What spacing should I use for decking joists?

For standard 25mm thick decking boards, joists should be spaced at 400mm centres (the distance between the centre of each joist). This prevents boards from sagging and provides adequate support. Wider spacing of 450mm may be used for lighter-use decks, but never exceed 450mm for 25mm boards.

How many decking screws do I need per square metre?

You need approximately 15 decking screws per square metre for standard 120mm wide boards. This allows for 2 screws per board at each joist crossing. Use 65mm stainless steel or galvanized screws for best durability outdoors.

What size joists do I need for decking?

Joist size depends on span (distance between supports): 47x100mm joists can span up to 2m, 47x125mm up to 2.5m, and 47x150mm up to 3m. Always use C24 grade treated timber for outdoor use. Add intermediate posts or bearers for longer spans.

Should decking boards run parallel or perpendicular to the house?

Boards can run either direction, but there are considerations: running boards away from the house (perpendicular) helps with drainage and creates a visually larger space. Running parallel to the house may require fewer cuts if board length matches deck width. Joists always run perpendicular to the boards.

Do I need to leave gaps between decking boards?

Yes, leave 5-8mm gaps between decking boards to allow for drainage and natural expansion of the timber. Use spacers or screws as guides when laying boards. Boards will swell when wet and shrink when dry, so the gap helps prevent buckling.

How do I calculate how many decking boards I need?

Divide the width to be covered by the board width plus gap (125mm for 120mm boards + 5mm gap). Multiply by deck length to get total linear metres needed. Then divide by board length and add 10% for waste. Our calculator handles this automatically.

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