Deck Board Calculator

Divide the deck width by the board pitch — board width plus gap — and round up to get the rows. Then work out how many board lengths each row takes.

Around 5 to 6 mm for timber, less for composite. Timber moves with moisture, and boards fitted tight when wet will cup and buckle as they dry — or the reverse if fitted dry.

It lets water drain rather than sitting on the joists. Water trapped between boards and joists is what rots a deck from underneath, long before the visible surface fails.

Commonly 400 mm for standard boards and closer for thinner or composite boards, or where boards run diagonally. The board manufacturer's span figure is the one to follow.

Either works structurally, since the joists run the other way. Boards running away from the house shed water more readily; boards running across it are more common visually.

Grooved side down for most profiles, despite the intuition — the grooves are for ventilation and drainage underneath, and a grooved upper surface traps dirt and holds water.

Around 5 to 6 mm for timber fitted at typical moisture content. Boards fitted wet will shrink and open the gap; boards fitted very dry will swell and close it, so the fitting condition matters.

Screws, in stainless or a coating rated for the treatment used in the timber. Nails work loose as boards move seasonally, and modern preservative treatments are corrosive to ordinary steel.

It varies with timber, treatment, drainage and maintenance, and the failure is almost always at the joists where water sits rather than in the visible boards. Joist tape over the top of each joist substantially extends it.

It costs more up front and needs no staining, and it moves more with temperature and gets hot in sun. Over a long life the total costs are closer than the purchase prices suggest.

A slight fall away from the building helps water clear, though a properly gapped deck drains between the boards. What matters more is that water does not sit on the joists.

Yes — end grain absorbs many times more moisture than the face, and cut ends have lost any factory treatment. End sealer on every cut is one of the highest-return steps in building a deck.