To fit garden decking, set a 1:80 fall, fix treated joists at 400 mm centres, gap boards 5–8 mm, and seal freshly cut ends.
Done well, a deck turns dead ground into a hard-wearing spot for meals, play, or planters. This How To Fit Garden Decking guide shows the method that trades use, with clear sizes, safe gaps, and checks that match UK guidance. You can adapt the layout to softwood, hardwood, or composite boards.
Plan The Area And Check Rules
Start with a scale sketch and a tape. Mark the footprint, add 300 mm clearance for access where a wall or fence sits near the edge, and pick a board direction that suits the view. Set the deck’s top level below the damp-proof course by at least 150 mm.
Most small decks fall under permitted development. In England, the deck must sit no higher than 30 cm and, taken with other garden works, cover no more than half of the garden area. Read the Planning Portal rules for decking before you start, and check local conditions where they apply.
Tools And Materials Checklist
Pick treated structural timber for the frame and corrosion-resistant fixings. Composite boards follow the maker’s kit; timber boards need screws with deep thread and a twin-cut point.
| Item | Purpose | Notes |
|---|---|---|
| String Line & Pegs | Set footprint | Square the corners with 3-4-5 method |
| Laser Or Long Level | Set fall | Target 1:80 (12.5 mm per metre) |
| Ground Spikes Or Posts | Support frame | Concrete foots for soft ground |
| C16/C24 Joists | Main frame | 150 x 50 mm is common |
| Decking Screws | Fix boards | Stainless or coated, 60–75 mm |
| Postcrete Or Ready Mix | Seat posts | Use for corner and beam posts |
| Weed Membrane & Gravel | Drain & tidy | Sharp gravel 30–50 mm deep |
| Preservative End-Seal | Protect cuts | Use on every fresh cut |
| Hidden Clips (Composite) | Board fixings | Follow the system used |
| Joist Hangers/Angles | Join frame | Galvanised or stainless only |
How To Fit Garden Decking: Framing, Fixing, And Gapping
This section covers layout, posts, beams, joists, noggins, and bracing. It works for a freestanding deck and for a low platform beside a house wall. Where you bolt a ledger to brick, use resin anchors and leave a drainage gap behind with nylon spacers.
Set Out And Prepare The Base
Mark the rectangle with a string line. Scrape turf, lay geotextile, and top with gravel so water runs through fast. On clay, raise the whole frame on posts or pedestals to keep air under the joists. Leave air bricks clear.
Install Posts And Beams
Dig post holes to firm ground and cast concrete to just above soil level so timber never sits in wet soil. Use UC4-rated posts for ground contact. Clamp a beam to the posts, set the fall at 1:80, then bolt it off. Repeat on the far side.
Add Joists And Noggins
Fix 150 x 50 mm joists across the span. For softwood boards, 400–600 mm centres give good support; heavier composite boards usually need 300–400 mm centres. Keep joists straight with a string line and fix noggins mid-span to stop twist.
Moisture Protection For The Frame
Use Use Class 3 timber for joists and beams, Use Class 4 for posts, as set out in BS 8417. Brush on end-seal to every cut and notch. A butyl joist tape strips water from the top face and adds grip for the boards. Stainless A2 or A4 screws prevent staining near the coast.
Board Laying: Timber And Composite
Let boards acclimatise in the shade. Start from the straightest edge, usually the house. Run a chalk line for the first course and use 2 mm spacers at butt joints to keep the line true.
Set The Gaps
Timber boards need a 5–8 mm side gap and an 8 mm end gap to shed water and allow seasonal movement. Composite systems set their own clip gap; follow the maker’s sheet for both side and end spacing. Leave a 10–12 mm gap to walls and posts for airflow.
Fix Boards Cleanly
Drive two screws per bearing line, 20–25 mm in from each edge. Keep the drill square so the head sits flush, not buried. Stagger joints so they land on different joists and avoid thin slivers at the border by ripping the last course to an even width. Seal cut ends on timber straight away.
Edge Details And Steps
Finish edges with a picture frame border on its own blocking so screw lines stay neat. For steps, use 47 x 225 mm stringers or prefabricated steel units; cap risers with a scrap of board to protect the grain.
Fitting Garden Decking Step By Step (Close Variant)
Here is the order that keeps things smooth from start to sweep-up. It aligns with trade checklists and avoids rework.
- Confirm levels and the fall with a laser. Mark the top line on posts.
- Cast posts, bolt beams, and brace corners.
- Set joists at 400 mm centres for softwood, tighter for composites.
- Add noggins and perimeter blocking for borders and stairs.
- Roll out membrane, then gravel so puddles cannot form.
- Chalk the first line, fix the starter course, and sight every five rows.
- Gap boards 5–8 mm on timber; fit clips where a system requires.
- Rip the last course to a clean width; ease sharp edges with a block plane.
- Seal timber cuts; wipe composite swarf before it marks.
- Sweep, rinse, and test drainage with a hose.
Sizes, Spacing, And Fall That Work
Reliable sizes mean fewer squeaks and a deck that drains fast. The figures below are drawn from UK trade guides and maker sheets, and they suit most home decks. Always check the sheet that ships with your boards.
| Item | Typical Figure | Where It Applies |
|---|---|---|
| Fall | 1:80 | Whole deck frame |
| Joist Centres (Softwood) | 400–600 mm | 28–32 mm boards |
| Joist Centres (Composite) | 300–400 mm | Clip systems |
| Side Gap, Timber | 5–8 mm | Between boards |
| End Gap, Timber | 8 mm | Board ends |
| Perimeter Gap | 10–12 mm | To walls and posts |
| Damp-Proof Course | 150 mm below | Deck surface level |
Care And Longevity
Good drainage and clean gaps keep any deck in shape. Sweep leaves before they clog, rinse in spring, and scrub algae with a mild deck wash. Spot sand raised fibres on softwood and re-coat with a water-based stain when colour fades. Many composites need only a soft-bristle brush and soap.
Timber Treatment And Ratings
In the UK, joists and beams for outdoor use sit in Use Class 3; posts in soil are Use Class 4. These classes tie to the level of treatment called out in BS 8417. Your yard should mark the class on the tag. For wider context and best practice on sizing and build-up, see the Timber Decking Handbook.
Board Movement And Fasteners
Wood swells across its width and shrinks as it dries. That is why a fixed 5–8 mm side gap matters. Composites move length-wise more, so follow the maker on end spacing. Use quality screws with a corrosion rating that suits your site. Near salt air, pick A4 stainless.
Common Pitfalls And Easy Fixes
Most snags come from poor drainage, wrong gaps, or over-wide joist centres. The cures are simple and can be done on a weekend.
Water Pools On The Surface
Check the fall with a level. If the deck is dead flat, add tapered shims to the frame to regain a small slope. Clear the gaps with a plastic scraper.
Squeaks Or A Bouncy Spot
Add a joist between the two that carry the bounce, screw noggins tight, and refix boards with fresh screws into new timber.
Black Stains Around Screws
This is steel reacting with tannins. Swap to stainless screws and sand the stain back, then oil the patch to blend it in.
Project Timings And Budget Tips
A small 3 x 3 m ground-level deck takes a weekend. Pre-cut joists to speed things up, lay out all fixings in trays, and keep blades sharp. Buy an extra two boards to cover waste on rips and defects. Off-cuts make perfect blocking for borders and steps.
Safety And Compliance Checks
Decks above ground may need guarding; local rules set heights and loads. Keep steps even and add anti-slip strips on treads. Where a deck sits near a door, fit a low threshold ramp to prevent trips. Check the Planning Portal link earlier for height and area limits on raised platforms.
Where The Specs Come From
The Planning Portal sets the height and area limits for most homes. The Timber Decking and Cladding Association gives the sizes, treatments, and gaps used by trades, and composite makers publish joist and clip details for their systems.
Wrap-Up: Build Once, Enjoy For Years
Follow the fall, set the centres, and keep the gaps clear. With that, the surface will drain, screws will stay tight, and the deck will last. If you want the exact phrase to search later, bookmark this page under “How To Fit Garden Decking” so you can find these sizes when you need them next season. You have every step, from layout to the last screw.