Building a timber deck: substructure, gaps and fixings that last

DB David Bohler · Published 23 October 2026

A deck lies flat in the rain, dries slowly and is walked on every day – it is not an indoor floor taken outside. This guide summarises what the UK Timber Decking and Cladding Association (TDCA) and other trade guidance say about timber choice, joists, ventilation, gaps, fixings and maintenance, and where the planning rules in England draw the line.

What a timber deck has to withstand

Horizontal timber outdoors takes rain, sun and dirt from above and dries more slowly than a wall. According to the TDCA, wood starts to decay when its moisture content stays above about 22 % for long periods. Good deck design therefore has two basic principles, in the association’s words: a well ventilated and free-draining structure.

The TDCA recommends using only timber that can provide a service life of at least 15 years – the industry default specification. Longer service lives (30 or 60 years) can be specified for pressure-treated softwood, but usually have to be requested explicitly.

Let water run off: gaps between boards, a fall along the grooves, no hollows under the deck
Let air in: keep the substructure open and ventilated
Keep the substructure as durable as the boards: it is hidden, but it carries everything

Choosing timber: durability, treatment and sapwood

The TDCA names three ways to reach that service life:

a hardwood that is naturally very durable or durable – for deck boards out of ground contact at least moderately durable (durability class 3);
a softwood that has been industrially pressure treated with an approved preservative – deck boards at least to Use Class 3;
a less durable species that has been thermally or chemically modified – modified boards out of ground contact rated at least durability class 3.

Natural durability only applies to the heartwood, not the sapwood. Because ground-contact ratings vary for modified wood, check the manufacturer’s data. Ask your supplier for written confirmation of the standard the timber meets.

Pressure-treated timber is treated in its finished form. Rip-sawing along its length is not allowed – return it to the treatment plant instead. Where you cross-cut, notch or drill it, give every new surface two liberal brush coats of end-grain preservative, and never put a cut end into the ground.

The substructure: joists, Use Class 4 and strength class

Since January 2020 the TDCA has recommended that softwood deck joists, posts, beams and any timbers providing exterior structural support are pressure treated to Use Class 4 – whether they are in ground contact or not. This is in line with BS 8417 and the Wood Protection Association’s code of practice. Joists made for indoor floors or roofs are not suitable.

For the strength of the frame the TDCA names strength classes:

Deck height	Softwood strength class
below 600 mm	C16 recommended
above 600 mm	C16 as a minimum – a requirement of building regulations for raised decks
smaller sections, longer spans, commercial use	C18 or C24

Joist sizes and spans come from span tables, for example in the TDCA/TRADA manual Timber Decking: The Professionals’ Manual. They depend on the loads – a hot tub, for example, needs a far stronger frame. This guide gives no structural design; for a raised deck, ask a professional.

Ventilation and drainage

The most common well-meant mistake, according to the TDCA, is to wrap joists in plastic sheeting “to protect” them: it traps moisture and shortens the life of the structure. Likewise, a weed-suppressant membrane must not lie between the boards and the joists.

Timber deck in section along a joist: boards with gaps, joist on pedestals, air beneath and a fall on the base – values from the VEH guidance sheet.

The European planing industry association VEH, together with Holzforschung Austria, gives these values in its guidance sheet on terrace boards: more than 150 mm between substructure and ground for ventilation; the substructure at least 10 mm (better 20 mm) above any water-carrying layer such as a slab; and a fall of at least 2 % on a closed surface beneath the deck. With grooved boards, lay the fall along the grooves – the TDCA notes that grooves are there to channel water, not to improve grip.

Gaps and joist spacing

Timber swells when wet and shrinks when dry, so the gaps have to take up that movement and let water and dirt through. The VEH guidance sheet gives:

Gap between boards: 7–10 mm at installation, depending on the material and its current moisture content – so measure the moisture before laying.
Butt joints: always over two supports, with at least 7 mm between board ends.
Distance to walls and other building parts: at least 20 mm.
Joist spacing: as a rule of thumb at most 20 × the board thickness – 28 mm boards, for example, on joists at most 560 mm apart. For hidden fixing systems, the manufacturer’s maximum spacing applies.

These are guide values for a ground-level deck, not a structural design.

Fixings: screws, pilot holes and connectors

All fixings must be corrosion resistant; otherwise rust stains and failing fixings follow. For screws the TDCA recommends:

Length: at least 2.5 times the thickness of the board (its fixings page gives 2–3 times, unless the manufacturer substantiates otherwise).
Position: at quarter points, i.e. 25 % in from the board edges, and 25 mm in from the board ends.
Pilot holes: good practice to avoid splitting, especially at board ends; in hardwood boards pre-drill every screw point with a 2 mm oversize pilot hole so the board can move.
Flush heads: a screw head standing proud or sunk too deep holds water.

Joist hangers, ties and post anchors make the joints stronger; their fixings should be of the same metal as the connector to prevent corrosion.

Maintenance: brush often, clean once a year

The TDCA advises regular brushing with a stiff brush to remove dirt, algae and leaves that make the deck slippery when wet, and at least once a year – preferably in spring – a more thorough clean with a mild soap solution or a deck cleaner. A pressure washer can be used, but at lower pressure and without holding the lance too close, as too much pressure damages the surface.

If you use a water-repellent coating, the TDCA suggests renewing it every other year, preferably at the end of summer. Coloured stains need refreshing from time to time, depending on sun and wear.

Do you need planning permission?

In England, the Planning Portal says that decking or other raised platforms in a garden are permitted development – no planning application needed – provided that:

the decking is no more than 30 cm above the ground;
together with other extensions, outbuildings etc. it covers no more than 50 % of the garden area;
no part of it is forward of the wall forming the principal elevation of the house.

Stricter limits apply in National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites, and no decking is permitted development within the curtilage of a listed building. These rights apply to houses, not to flats and maisonettes. Building Regulations should be assumed to apply to every deck that requires planning permission. The policy in Wales may differ, and the TDCA points out that the rules vary across the UK – check with your local planning authority before you start.

For all but the simplest low-level deck, the TDCA recommends planning a balustrade as part of the project from the start; where it serves as a safety barrier, it must meet the Building Regulations.

Sources and standards

Standards are not free to read and are named here for verification only. What applies is always the edition in force at the building location.