Decking is an outdoor platform made from timber or composite boards fixed to a structural sub-frame of joists supported on posts or bearers, used to create a flat, level surface for sitting, entertaining, or access in a garden. It is particularly useful on sloping ground where a level paved area would require expensive groundworks. Decking boards are typically 90-140mm wide, 25-32mm thick, and either pressure-treated softwood (e.g. C24 treated redwood), hardwood (e.g. ipe, teak, or treated oak), or composite (wood-polymer blend).

Does garden decking need planning permission?

In England, garden decking at the rear of a house is generally Permitted Development (no planning permission needed) if: it is no higher than 300mm above ground level; it does not extend beyond 50% of the total garden area (including any existing outbuildings); and the property is not in a conservation area, national park, or AONB. Raised decking over 300mm above ground - which may need a balustrade under Part K - is more likely to need planning permission. Front garden decking visible from the highway always requires a greater scrutiny.

How is a timber decking sub-frame built?

A typical decking sub-frame uses 100x100mm (or 150x150mm for raised decks) treated softwood posts set on concrete pad foundations or proprietary post bases. Bearer beams (75x150mm or 100x200mm treated timber) run between the posts. Joists (typically 47x100mm or 47x150mm at 400mm centres) span between the bearers at right angles to the final board direction. The deck boards are fixed to the joists with decking screws or hidden clips, with a 5-6mm gap between boards for drainage and movement. All timber should be minimum C24 grade, pressure-treated to Use Class 3 or higher.

What is composite decking?

Composite decking boards are made from a blend of wood fibre and plastic polymer (PVC or polyethylene), giving the appearance of timber with greater durability and lower maintenance. Composite boards are resistant to rot, insect attack, and fading, and do not require annual oiling or staining as timber does. They are fixed to a conventional timber or aluminium sub-frame in the same way as timber boards, using manufacturer-specific hidden clip systems. Composite decking costs significantly more than timber but is claimed to have a 25-year+ service life.

External Works noun

Decking

/ ˈdɛkɪŋ /

Also known as: garden decking, timber deck, composite decking, raised deck, deck boards

Definition

Decking is an outdoor platform of boards fixed to a structural framework of joists and posts, used to create a level garden surface for sitting and entertaining - particularly useful on sloping ground. Boards are typically pressure-treated softwood, hardwood, or composite (wood-plastic blend). Raised decking (over 300mm above ground) may require planning permission and must be fitted with a guardrail under Building Regulations Part K to prevent falls.

In practice

A standard ground-level deck is built on a sub-frame of 100x100mm treated softwood posts set on concrete pads or post bases, with 75x150mm bearer beams and 47x100mm joists at 400mm centres. Boards (typically 120x28mm grooved decking board) are fixed across the joists with 5-6mm drainage gaps. All timber must be pressure-treated to Use Class 3 (exterior, above ground) as a minimum - Use Class 4 (ground contact) for posts and bearers in soil or close to ground level.

Composite decking boards use manufacturer-supplied hidden clip systems rather than screw-fixed fixing. For raised decks, structural calculations may be needed for the post size, bearer spans, and joist spans. Any deck over 300mm above finished ground level on the lower side requires a guardrail at least 1100mm high (900mm in domestic use) with no climbable elements at lower than 1000mm. The deck surface itself should have a slight fall (1:80 to 1:50) to drain water away from the house.

Building Regulations

Planning (rear garden)Permitted Development if under 300mm and under 50% of garden

Part K (fall protection)Guardrail required if deck edge is more than 600mm above ground

Guardrail heightMinimum 1100mm (domestic), with no climbable rails below 1000mm

Timber treatmentUse Class 3 minimum above ground, Use Class 4 for ground contact

Approved Document K (Protection from falling, collision and impact) requires guarding at deck edges where a fall of more than 600mm is possible. Domestic guardrails must be at least 1100mm high. Decking that is part of a house extension or that changes the exterior appearance of the property in a conservation area may require planning permission regardless of height. Building Regulations approval (for the structural sub-frame) is required for raised decks attached to a dwelling in most cases.

Full Building Regulations guidance

Decking

Just so you know...