Garden decking installation needs a stable base, well-spaced joists, drainage, and careful board fixing with gaps for airflow.
Adding a timber or composite platform can turn unused ground into a clean, level zone for seating, pots, or a grill. This guide keeps things simple, from planning to the final screw. You’ll see what to buy, how to prep the base, and the exact order that avoids squeaks, puddles, and warped boards.
Plan First: Rules, Layout, And Materials
Start with the boring bit that saves headaches. Check local rules on height and plot coverage. In many UK cases, low platforms at or near ground level count as permitted development; once you go higher than about 30 cm or cover over half the garden with outbuildings and platforms, consent may be needed. Read the Planning Portal rules and call the council if you’re unsure. If your plot sits in a protected area or the home is listed, speak to the council early.
Map the shape with string lines and pegs. Note doors, air bricks, inspection covers, trees, and drains. Pick a board type that suits your time and budget. Softwood is cheaper and easy to cut. Hardwood resists dents. Composites need low care and steady gaps. Match fixings to the board type the maker states.
| Item | Why It Matters | Notes |
|---|---|---|
| Sub-base (hardcore or gravel) | Spreads load and sheds water | 40–50 mm layer over membrane |
| Weed membrane | Stops growth under the frame | Overlap edges by 100–300 mm |
| Joists (treated timber) | Carry the frame | Common sizes: 150×50 mm |
| Posts/sleepers or pads | Bear the joists | Use UC4 ground-rated where in soil |
| Deck boards | Walking surface | Softwood, hardwood, or composite |
| Fixings | Holds boards down | Exterior screws or hidden clips |
| Edge trim | Covers cut ends | Neat finish |
| End grain sealer | Protects cut wood ends | Use on treated timber cuts |
| Bitumen tape | Shields joist tops | Adds life, reduces squeaks |
| Spacers | Controls gaps | 5–8 mm between boards |
Two quick checks help right here. First, the base needs fall so rain drains away from the house. Second, the top of the platform should finish a shade below the door threshold so water can’t creep inside.
Ground Prep: Clear, Level, And Drain
Strip turf and roots. Rake off sharp debris. Aim for a flat, firm bed that is 100–150 mm below your planned finished height. Compact the soil. Roll out a weed barrier with overlaps. Add a layer of compacted gravel to help drainage and keep the frame off wet mud.
Set paving slabs or plastic pads on the gravel where posts or bearers will sit. Space them on a grid so loads spread evenly. On soft spots, dig a bit deeper and compact more sub-base until the slab stops rocking.
Framing: Joists, Spacing, And Airflow
Build a rectangle from treated timbers and square it: measure diagonals until they match. Fix inner noggins to stop twist. Joist spacing depends on board type and pattern. Softwood boards often sit on centres around 400–600 mm. Composites are tighter, often near 300–400 mm, and even closer for angled layouts. When in doubt, follow the maker’s span chart.
Keep at least a small void under the surface so air can move. That gap helps dry the frame after rain. Where joists meet the house, leave a slim gap against the wall and keep air bricks clear.
Where any timber touches soil or sits in damp ground, choose UC4-rated sections. For cut ends, brush on end grain sealer before you fix them. Tape along joist tops keeps water off screw lines.
Fixing The Frame To The Ground
On a free-standing pad, set posts or bearers onto slabs. Check levels with a long straightedge and a spirit level. Shim with plastic packers as needed. If tying into a wall plate, use masonry anchors and spacers so the plate sits off the brick by about 10 mm. That air gap keeps water from lingering.
Double up joists under board ends if the pattern needs it. Add noggins at mid-span where bounce feels too springy. The frame should feel solid underfoot before any boards go down.
Board Layout: Direction, Gaps, And Fixings
Run boards along the direction that suits sight lines and runoff. Many folks run them away from the house to draw the eye outward. Stagger joints so end seams don’t line up in a single row.
Leave a small gap between board edges. A 5–8 mm space works for many timbers in the UK climate. Leave a little gap where a board meets a post or wall. Composites need the gap the maker states as they move with heat. Use spacers during fixing so the lines stay even. For gap sizes and edge allowances, the TDCA guidance gives clear numbers that keep water moving and boards straight.
Pre-drill near cut ends to avoid splits on timber. Use exterior-grade screws or the maker’s clip system for composites. Drive heads flush, not proud, and not below the surface. Keep fixings in straight lines. Two screws per bearing line on timber boards looks neat and holds well.
Stairs, Edges, And Drainage Details
For steps, cut stringers from treated timber or use metal brackets. Treads need solid landing points and even rises. Edge trim or a picture-frame border hides cut ends and helps keep the layout crisp. Add small drip gaps at edges so water can fall clear of the frame.
Care And Longevity
Sweep off leaves. Keep gaps clear so water moves off the surface. Wash mud and algae with deck cleaner or soapy water. Sand splinters on softwood when needed and refresh finish coats on a calm, dry day. Replace damaged screws so boards don’t move.
Installing Deck Boards In A Garden: Step-By-Step
Here’s a simple order you can follow from first mark to last screw.
1) Measure And Mark
Mark the outline with string lines. Check square off the house wall with the 3-4-5 triangle trick or a laser. Decide on board direction and stair placement now.
2) Clear And Set The Base
Remove turf. Level the ground. Lay membrane with overlaps. Add and compact gravel. Set slabs or pads on a grid that matches your frame plan.
3) Build The Frame
Cut and lay the outer rectangle. Square it. Fix joists at the centres your board type needs. Add noggins. Tape joist tops. Brush end grain sealer on every fresh cut.
4) Set Levels
Prop the frame on pads and check levels across the whole area. Allow a small fall away from the house. Lock heights with fixings or plastic shims.
5) Lay The Boards
Dry-lay the first run to set your sight line. Use spacers to hold side gaps. Fix each bearing line. Keep cut ends neat and sealed. Stagger joints. Check every few rows that gaps and overhangs match.
6) Finish Edges And Steps
Add trims and the stair set. Cap exposed ends. Fit a handrail where drops suggest it.
7) Final Checks
Walk the surface and listen for squeaks. Back out and refit any screw that spins. Rinse off dust and you’re done.
Cost, Quantities, And Time
Budgets vary with size and board type. Softwood costs less up front but needs finish coats. Composite costs more but cuts time on upkeep. The grid below helps you plan orders for a simple rectangle. Adjust for angles or curves.
| Deck Size | Board Count* | Build Time** |
|---|---|---|
| 3 m × 3 m | 40–50 (120 mm boards) | 1–2 days, two people |
| 4 m × 3 m | 55–65 | 2–3 days, two people |
| 6 m × 4 m | 95–110 | 3–4 days, three people |
*Varies with width, pattern, and offcuts. **Build time assumes a clear site, simple shape, and easy access.
Code Notes And Good Practice
Low platforms near ground level often sit within basic home rules. Taller builds and wide spans need checks on loads and railings. The Planning Portal rules set the common 30 cm height threshold and the 50% garden coverage limit for permitted development.
Where a board meets a wall, keep a 10 mm gap. Between boards, keep 5–8 mm. For composite spans and patterns, stick to the maker’s chart and keep joist centres tight for diagonals. Timbers that sit in or on soil need a UC4 rating. Joists and beams that stay off the ground usually sit in UC3. Fresh cuts on treated wood need end grain sealer. These small steps add years to the frame.
Common Mistakes To Avoid
- Laying over grass without clearing and leveling first.
- Skipping a membrane and sub-base in damp soil.
- Joist centres that are too wide for the board type.
- No gaps between boards or against a wall.
- Blocking air bricks or sealing vents.
- Fixing screws too close to cut ends.
- Forgetting drip gaps at the outer edge.
Useful References
Before you start, read the UK Planning Portal page on garden platforms, and the TDCA guidance on board spacing and clearances. For composite spans and patterns, check a current maker guide.