Decking Substructure & Joists

Decking substructure and joists

A deck is only as good as the frame beneath it. The substructure carries the load, keeps the boards flat and level, and lifts the deck off the ground so air can circulate. Using durable or properly treated joists is important, because the frame is hidden and hard to replace later.

What the frame includes

• Joists running at right angles to the boards.
• Bearers or beams supporting the joists on posts, pads or a prepared base.
• Noggins or blocking to stiffen the frame.

Sizes and spacing

For 28mm boards, joists are normally set at 400mm centres, and for 32mm or thicker boards at up to 500 or 600mm. Typical joist sections are 47 by 100mm or 47 by 150mm, with the size depending on the span. Support the frame on a firm, level base, over a weed control membrane, with a slight fall of around 1 in 100 away from the house for drainage. Always double joists where boards butt together, and where the deck changes direction.

Treatment and ventilation

Use timber treated for ground contact or use durable timber for the lowest courses. Cut ends should be re-treated. Leave an airflow gap around the perimeter, and avoid enclosing the frame, as trapped moisture shortens life. A protective joist tape on top is an added measure.

Ordering

Plan the joist layout, total the lengths, and add 10% for waste. Fix with suitable stainless or coated screws from screws, nails and accessories. Pair the frame with our decking boards, decking kits or anti-slip decking. For advice, use the quote form.