How far apart should decking joists be?

How far apart should decking joists be?

Decking joists are spaced by their centres: measure from the centreline of one joist to the centreline of the next. You may see 400 mm centres suggested for domestic decking, but that is a layout example, not a universal UK rule. The permitted distance depends on the board, its thickness, the direction it is laid and the manufacturer's installation instructions. Treat the smallest applicable limit in those instructions as your maximum.

Board support and joist strength are separate checks. A board specification tells you how frequently the boards need support; it does not establish how far a joist can span between its own supports. Decide which boards, joists and supports you are using before marking out the frame. If the instructions do not cover your proposed arrangement, obtain a suitable specification rather than assuming a familiar spacing will work.

• Record the exact decking board product and thickness.
• Find its maximum permitted joist centres in millimetres.
• Check separately how the joists will be supported.

Step 1: Read the board installation instructions

Use the current installation instructions for your exact timber or composite board, including any requirements for the intended use. Check whether the stated spacing changes when boards run diagonally across the joists, meet at a joint or form a picture-frame border. Also look for requirements at board ends and for fixing distances from edges. Do not transfer a spacing from another product simply because the boards appear to be the same size.

Sketch the direction of the boards and draw the joists beneath them before ordering materials. Mark every place where a board ends, changes direction or meets a feature such as a hatch or step. Those locations may need additional framing even when the regular joist spacing is within the board limit. Keep the instructions with your sketch so you can check the layout again when the materials arrive.

Step 2: Check the joist spans and intended loads

Measure how far each joist will run between its supports, such as bearers or other appropriately designed supports. Record the timber size and grade, or the specified framing system, and check the proposed span against information suitable for that system and its intended loads. Changing joist centres does not automatically make an unsupported joist span acceptable. If you cannot establish an appropriate span, seek structural advice before building.

Identify concentrated loads before setting out the frame. A hot tub, heavy planter or outdoor kitchen can demand a different support arrangement from an ordinary deck used for walking and seating. Raised decks, connections to a building and ground conditions also affect the design beyond board spacing. The UK government's Approved Document A provides structural guidance for England, but it is not a decking-board spacing table; requirements and guidance differ across the UK.

Step 3: Measure the run and calculate the bays

Measure the distance along the direction in which the joist positions will be repeated, perpendicular to the joists themselves. Establish where the centreline of the first and last joists must fall after allowing for the board edges and the manufacturer's overhang limits. Use the distance between those two centrelines for the calculation, not the deck's overall outside width. Mark both end positions on the framing plan.

Divide that centre-to-centre distance by the maximum permitted spacing, then round the number of bays up to a whole number. Divide the distance by that whole-number bay count to find an even setting-out distance; the number of joists is one more than the number of bays. Mark the positions in millimetres and measure each gap from centreline to centreline. Rounding a mark to a convenient millimetre is acceptable only if the resulting gaps stay within the specified maximum.

• Measure between the first and last planned joist centres.
• Round the calculated bay count up, never down.
• Count one more joist than the number of bays.

Step 4: Add support at edges and board joints

A regular grid is only the starting point. Check where each board end will land and provide the support required by its installation instructions. If two board ends meet, the fixing requirements may call for separate support for each end rather than forcing both sets of fixings into one narrow joist. Plan these extra members before cutting boards so joints do not finish over an unsupported gap.

Look again at steps, corners, borders and openings. A short border board or a board laid in a different direction may need support that the main joist layout does not provide. Check the specified edge overhang and fixing positions against the actual board widths and the framing you have marked. Extra framing at a detail does not permit wider centres across the rest of the deck.

Step 5: Check the frame before fixing boards

Set out the joist centrelines on the installed supports and measure every bay with a tape measure. Compare the largest measured gap with the board manufacturer's limit, including any tighter limit for diagonal boards or other details. Check that joists remain in their intended positions and that board ends and borders have their planned support. Correct the framing now, while the centrelines and fixing locations are visible.

Confirm that the specified fixings and connectors suit the timber or framing system and the outdoor exposure. Follow the board instructions for gaps, fastener placement and any required clearance or ventilation. Keep a dated copy or photograph of the board specification and your measured layout with household project records. That record makes later repairs easier, particularly if a board is replaced with a different product.

Final verification checklist

Before laying boards, identify the exact product and read its current installation instructions once more. Check the maximum centres for the actual board direction, then measure the widest installed centre-to-centre gap. Confirm that joist spans and supports have been checked independently of the board spacing. Make sure every board end, border and change of direction has the support its detail requires.

Finally, check the wider project requirements for the deck's location and height. Planning and building-control considerations are separate from the joist-spacing calculation, and the applicable rules depend on which UK nation the property is in. For a property in England, the UK government's householder permitted-development guidance is a starting point for the planning check; contact the relevant local authority where the position is unclear. Record the finished measurements before the framing is covered.

• Exact board instructions checked against the installed layout.
• Every joist bay measured and within the applicable limit.
• Joist spans, supports and any heavy loads checked separately.
• Board ends, borders and openings supported as specified.
• Planning and building-control position checked where relevant.

Scope and limitations

This note helps householders organise measurements and understand the calculation. It does not replace an on-site decision by a designer, installer, contractor, or manufacturer. Fabrication, structure, electrical work, gas, waterproofing, and personal safety require suitably qualified verification.