up on our footings page.
A membrane is not a joist support. Joists on a sheet are still joists on the clay. We do not call that a build.
How we set the joists
Joists run the short way under the boards, at a spacing the board can live with. Composite needs them closer than many timber boards. We follow the board maker. We do not stretch the gap to save a length of timber.
Each joist bears on a bearer or on posts, not on a skim of soil. Ends are cut square and kept off the house wall. Where a joist meets a ledger on the house, the fixing is into sound brick, and the ledger itself stays clear of the damp-proof course.
We set a slight fall so rain walks off the boards. On a flat clay yard, water will sit if we do not give it a way out. The fall is small. You will not feel it as a slope under a chair. You will see it in the first heavy shower.
Alleys limit the length we can carry. A full joist may not turn the corner by the gate. We join on a post, with the joint over support, not in mid-span. The join is planned. It is not a cut made in a panic in the alley.
Air under the boards
Air is the second defence after the pad. Clay sends damp upwards. If the deck is a lid, that damp stays in the frame. We leave a clear void. We do not close the sides with a tight skirt that turns the void into a box.
Shade makes this slower. A north-facing yard, or one under a tree, keeps the void cool. Cool and damp is where green growth starts on the board edge. Air flow is how we fight that. Not a sealed edge with a pretty fascia and nowhere for a breeze.
We keep air bricks clear. A deck that covers one is a damp problem for the room as well as the joist. Leaves in a shaded corner hold water against the timber end. We leave a way to sweep underneath. A deck you cannot clean below will fail on this clay.
Shade, alleys and the height left over
Joist depth spends the height you may be trying to keep. If the walking surface must stay within 30 cm of the soil, the pad, the post and the joist all have to fit in that space, with air still left. Often they will not. We say so. The height rules explain the ruler. This page is about the timber inside that ruler.
A shaded alley is the awkward mix. The ground is clay. The sun barely arrives. The working room is tight. We still refuse to drop joists onto the soil to "gain a bit of height". That gain is the rot. A thinner board, or an honest application for a slightly higher deck, is the cleaner choice.
A wide joist gap lets a composite board flex, and flex holds water in the groove. We tighten the centres. The shade and boards guide covers the surface. A quote shows centres, timber sizes and the post grid. Ask for a quote if the garden already holds water. We still open a hole. The map is only a hint.
Your questions about joists on clay
Can the joists rest on the ground if we use a membrane?
No. A membrane is not a foundation. Joists on a sheet are still joists on clay. They stay wet and they move. We carry them on posts and pads, with air all round the timber.
Do shaded yards need closer joists?
Often, yes, when the board is composite or the span is long. Shade keeps the surface damp, and a flexing board holds that damp in the grooves. We follow the maker's centres and tighten them if the board asks for it. We do not open the gap to save timber.
Will a joist layout push us over 30 cm?
It can. Joists have a depth. Add a board and a pad and the walking surface may stand more than 30 cm off the soil. If you need to stay under that line, we design for it at the start. If we cannot, we tell you.
How do alleys change the joist plan?
They limit the length we can carry in. Joists may be joined over a post instead of run in one piece. The joint is never left in mid-air. We check the alley width on the visit and write the piece lengths into the quote.