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Decking

Heights, joist spans, fall, fixings, and where decking becomes a planning matter.

All guides
Stub — growing
Last reviewed
2 sources
Regulations
  • 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]
  • 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]
Hints & tips
  • 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.

This app provides general UK guidance and material estimates only. It is not legal, planning, engineering or building-control advice. Always confirm requirements with your local planning authority, building control, utility providers, manufacturers or qualified professionals.