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Decking Specialist terms explained — Horsham glossary

The 300mm Rule(300mm)

The height above natural ground level below which decking is normally permitted development. Above it, planning permission is usually needed — largely because a raised platform creates overlooking, which is the objection neighbours actually raise.

Composite Decking(Composite)

A wood-and-polymer board that does not need staining or sealing and resists rot. Higher upfront cost, far lower maintenance, and it expands and contracts more than timber — so the fixing gaps specified by the manufacturer are not optional.

Joist Spacing(Centres)

The distance between substructure joists, which governs how much the deck flexes underfoot. Composite boards generally need closer centres than timber, and the commonest cause of a bouncy new deck is timber spacing used under composite boards.

Ground Preparation(Substructure)

The membrane, footings and ventilation gap beneath the deck. A deck sitting directly on soil with no airflow rots from underneath, where nobody sees it until a board gives way.