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.