This calculator works out how many joists your deck needs and the total lineal metres of joist timber to buy, based on the deck size and the spacing you are building to. Joists are the parallel timbers that sit on top of the bearers and carry the decking boards, and their spacing matters for both cost and performance. Space them too far apart and the deck feels bouncy and the boards can sag or cup; space them too close and you waste timber and money. In New Zealand, decking joists are commonly set at 400mm or 450mm centres for standard timber decking, while some composite and thinner boards call for closer spacing, so always check the decking manufacturer's span and fixing requirements and the relevant span tables in NZS 3604. You enter the width of the deck measured across the direction the joists run, the spacing you intend to use measured centre to centre, and the length of each joist, which is the deck dimension the joists span. The calculator returns the number of joists required, including the joist at each end, and the total lineal metres of timber so you can price the order. Knowing the joist count early helps you check that your bearer layout and pile spacing will support them, and lets you compare the timber cost of 400mm versus 450mm centres before you buy. It is a planning tool to get your materials list and budget right; the structural design of a deck, especially anything raised or attached to the house, should follow NZS 3604 or an engineer's specification, and a building consent is often required.
Includes a joist at each end. Confirm spacing against your decking and NZS 3604 span tables. An estimate only.
The number of gaps across the deck is the deck width divided by the spacing, rounded down. The number of joists is one more than the number of gaps, because there is a joist at the start and at the end of the run. Total lineal metres is the number of joists times the length of each joist.
A deck 4000mm wide with joists at 450mm centres has 4000 divided by 450, which is 8.9, rounded down to 8 gaps. Adding the end joist gives 9 joists. If each joist is 5 metres long, that is 45 lineal metres of timber. Tightening the spacing to 400mm would need 11 joists and 55 lineal metres.
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