How to Build a Garden Deck
A complete weekend project that transforms your garden - plan it right and build it once.
Difficulty
Hard
Time
2-4 days
Cost
£300 - £1,500
Before You Start
Check whether you need planning permission. Decking does not usually need permission unless it is more than 300mm above ground level, covers more than 50% of the garden, or is in a conservation area. Check for underground services before digging. Wear gloves when handling pressure-treated timber - the preservative chemicals irritate skin.
A well-built deck adds genuine living space to your garden and is one of the best value improvements you can make. The frame is the most important part - get this level and square, and everything else is straightforward. Use pressure-treated timber for the subframe no matter what you choose for the deck boards. Composite boards cost more upfront but need virtually no maintenance, while timber boards need oiling annually.
What You Will Need
Tools
- - Circular saw or mitre saw
- - Drill/driver with countersink bit
- - Spirit level (long)
- - Tape measure
- - String line and pegs
- - Spade
- - Set square
- - Socket set (for coach bolts)
Materials
- - Decking boards (pressure-treated softwood or composite)
- - Joists (47x150mm treated timber)
- - Bearers/beams (same or larger than joists)
- - Concrete deck blocks or post anchors
- - Decking screws (stainless steel)
- - Weed membrane
- - Gravel (for base drainage)
- - Decking oil or stain
Step-by-Step Instructions
Plan the Layout
Decide on size and shape. Draw a plan with measurements. Calculate joist spacing (400mm centres for standard decking boards). Mark out the area with string lines and pegs. Check diagonals to ensure the area is square.
Prepare the Ground
Clear the area of turf and vegetation. Level roughly with a spade. Lay weed membrane over the entire area and cover with 50mm of gravel. This prevents weeds growing through the decking and improves drainage.
Position the Supports
Place concrete deck blocks at 1.2 metre intervals across and along the deck area. These sit on the gravel and support the bearers. Check they are level with each other - add or remove gravel underneath to adjust. A long spirit level and a straight-edge timber help here.
Fit the Bearers (Beams)
Lay the main bearers across the deck blocks. These run in the opposite direction to the decking boards. Check they are level along their length and across from one bearer to the next. Shim with offcuts if needed.
Fit the Joists
Fix joists across the bearers at 400mm centres. Use joist hangers or skew-nail them at each junction. Check the frame is level and square one more time. Add noggins (short blocks between joists) at the edges for board support.
Lay the Decking Boards
Start at the edge nearest the house, leaving a 10mm gap between the deck and the wall for drainage and airflow. Fix each board with two screws at every joist point. Pre-drill to prevent splitting. Leave a 5-8mm gap between boards for water drainage and timber expansion.
Trim the Edges
Once all boards are laid, snap a chalk line along the overhanging edges and cut them flush with a circular saw. This gives a clean, straight edge. Fit a fascia board around the edges for a finished look.
Treat and Protect
Apply decking oil or stain to all exposed surfaces. Pay extra attention to end grain where moisture enters most easily. Reapply annually for timber decks. Composite decking only needs washing.
Pro Tip: Pre-Drill Everything
Always pre-drill screw holes in decking boards, especially near the ends. Without pre-drilling, the wood splits and looks terrible. Use a countersink bit so screw heads sit flush.
Pro Tip: Leave Expansion Gaps
Use an 8mm spacer (a spare pencil works) between boards. Timber expands when wet. Without gaps, the boards push against each other and buckle upward.
Common Mistakes to Avoid
- - Not using pressure-treated timber for the subframe - untreated timber rots within 2-3 years
- - Joists too far apart - the decking boards flex and feel bouncy underfoot
- - No gap between boards - water pools, boards swell and the deck buckles
- - Screwing too close to the end of boards - the wood splits
- - Placing the deck directly on soil - moisture is trapped underneath, accelerating rot
- - Forgetting to allow for airflow underneath - poor ventilation causes the underside to rot
When to Call a Professional
If your garden slopes significantly and the deck will be raised more than 300mm off the ground, you need proper structural posts sunk into concrete and may need Building Regulations approval. A raised deck also needs a balustrade for safety. This level of construction is best done by a professional carpenter or decking specialist.