Framing

6×2 (47x150mm) is the safer choice for most decks. It can span around 2.4m between supports, while 4×2 (47x100mm) is limited to about 1.5m. If your deck sits close to the ground on plenty of supports, 4×2 will do and keeps the frame low. For a raised deck or longer spans, go to 6×2 or larger. Mick George stocks treated C24 carcassing from 47x100mm up to 47x225mm.

Build a rectangle of outer joists, then fit inner joists between them at regular centres. Start by making the area and setting posts or pads at each corner and along the span. Fix the outer frame to the posts, check it is square and level, then add the inner joists using joist hangers or screws. Add noggins between joists on larger decks to stop them twisting. The boards go on last.

400mm centres for timber decking boards laid straight across the joists. Reduce this to 300mm if the boards run diagonally, as each board then spans further between supports. Measure from the centre of one joist to the centre of the next, not the gap between them. Closer centres give a firmer deck but use more timber.

Usually yes. Composite boards flex more than timber over the same span, so most makers specify 300mm centres for straight boards and 200mm to 250mm for diagonal boards. Check the spacing for the Core composite decking board before you build the frame, as adding joists afterwards is awkward. The frame itself can still be treated timber.

Yes. treated C24 carcassing is the standard timber for decking joists and is what Mick George supplied for deck framing. C24 is a strength grade, so it spans further than C16 at the same size. The treatment protects it against rot and insects outdoors. Seal any cut ends with an en-grain preservative, as cutting exposes untreated timber.

For a raised deck, yes. Posts that carry the frame off the ground should be set in Postcrete so they cannot shift or lean. A low deck sitting on firm, level ground can rest on concrete pads or paving slabs instead, with the posts fixed to them. Never set the frame directly on soil, as the timber will stay damp and rot.

When the deck is fixed to the house or another building. A ledger board is a joist bolted to the wall that carries one side of the frame, so you need fewer posts. It must be bolted to solid masonry, not cladding or render, and sit at least 150mm below the damps proof course. Leave a small gap behind it with spacers so water can drain away from the wall. A freestanding deck does not need one.

Leave a gap of at least 150mm between the underside of the joists and the ground. This lets air move through and dries the timber after rain, which is what stops rot. On a low deck, clear the ground, lay a weed membrane and cover it with gravel so the space stays dry. Avoid boxing the sides in completely, as that traps damp air under the frame.