How to build decking

How do you build decking?

In five steps: rules, ground, frame, boards, edges. Each depends on the one before, and the frame needs most of the effort.

1. Check the rules first

In England, decking up to 30 cm above the ground normally needs no planning permission, as long as it, other extensions and outbuildings cover no more than half the garden, and no part is forward of the principal elevation. Scotland, Wales and Northern Ireland differ; see planning permission. If the drop is more than 600 mm, guarding is expected; see balustrades.

2. Prepare the ground

Clear vegetation and lay a geotextile weed membrane so weeds don’t push through the gaps six weeks after you finish, as one UK builder warns. Set the supports on firm ground: concrete pads, posts, ground screws or pedestals. The ground guide covers each surface.

3. Build the frame

Decide which way the boards will run before you build, because the joists run across them. Build the outer frame first and level it, then fill in the middle, which is much quicker. A builder working on a raised deck cuts the posts short by the thickness of the boards, so the surface meets the door threshold at the right height.

• Use pressure-treated graded timber and add noggins between joists to stop twisting. Two UK builders use C24 and C16 respectively; check what your plan needs.
• Space joists as the board maker says: Millboard 400 mm for composite, Composite Prime 300 mm, Cladco Signature 400 mm. Timber boards allow more. Our install guide has a table.
• Double the joists where two boards meet, and support every board end.
• Give the frame a slight fall so rain runs off, not a dead-level surface: makers quote anything from 0.5 percent (5 mm per metre) to 1 percent (10 mm per metre).
• Check it’s square by measuring both diagonals and nudging until they match.
• Treat every cut end with preservative or bitumen primer, because the factory treatment only soaks in a short way.
• Put joist tape on the joist tops; see joist tape.
• Add extra bracing where people walk out of the door, since a bouncy deck shows there most. Don’t board over drain covers: leave an access hatch.

4. Lay the boards

Start from the house or garden wall and work outwards, leaving a small gap to the wall. For timber, follow Wickes’ advice to treat all sides with deck seal, oil or stain. For composite, follow the maker’s clips and gaps; the install guide gives them by maker. Check straightness with a string line every few boards, and leave an access hatch where you need one.

5. Finish the edges

Picture-frame borders use 45 degree mitres. NeoTimber’s detail shows 4 mm expansion gaps and a double-joist arrangement for a perpendicular border. Add fascia boards, and plan stairs and handrails early: see decking steps.

What does it cost to build?

One UK DIYer’s 2024 timber build, 23.5 m², cost about £1,750 in materials, including VAT and excluding his own labour. The subframe was the dearest part at about £751, against about £563 for boards. These are his figures. For benchmarks, MyJobQuote’s October 2026 installed prices run £40 to £57 per m² for treated pine and £65 to £110 for composite, and BestBuilders’ supply and build range is £80 to £220. Use the calculator for your own deck.

How do you lay decking boards, step by step?

The steps below come from Wickes’ how-to, the Timber Decking and Cladding Association (TDCA) specification and its raised-deck code, Howarth Timber and Cladco. They agree on the order and differ on some numbers, which are shown.

1. Dry-lay first. Howarth says to lay boards across the joists, starting at the outer edge, and check the layout before you fix anything.
2. Set the gaps. TDCA says 5 to 8 mm between boards, 10 mm where a board meets a building and 5 mm where a board end meets a post. Howarth gives 5 to 8 mm along the length and 3 mm at board ends. Wickes uses a spacer for a 5 to 8 mm gap.
3. Butt joints. TDCA says to butt board ends together without forcing them tight. Its raised-deck code puts the join over a joist with an extra batten or section so each board end has support.
4. Drill and fix. Pre-drill and countersink, using a 2 mm bit on Wickes’ method, and fix with 64 mm decking screws into the second groove from the edge. TDCA specifies two fixings at each joist crossing, at the outer quarter points. Howarth puts two screws per joist, 15 mm from the board end and 20 mm from each edge, and says to stagger joints and wait four to six months of weathering before oiling or staining.
5. Start edge. Wickes lets the first board overhang the frame slightly if a fascia will be added later.
6. Fall. TDCA says to set the front beams slightly lower than the rear, or raise the joists at one end with durable packing, and to lay grooved boards lengthways along the direction of fall. It gives no number. For composite, Ecoscape says 1 percent, Wickes 1:100 and Cladco 1 in 80, so follow your board maker.
7. Fascia. Wickes marks the fascia 50 mm from each end and then every 600 mm, holds it level with packers, pre-drills and screws it into the second groove.
8. Treat cuts. Wickes says to give every cut end, sawn surface and drill hole two generous coats of wood preservative. TDCA’s code says the same for unavoidable cross cuts, notches or bores, using an end-grain preservative.

What joist spacing do timber boards need?

Wickes puts internal joists at 400 mm centres with noggins no more than 1,200 mm apart. Howarth says 450 mm, and Cladco gives a maximum of 400 mm for composite and 300 mm for PVC boards. The TDCA code for raised decks on new homes suggests joists of 170 by 44 mm or larger at 300 to 600 mm centres depending on board size and grade, but that applies to decks over 600 mm high, not a ground-level garden deck.

For the full run of tasks before boards go down, see the sections above on rules, ground and the frame, and joists for composite board makers’ centres.