Offcut WorksDecking subframe calculator
Count the joists, rim joists, noggins, bearers and posts for the frame under your deck. It counts timber but does not size it.
What the frame is made of
Under the boards, a deck frame is a set of parts that pass the load down to the ground:
- Joists carry the boards. They run in one direction and are spaced at the centres your boards require.
- Rim joists close the frame across the ends of the joists and keep them from twisting.
- Noggins are short blocks between joists that stiffen the frame. They can also support board ends in some layouts.
- Bearers carry the joists. They run across the joists at a spacing that depends on the joist size and timber grade.
- Posts or pads carry the bearers down to the ground at a spacing that depends on the bearer size.
How the quantities are worked out
Joists = (distance they are spaced across ÷ joist spacing), rounded up, plus one. Each joist is as long as the span it crosses. If that is longer than the timber you buy, each joist needs more than one piece, and that is flagged.
Bearers = (joist span ÷ distance between bearers), rounded up, plus one line of bearers. Posts = the number of bearer lines × (bearer length ÷ post spacing, rounded up, plus one).
Noggins = rows × (joists − 1). The length of each is the joist spacing less the joist thickness, so for 400 mm centres and 47 mm timber that is about 353 mm.
What this tool does not do
It counts timber. It does not size it. How far a joist or bearer can span depends on its depth, the timber grade, the loads and the deck boards, and published span tables exist for that purpose. Take the bearer and post spacings from the span tables for your timber, or ask your supplier or an engineer. A wrong span is a safety problem, not just an ordering mistake.
Other things to plan for
- Joist spacing. Match it to your boards. One composite joist maker recommends 300 mm centres, and 250 mm for diagonal boards or above ground level.
- Joints under boards. Where boards join end to end, the joist under the joint is usually doubled. See the decking calculator.
- Timber treatment. Parts close to the ground need timber treated for that use. Ask your supplier which treatment suits each part.
- Ventilation and drainage. Keeping air flowing under the frame helps it dry out. Check how your supplier recommends preparing the ground.
- Height. Decks more than 30 cm above the ground are outside permitted development in England and may need building regulations approval. Use the planning permission checker.
Questions
How many joists do I need for a deck?
Divide the distance the joists are spaced across by the joist spacing, round up, and add one. For a 3.6 m length at 400 mm centres that is 9 + 1 = 10 joists.
How far apart should bearers and posts be?
It depends on the timber size and grade, so use span tables for your timber or ask an engineer. The calculator leaves these blank because we could not find a published UK figure that fits every deck.
Do I need noggins?
Not always. They stiffen the frame and can support boards at joints or in diagonal layouts. Follow your board maker's guide.
Does this work for composite joists?
Yes for quantities. Enter the composite joist spacing and length from the maker's guide.
Where the defaults come from
- Composite decking joist listing (PBSL Group). Joist centres no greater than 300 mm, 250 mm for diagonal boards or above ground, and double joists under butt joints with an 8 mm end gap.
- eDecks composite decking installation guide (PDF). Joists at 300 mm centres, hidden clips, and a 5 mm gap against solid structures.
- BM Steel, how to fit composite decking. Joists at 400 mm centres and one T-clip per joist.
- BSW Timber, timber decking installation guidance (PDF). Joists at 400 mm centres, noggins staggered at about 1.2 m centres, a 6 to 9 mm gap between boards, two screws per joist, and board joints over a joist.
- Planning Portal: decking. Permitted development rules for decking in England.
- Bearer and post spacings have no default. We searched for UK published values and found none that apply to every deck. The Timber Development UK C16 tables cover joists in dry domestic floors rather than exposed decks, and the figures we found for bearers were Australian (a different standard and different timber) or from commercial blogs. The TDCA's Timber Decking Professionals' manual has deck span tables, but it is a paid publication we could not read.
- The 47 mm joist thickness for noggin length is a common timber size. Change it if yours differs.