Deck materials calculator

Start with the finished footprint: how long and how wide the deck will be, in metres. Measure the actual area you are decking, not the ground around it. If the deck is an odd shape, break it into rectangles, add them up, and use the total as your length by width.

This calculator assumes your decking boards run across the width and the joists run the other way, underneath them, which is the usual layout. If you plan to run boards the long way instead, just swap your length and width figures in the fields. The maths is the same, the direction of the grain is the only thing that changes.

Two numbers decide how much timber you buy: the gap between boards and the spacing of the joists under them.

• Gap: 5 to 7mm. Around 5mm for a 90mm board, 6 to 7mm for a 140mm board. Timber swells when wet and shrinks as it dries, so a gap lets it move and lets water drain. Butt boards too tight and they cup or lift.
• Joist spacing: 450mm for 19mm boards. That is the NZ standard for the common 90x19 and 140x19. Thicker 32mm boards can span out to 600mm. Tighten to 400mm if you want a firmer deck underfoot or you are laying boards on the diagonal.

Get the joist spacing wrong and the deck feels springy, or worse, the boards fail over time. When in doubt, tighter is safer than wider.

Treated timber is graded by where it is allowed to sit, and mixing it up is a durability failure the Building Code does not forgive. For a deck it comes down to two grades.

• H3.2 is for timber above the ground but exposed to the weather. That is your decking boards, your joists and your bearers. All the framing you can see.
• H4 is for timber in or near the ground. That is your posts and piles, the bits that go into the soil. They cop the moisture and the bugs, so they need the heavier treatment.

The rule of thumb: if it touches the dirt, it is H4. If it lives in the air, H3.2. Use H3.2 where H4 belongs and the posts will rot out years early.

Nobody uses every millimetre of every board. You cut boards to length and the offcuts do not always fit the next run. You reject the odd bowed or split board. You trim the ends flush after laying, which throws away a strip off the whole deck. A 10% margin covers all of that on a normal rectangular deck. Bump it higher if your deck is an odd shape, you are laying a pattern, or your boards are long and the offcuts will be hard to reuse. Running a board short on the last row is a trip back to the yard you do not want.

This calculator sizes your decking, joists and fixings for a straightforward ground-level or low deck. It does not size your piles, bearers or bracing, and it does not replace a plan. In New Zealand a deck is generally exempt from building consent only if you could not fall more than 1.5m from it, even if it collapsed. Once you are higher than that, you need a consent, and often an engineer's design for the structure holding it up. A safety barrier is required for any drop of 1m or more regardless. If your deck is elevated, attached to the house, or carrying anything heavy like a spa, get it checked before you order timber. Our building consent checker is the quick first look.