Decking Joist Spacing & Frame Guide

Joist spacing

Layout	Joist centres
Standard boards (timber & composite), laid straight	400mm centres
Diagonal / 45° patterns	300mm centres

Centres means centre-of-joist to centre-of-joist. 400mm is the TDCA's standard spacing for straight-laid boards (see sources below) and stops them flexing underfoot; it's also the spacing HomeCalc's decking calculator assumes. For a diagonal pattern, drop to 300mm and add joists to suit (the calculator estimates at 400mm straight).

Build the frame off the ground

• Never lay boards straight onto soil — they rot within 1–2 years from ground contact.
• Lay a weed membrane over levelled ground, then set concrete pads or adjustable pedestals at the joist intersections.
• Build the subframe in treated joists and keep a clear airflow gap beneath the deck — the TDCA advises ventilating the underside to prevent rot and mould. For ground-level decks, raise the frame on pads or add ventilation gaps.
• Budget method: 47 × 100mm treated joists at 400mm centres on concrete deck blocks, rather than digging footings.

Boards & coverage

Board	Covers
Standard 145mm wide × 3.6m long	~0.52 m² per board
Rule of thumb	~2 boards per m² (before cut wastage)

Do you need planning permission?

In England, garden decking is usually permitted development. You do need permission if any of these apply:

• the decking is more than 300mm (30cm) above ground level, or
• together with other extensions and outbuildings it covers more than 50% of the garden, or
• any part sits forward of the principal elevation — roughly, the front wall of the house facing the road.

Tighter limits apply on designated land (National Parks, Areas of Outstanding Natural Beauty, the Broads, World Heritage Sites), and decking is not permitted development at all within the curtilage of a listed building — that always needs full planning permission. Rules differ in Wales, Scotland and Northern Ireland — always check with your local planning authority before you build.