Decking

1. 1

   Check the height rule

   Keep the deck surface under 300 mm above ground to stay within permitted development, and check it does not cover more than half the garden.

2. 2

   Set out and clear

   Mark the deck, strip turf, lay weed membrane and set the levels so the deck falls very slightly away from the house.

3. 3

   Build the foundations

   Set concrete pads or ground screws at the spacing the frame needs, typically 1.2 to 1.5 m apart under the bearers.

4. 4

   Build the frame

   Build a treated joist frame with joists at 400 mm centres, or 300 mm for diagonal boards, and double up under any board joint.

5. 5

   Keep it clear of the house

   Hold the deck at least 150 mm below the damp proof course and leave a ventilation gap rather than fixing hard against the wall.

6. 6

   Lay the boards

   Fix boards with a 5 to 8 mm gap for drainage, two screws per joist, staggering end joints over a joist.

7. 7

   Finish the edges

   Add fascia boards, a slip-resistant finish on shaded decks, and a handrail wherever the drop exceeds 600 mm.

Most garden decks are permitted development, but two numbers trip people up: 300 mm above ground and 50 % of the garden. Both are measured generously by planning officers, so measure honestly.

• Decking is permitted development unless it's over 300 mm above ground, or together with other extensions covers more than 50 % of the garden. ^[1]
• The 300 mm is measured from the highest point of the natural ground at any edge, on a sloping garden a deck level with the house floor can easily breach it at the downhill end. ^[1]
• Timber structures over 600 mm with a fall risk should have a guardrail in accordance with Approved Document K (1.1 m high domestic). ^[2]
• Decks on listed buildings, in conservation areas or in Article 4 areas can need permission even under 300 mm, check with the local planning authority before ordering timber. ^[1]
• A deck that looks over a neighbour's garden can be refused on privacy grounds even when it passes the height rule, screening it after refusal is far harder than designing it in. ^[1]

Numbers that come from real decks, not brochures. Timber moves with the seasons; the details below are what stop it moving badly.

• Before digging pad footings or driving screw piles, CAT & Genny sweep first (HSE HSG47). Decks often sit over garden lighting, irrigation and old waste runs that don't show on any drawing. Full method in the Safety guide.
• Joist spans for C16 softwood at 400 mm centres: 47×150 spans up to ~2.4 m; 47×200 up to ~3.0 m.
• Composite boards usually need joists at 300 mm centres, check the manufacturer's span tables before ordering.
• Hardwood (oak, ipé), pre-drill every fixing or boards split. Use stainless A4 screws so tannins don't bleed black streaks.
• Slope the deck 1:100 away from the house for drainage and to stop standing water on the boards.
• Use stainless or A4 coated screws for hardwood and composite; galvanised fix into pressure-treated softwood.
• Allow 50–100 mm ventilation gap below joists, sealed-in decks rot.
• Decks on clay subgrades need taller pad footings (≥300 mm) or screw piles, surface concrete heaves with seasonal movement.
• Leave 5–8 mm gaps between softwood boards (they shrink as they dry) and 3–5 mm for composite, which barely moves in width but grows in length on hot days.
• Keep the deck frame at least 75 mm below the house DPC; if the design needs it higher, hold the deck off the wall on a ledger-free frame with a gap for airflow.
• Stagger board end joints across at least three joists and land every end on a joist, floating ends cup and creak within a season.

A deck is a small suspended floor: posts or pads, beams, joists, then boards. Each layer has a job and a minimum size; skip one and the spring appears within a year.

• Supports: concrete pad footings (about 300×300×300 mm) or adjustable pedestals on a compacted base. Timber posts should be UC4-treated and never buried in soil, post-in-ground rots at the collar within 5–8 years.
• Beams: doubled 47 mm sections bolted either side of posts carry the joists over longer spans and remove most bounce from the frame.
• Joists: C16 or better C24 softwood at 400 mm centres for softwood and hardwood boards, 300 mm for composite. Lay them with the crown (natural bow) facing up.
• Noggins between joists at max 1.2 m centres stiffen the frame and stop joists twisting as the timber dries.
• Boards: run them across the joists, fixing with two screws at every crossing. On long decks, picture-frame borders hide cut ends and look finished.
• Ledger boards bolted to the house are the classic failure point: they trap water against the brickwork. Prefer a free-standing frame held 25 mm off the wall.

The failures we see on surveys are almost always the same five, and all of them are cheaper to avoid than to fix.

• No airflow: skirting boards fitted tight to the ground seal the under-deck void. Moisture has nowhere to go and the frame rots from below. Leave ventilation gaps on at least two sides.
• No weed membrane and no weed kill: a gravel-and-membrane layer under the frame stops growth punching up between boards.
• Boards laid flat and level: without the 1:100 fall, water sits on the boards, algae follows, and the deck becomes an ice rink in autumn.
• Under-sized joists to save money: a 47×100 joist at 400 mm centres feels fine on day one and bounces by year two. 47×150 is the honest minimum for a domestic deck.
• Galvanised screws in hardwood: tannins react and bleed black streaks within weeks. A4 stainless only.
• Oiling a deck that is still wet: moisture trapped under oil goes mouldy. Wait for two or three dry days and test a hidden corner first.

Installed prices vary by access, ground and region, but these ranges (supply and install, 2026) are a fair budgeting start.

• Softwood decking: roughly £100–160 per m² installed, the cheapest entry but needs oiling every year or two.
• Composite decking: roughly £160–240 per m² installed. Higher upfront, near-zero maintenance, and boards outlast the frame.
• Hardwood decking: roughly £180–280 per m² installed, with pre-drilling labour pushing the top of the range.
• Sub-frame and groundworks typically make up 30–40 % of the quote, which is why 'just swap the boards' jobs rarely save what people expect.
• Screw piles add around £30–60 each but remove excavation and spoil entirely, often cheaper than pads on awkward access plots.