Last updated: January 2025

Deck Joist Calculator: Spacing, Sizing, and Installation

The subframe is the invisible skeleton of your deck — get it right and your boards will sit flat, drain properly, and last for decades. Get it wrong and you will have bouncy boards, sagging spans, and premature failure. This guide covers everything you need to know about deck joist calculations: how many joists you need, what spacing to use, how to size your bearers and posts, and how to plan the subframe for different deck shapes and board layouts.

Joist Spacing Explained

Joist spacing — measured centre to centre — determines how well your deck boards are supported. The three standard spacings used in UK domestic decking are 400mm, 450mm, and 500mm. The correct spacing depends on your board material and thickness, the layout pattern, and the expected load on the deck.

For timber decking boards (typically 28–32mm thick), 450mm joist spacing is standard. This provides adequate support without excessive material cost. Composite boards are generally less rigid than timber at the same thickness, so manufacturers typically recommend 400mm joist centres. For herringbone or diagonal layouts where boards cross joists at angles, use the tighter spacing to compensate for the increased effective span.

500mm spacing is the absolute maximum for domestic decking and should only be used with thick (38mm+) timber boards on low-traffic decks. At this spacing, some boards may develop a slight bounce underfoot, particularly if they are softwood. If in doubt, go tighter — extra joists are cheap insurance against a bouncy deck.

Calculating Joist Quantities

The formula is straightforward: divide the deck length (the direction the joists run) by the joist spacing, then add one for the end joist. For a 4-metre deck with 450mm spacing: 4,000 ÷ 450 = 8.9, rounded up to 9, plus 1 = 10 joists. Each joist needs to span the full deck width, so if your deck is 3 metres wide, you need 10 joists at 3 metres each — 30 linear metres of joist timber.

Our decking calculator works this out automatically in Joists mode or Full Build mode. It also calculates the number of bearers and posts needed to support the joists, giving you a complete subframe shopping list.

Joist Sizing

Standard domestic deck joists in the UK are 47mm × 150mm treated softwood (C16 or C24 grading). This size comfortably spans up to 2.4 metres between bearers for normal domestic loading. For longer spans, wider or deeper joists are needed — 47mm × 200mm can span up to 3 metres. If your deck width exceeds 2.4 metres, you will need intermediate bearers to keep joist spans within limits.

Always use pressure-treated timber rated for ground contact (Use Class 4) for joists, as they are close to the ground and exposed to moisture. Untreated timber will rot within a few years in UK conditions, regardless of how well the deck surface is ventilated.

Bearer and Post Calculations

Bearers sit perpendicular to the joists, typically on top of posts or concrete pad foundations. A deck up to 1.8 metres wide needs only two bearers (one at each end). Wider decks need intermediate bearers spaced at no more than 1.8-metre intervals to prevent joist sagging.

Posts support the bearers and transfer the load to the ground. Space posts at no more than 1.5-metre intervals along each bearer for adequate support. Each post needs a stable foundation — typically a concrete pad or post spike driven into firm ground. For raised decks over 600mm high, posts should be bolted to concrete pad foundations and may require cross-bracing for lateral stability.

Special Considerations for Different Layouts

If your decking boards run at 45 degrees (diagonal layout), the effective span between joists increases by approximately 40%. This means you should reduce joist spacing accordingly — use 350mm centres for composite on a diagonal, or 400mm for timber. See our herringbone guide for the specific joist requirements of that pattern.

For L-shaped decks, plan the joist layout for each rectangular section separately. Where the two sections meet, you will need doubled joists or a wider bearer to handle the transition. Our L-shaped deck example shows how to handle this in practice.

Common Joist Mistakes

• Spacing joists too far apart, leading to board bounce and eventual cracking.
• Using untreated or interior-grade timber that rots within years.
• Failing to crown joists (install with the natural bow facing upward) so they deflect flat under load.
• Not checking for level — even small variations in joist height create a wavy, uneven deck surface.
• Forgetting to add noggins (blocking) between joists at board end joints.

Frequently Asked Questions