Question 1

What is the standard UK decking board size?

Accepted Answer

The most common softwood decking board size is 32 x 150 mm (finished approximately 28 x 144 mm) in 3.0 m, 3.6 m and 4.8 m lengths. The 32 x 125 mm size is also widely available. These are typically pressure-treated (tanalised) softwood with a grooved or smooth profile on each face.

Question 2

What spacing should I leave between decking boards?

Accepted Answer

Leave 5-8 mm gaps between softwood decking boards for drainage and expansion. For composite decking, follow the manufacturer guidance — typically 5-6 mm side-to-side gaps and 3-5 mm end-to-end expansion gaps. Insufficient gaps cause water pooling and rot in timber decks, or buckling in composite decks.

Question 3

What are the advantages of composite decking?

Accepted Answer

Composite decking (wood-plastic composite or WPC) requires no annual treatment, does not splinter, resists rot and insect damage, and typically carries a 10-25 year warranty. It is more expensive initially (typically 3-5 times the cost of softwood) but lower maintenance over its lifetime. Common brands in the UK include Trex, Millboard, TimberTech and Eva-Last.

Question 4

What joist spacing do I need for decking?

Accepted Answer

For 32 mm softwood decking boards, maximum joist centres are 400 mm. For 28 mm boards (finished thickness), 350 mm centres are recommended. Composite decking varies by manufacturer — typically 300-400 mm centres. At a 45° diagonal lay pattern, reduce joist centres by 25% (e.g. 300 mm instead of 400 mm).

Nominal Size (mm)	Finished Size (mm)	Lengths (m)	Profile
32 x 100	~28 x 95	2.4, 3.0, 3.6, 4.8	Grooved / smooth reversible
32 x 125	~28 x 120	2.4, 3.0, 3.6, 4.8	Grooved / smooth reversible
32 x 150	~28 x 144	2.4, 3.0, 3.6, 4.8	Grooved / smooth reversible
38 x 150	~34 x 144	3.0, 3.6, 4.8	Heavy-duty, commercial use

Species	Typical Size (mm)	Durability Class	Notes
Yellow Balau	21 x 145	Class 2	Dense, naturally durable, smooth finish
Ipe	21 x 145	Class 1	Extremely hard and durable, pre-drill required
Garapa	21 x 145	Class 2	Light colour, good durability
European Oak	27 x 150	Class 2	UK-sourced, use stainless steel fixings

Type	Typical Size (mm)	Profile	Fixing
Solid composite	20–25 x 140–150	Grooved or smooth	Hidden clips or face screws
Hollow-core composite	22–28 x 140–150	Grooved edges	T-clips in grooves
Capped composite	24 x 140	Polymer-wrapped surface	Hidden clip system

Joist Size (mm)	Max Span (mm)	Notes
47 x 100	~1200	Low-level decks only
47 x 150	~2000	Standard domestic decks
47 x 200	~2800	Larger spans, raised decks

UK Decking Board Sizes & Profiles

Softwood Decking Boards

Hardwood Decking Boards

Composite Decking Profiles

Decking Joist Sizes

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