How it's done
Decking Substructure: Joists, Spacing and Ventilation
Boards are the visible half. Joist centres, airflow and drainage decide whether a deck stays flat and dry.
If you only inspect one thing on a decking quote, inspect the frame specification. Everything that goes wrong with decking — bounce, cupping, rot, and boards that pop their fixings — starts underneath.
Joist centres
Softwood boards laid square to the joists tolerate wider centres than composite. Composite manufacturers specify tighter centres, and laying boards diagonally reduces the permitted spacing further again. We build to the board manufacturer's published spacing, which is also what protects any product warranty.
Ground preparation
Turf and topsoil come off, a weed membrane goes down, and the frame sits on compacted hardcore, paving slabs or concrete pads — never directly on soil. On clay we allow for seasonal movement; on made ground we increase bearing points.
Ventilation and drainage
A deck needs air moving beneath it. We keep a clear void, avoid boxing in every side solidly, and set a slight fall so water runs off rather than pooling in the board grooves. A deck that cannot breathe rots from below while looking perfect from above.
Detail points that matter
- Cut ends of treated timber re-treated before fixing
- Joist tape over the top of each joist to stop water sitting in screw holes
- Correct expansion gaps at board ends, particularly for composite
- Deck kept clear of the damp-proof course on the house wall
- Stainless or coated fixings sized for the board type
Frequently asked
Can decking go straight onto grass?
No. Without a prepared, drained base the frame sits wet and the deck moves.
How high should a deck sit off the ground?
Enough for airflow beneath, and always below the damp-proof course where it meets the house.
Why does my deck bounce?
Joist centres are too wide or bearers are too far apart for the span — a frame problem, not a board problem.
Do you use joist tape?
Yes on every deck we build; it is inexpensive and adds meaningful life to the frame.
Want this priced properly for your garden?
Free site visit across London and Hertfordshire, a written specification, and a fixed price. No obligation.