How to Build Decking: Ground Prep, Frame, Joist Spacing and Boards

Building decking that lasts comes down to what's underneath: prepare and level the ground, set the frame on solid footings clear of the soil, space the joists to suit the boards, and leave room for air and water to move. The boards are the easy part. Most decks that fail early fail because of the frame.

This is the sequence our team follows on a typical garden deck, with the dimensions that matter.

Step 1: Plan the size, height and fall

Mark out the deck and decide the finished height. If the deck is next to the house, the finished surface should sit below the damp proof course, with a gap to the wall. Keep every part of the deck no more than 30cm above the ground if you want to stay within permitted development.

Build in a very slight fall (around 1:100) away from the house, along the length of the boards, so rain runs off.

Step 2: Prepare the ground

Strip turf and topsoil from the deck area. Lay a weed-suppressing membrane and cover it with a layer of gravel. This stops weeds growing up through the boards and lets water drain away instead of sitting under the deck.

Step 3: Set the footings

The frame needs to stand on something solid that won't move. The usual options are:

• Concrete footings: posts set in concrete, or the frame sitting on concrete pads

• Ground screws: steel screws driven into the ground, quick and good on awkward sites

• Adjustable pedestals: on an existing hard surface such as an old patio

Timber must not sit directly on soil. Even pressure-treated timber rots when it's permanently damp.

Step 4: Build the frame

Use pressure-treated timber rated for ground contact where it's close to the ground. For a typical low deck, joists of around 47mm by 100mm to 47mm by 150mm are common, with deeper joists needed for longer spans; a raised deck needs a properly designed frame with posts and bearers.

Build the outer frame, then fit the joists inside it with joist hangers or screws designed for structural timber. Treat every cut end with an end-grain preserver, because cutting exposes untreated timber.

Decking joist spacing

Joist spacing depends on the board, and getting it wrong is the most common reason boards bounce, crack or cup.

• Softwood boards (around 28-32mm thick): joists at up to 400mm centres is the common standard.

• Composite boards: most manufacturers specify closer spacing, commonly 300-400mm centres, and closer again for boards laid diagonally. Always follow the board manufacturer's figure; a composite warranty often depends on it.

• Diagonal or herringbone patterns: reduce the spacing, because each board spans further across the joists.

Add noggins (short cross pieces) between joists on longer runs to stop them twisting.

Step 5: Lay the boards

• Start from the house side or the most visible edge, with a straight board.

• Leave a gap between boards, typically 5-6mm for timber, or whatever the composite manufacturer specifies (composite also needs expansion gaps at the ends).

• Stagger the joints so board ends don't line up in a row, and make sure every board end lands on a joist.

• Fix timber boards with two stainless or coated deck screws at each joist, pre-drilling near the ends to avoid splitting.

• Fix composite boards with the manufacturer's hidden clip system.

Step 6: Finish the edges

Fit fascia boards to hide the frame, a picture-frame border to cover cut ends, and steps where needed. On a deck high enough for a fall to hurt, fit a proper balustrade with posts bolted into the frame.

Building a raised deck

A raised deck is a different job. The frame sits on posts and bearers rather than on the ground, the posts need proper footings, and the whole structure needs diagonal bracing so it can't rack sideways. Joists usually need to be deeper to span between bearers. Stairs need consistent risers and treads, and the edge needs a balustrade strong enough to lean on.

Remember the planning position too. A deck with any part more than 30cm above the ground isn't permitted development, so a raised deck normally needs planning permission. That's worth settling before you buy materials.

How long does it take to build a deck?

A small, low deck on level ground with good access is often two or three days' work for a small team. A larger deck, a raised deck, or one with steps, balustrades, lighting and built-in seating can take a week or more. Ground screws and pedestals are quicker than concrete footings, which need time to set.

What tools you need

• Spade, mattock and wheelbarrow for ground prep

• Spirit level (a long one), string lines and pegs

• Mitre or circular saw, drill driver and impact driver

• Post-hole tools or a ground-screw driver if you're using screws

• Clamps, a square and spacers for consistent board gaps

Mistakes we fix most often

• Frame sitting on the soil, rotting within a few years

• Joists too far apart, especially under composite boards

• No gaps between boards, so water and leaves sit on the deck

• Untreated cut ends

• No ventilation under a low deck

• Deck built up against the house above the damp proof course

Timber or composite boards?

The frame is built the same way for both, but composite usually needs closer joists and a specific fixing system. Composite costs more up front and needs much less maintenance; timber is cheaper and repairable. Our comparison of composite decking vs wood covers the trade-offs.

When to bring in a contractor

A small, low deck on flat ground is a reasonable DIY project. A raised deck, a deck on a slope, anything with steps and balustrades, or a deck that has to meet the house and a patio at exact levels is where experience pays for itself. We build decking in Kettering and across Northamptonshire, and often combine a deck with paving from our patios team. Get in touch for an itemised quote.