Use pressure-treated (tanalised) softwood graded C16 or C24 for all structural members. Common joist sections are 47×100mm for ground-level decks with closely-spaced support, and 47×150mm or larger where joists span further. Posts for raised decks are typically 100×100mm. The larger the unsupported span, the deeper the joist needs to be — span tables from the timber supplier or a structural source should govern the section you choose, especially on raised structures.

Joist hangers and framing anchors speed up and strengthen the internal joist connections; fix them with the correct sherardised or galvanised hanger nails or structural screws, filling every hole. Coach bolts or structural coach screws fix the ledger to the wall and posts to beams. Use hot-dip galvanised or stainless structural fixings throughout — ordinary bright screws corrode outdoors and stain the timber. Don't substitute a few long woodscrews for a rated hanger; the hanger spreads load the way the joint needs.

For footings you'll need post-mix or ballast and cement, or pre-cast concrete pads. A weed-control membrane under the deck suppresses growth and a layer of gravel or sub-base aids drainage. A strip of damp-proof course (DPC) goes behind a wall-fixed ledger to stop a damp bridge. Joist protection tape applied along the top edge of each joist keeps water out of the fixing holes and noticeably extends timber life. Keep end-grain preserver to hand to re-treat every cut you make in pressure-treated timber.

Mark the deck footprint with pegs and string line, checking the rectangle is square by measuring the diagonals — equal diagonals mean square corners. Clear turf and vegetation across the area, dig out any soft spots, and roughly level the ground. Lay a weed-control membrane over the soil and add a layer of gravel or sub-base to help water drain away rather than sitting under the timber. Compact any made-up ground so it can't settle once loaded.

The support method depends on the deck height. A low ground-level deck can sit on concrete pads or paving slabs bedded level at regular intervals, supporting the frame just clear of the ground. A raised deck needs posts set in concrete footings: dig holes (commonly around 450mm deep in firm ground, deeper in soft or made ground), set the posts plumb, brace them temporarily, and concrete them in — checking each stays vertical as the concrete sets. Let footings cure before loading them.

Set out footing and pad positions to suit your beam and joist layout, not the other way round, so every joist and beam lands on solid support. On raised decks, position posts so beams sit directly over them.

If the deck abuts the house, a ledger board bolted to the wall carries one edge of the frame. Set its height so the finished deck surface sits at least 150mm below the building's damp-proof course, so the deck can never bridge it. Fit a DPC strip behind the ledger to keep moisture off the wall, then bolt the ledger securely into the masonry with appropriately spaced, corrosion-protected fixings — into solid brick or blockwork, not just the render. Check it's dead level along its length before final tightening, as everything references off it.

Construct the outer frame (the rim/band joists) that defines the deck's edges, fixing the corners with structural screws and reinforcing them with framing anchors where needed. On a raised deck the frame sits on the beams over the posts; on a ground-level deck it rests on the pads. Check the frame is square (diagonals again), level in both directions — allowing for your intended fall — and properly supported all the way round. Pack or adjust support points until the whole frame is true before you add a single internal joist.

Mark the joist positions along the frame at your chosen centres (400mm for straight softwood, closer for diagonal or composite — see section 3). Fix a joist hanger at each mark, then drop each joist in, checking it sits flush with the top of the frame so the board surface stays flat. Nail or screw every hole in each hanger with the correct fixings — half-filled hangers are a common and serious weakness. Keep the tops of all joists in the same plane; a high or low joist will show as a ridge or dip in the finished deck.

Noggins (short offcuts fixed between the joists) stop long joists twisting and bouncing and tie the frame into one rigid unit. Fit at least one row across the middle of the joist span, and a second row on wider decks, staggering them slightly so you can screw straight through the joist into each end. On raised decks, add diagonal bracing between the posts as well — this is what stops a tall deck swaying, and it isn't optional on anything significantly off the ground.

Before any boards go down, walk the whole frame. Confirm it's square and that the joist tops sit flat in one plane. Confirm the intended fall is present so water will run off. Check every hanger and structural connection is fully fixed and tight. Re-treat all sawn ends with end-grain preserver, and run joist protection tape along the top of every joist. Only once the frame passes this sign-off is it ready for the boards — fixing them is covered in our decking screw and board-laying guides.

A flat deck holds water; a deck with a slight fall sheds it. A gentle slope of around 1:80 — roughly running away from the house — lets rainwater drain off the surface and out from under the structure rather than pooling on the boards or sitting against the wall. Set the fall when you level the frame, not afterwards. It's small enough to be invisible underfoot but enough to make a real difference to how dry the timber stays.

Timber that can dry out lasts; timber sitting in still, damp air rots. Keep the frame clear of the soil, leave a ventilation gap around and under the deck so air can move, and don't seal the perimeter solid. The weed membrane and free-draining layer you laid earlier help here too, keeping ground moisture from wicking up into the joists. On enclosed or skirted decks, build in vents so the underside can still breathe.

Every board screw makes a hole in the top of a joist — and that hole is where water sits and rot starts. A self-adhesive joist protection tape run along the top edge of each joist seals around the screws and keeps the joist top dry, adding years to the frame for very little cost or effort. Equally important: pressure treatment only protects the timber's surface, so every cut you make exposes untreated wood. Brush end-grain preserver onto every sawn end and notch before it's built in.

A deck that flexes underfoot almost always has joists spaced too widely for the boards, or for a diagonal layout, or for the composite product used. The fix is prevention: set centres to suit the board type and direction before building — 400mm for straight softwood, closer for diagonal and composite — and never stretch the spacing to save a couple of joists. Once the boards are down, correcting it means lifting them and adding joists.

Building straight onto soft, uncleared or unconsolidated ground invites weeds growing up through the deck and supports that settle unevenly, pulling the frame out of level over time. Take the time to clear, level, membrane and properly support the frame, and on raised decks dig footings to a sensible depth in firm ground. The groundwork is invisible once finished, which is exactly why it gets rushed — and exactly why it causes problems.

A dead-flat deck with no airflow and bare sawn ends rots from the inside out long before the boards wear. Build in a fall so water drains, leave the structure ventilated and clear of the soil, tape the joist tops, and re-treat every cut. These cost almost nothing during the build and are the difference between a frame that lasts a few years and one that lasts decades.

Two connection errors do real damage. Joist hangers with only a few of their holes filled carry a fraction of their rated load — every hole must take the correct fixing. And a ledger bolted over the home's DPC, or sealed tight to the wall, bridges damp straight into the building. Fill every hanger, keep the deck surface well below the DPC, fit a DPC strip behind the ledger, and bolt into solid masonry. These details aren't where to save time.