Building a Timber Deck for a Small Garden

Start with the Ground, Not the Timber

A small garden deck earns its keep when it feels like a room rather than a platform bolted onto the back of the house. That feeling comes from the groundwork, which is the part nobody photographs but everybody notices when it goes wrong. Before you order a single board, spend an afternoon understanding what the ground beneath your proposed deck actually does when it rains.

Decks fail most often because water sits under them. Aim for at least 50mm of clear air between the soil and the underside of your joists, and preferably 100mm. If the finished deck will sit close to the house, keep the top of the boards at least 150mm below the damp proof course, and never let the deck bridge the damp course line. Check the level of any air bricks too — a deck that blocks ventilation under a suspended floor is asking for trouble with timbers inside the house.

Clear the footprint of turf and soft topsoil, then lay a weed-suppressing membrane over the bare ground. On heavy clay, dig out a little deeper and put down 50mm of compacted gravel or coarse bark. This isn't a drainage system so much as a way of stopping splashback, damp and a forest of nettles appearing between your boards by June.

• Set a fall of roughly 1:80 away from the house — about 12mm per metre — so surface water runs off rather than pooling.
• Keep the deck independent of the house wall where you can, leaving a 10–15mm gap behind any fascia or trim.
• Use a string line and spirit level rather than trusting your eye; small gardens show up errors at a glance.

Choosing Boards and Joists for a Modest Space

For most small gardens, 28mm thick boards are perfectly adequate, and they're the easiest to source in consistent quality. If you want a chunkier, more solid feel underfoot — useful if the deck will take heavy planters or a table and chairs that rarely move — step up to 32mm or 38mm. Wider boards cover ground faster and show fewer gaps, but narrow boards (around 100–120mm) tend to drain and dry better and are less likely to cup.

Softwood decking is the traditional choice and takes stain well. Hardwood costs more but holds its shape with less fuss. Composite boards are worth considering if you want to spend your weekends sitting on the deck rather than scrubbing it, though they expand and contract more with temperature, so follow the manufacturer's fixing guidance closely.

For the frame, 100mm x 47mm treated structural timber is the standard workhorse in the UK. Buy timber marked as suitable for ground contact or at least pressure-treated for exterior use — the extra cost is small next to the labour of replacing a rotted frame.

Setting Out the Frame and Joist Spacing

This is where the deck either feels springy or feels solid. Joist spacing depends entirely on board thickness, and getting it wrong is the single most common self-build error.

• 400mm centres for 28mm boards — the safe default.
• 600mm centres for 32mm boards and above.
• Reduce spacing to 300mm at the edges and anywhere a board end lands, so no board is left unsupported.

Run the joists perpendicular to the board direction. If your boards will run parallel to the house, the joists run out from the wall. Build a perimeter frame first, square it using the 3-4-5 method, then fit internal joists at your chosen centres. Add noggins — short blocks between joists — every 1.2–1.5m to stop the frame twisting, and double up joists wherever two board ends will meet.

Support the frame on concrete blocks, paving slabs or short posts set on a firm base, never directly on soft soil. Check the whole frame for level as you go; correcting it later means lifting boards you've already screwed down.

Fixing the Boards and Getting the Gaps Right

Board gaps are not decoration — they're drainage and ventilation. Aim for a consistent 5–8mm between boards, which allows air to move through and lets water drain away freely. In a small garden, use a spacer offcut cut to exactly the width you want and move it along as you work. Random gaps look scruffy and trap debris.

Fix boards with stainless steel or hot-dip galvanised screws — at least 50mm long for 28mm boards. Plain steel screws will stain the timber within a season. Pre-drill near board ends to avoid splitting, and drive two screws per joist crossing, positioned about 20mm in from each board edge.

Stagger the joints between rows so no two neighbouring boards end on the same joist. Leave a small expansion gap of around 10mm where boards meet a wall, a step or an edging trim. Work from a fixed edge and check your rows are parallel every few boards — a 2mm drift per row becomes 30mm of visible taper across a small deck.

Edging, Finishing and the First Winter

Finish the perimeter with a fascia board fixed to the outer frame, which hides the joist ends and gives the deck a neat, deliberate edge. If the deck is more than 300mm above ground level at any point, consider a low step or a chamfered edge board for safety.

Once the boards are down, sweep the surface and apply an exterior wood oil or stain. A breathable oil finish suits most softwood decking better than a film-forming varnish, which traps moisture and peels. Reapply annually in spring.

Finally, keep the gaps clear. A quick sweep and an occasional pass with a stiff brush will do more for the life of your deck than any product on the shelf. Done properly, a modest deck built over a weekend or two gives you somewhere to sit with a mug of tea and watch the garden do its thing — which is, after all, the whole point.