Too often, we see homeowners look at their uneven outdoor space and jump to the assumption that decking isn’t a viable option for them. The reality is that decking is definitely a possibility, with the right approach.
If you’re interested in laying decking outside, but are unsure of how to navigate the process on uneven ground, read on. This is how to build decking on a slope, helping you pick the right method before you actually go ahead with buying things.
Which Method Do You Need?
The method isn’t a one-size-fits-all one when it comes to uneven ground:
Ground-level deck (gentle unevenness, under 300mm variation)
Use adjustable deck feet on a compacted MOT sub-base. The feet screw up and down to compensate for dips and bumps without any post digging.
Raised deck on a slope (300mm+ drop or significant gradient)
You need a post-and-bearer frame. Posts are set at varying heights into concrete footings, carrying a level frame above the uneven ground below.
Materials Checklist
Decking boards (add 10% for cuts)
Joists: 47×100mm or 47×150mm depending on span
Bearers and rim joists
Adjustable deck feet (ground-level builds) or fence posts and adjustable post bases (raised builds)
MOT Type 1 sub base
Coach screws: M10 × 100mm
Joist hanger brackets
Post-mix concrete (raised builds)
End-grain sealant and wood preservative
Step-by-Step: How to Lay Decking on Uneven Ground
1. Prepare the site
Clear turf and vegetation, then lay weed membrane across the whole area. Working on a ground-level build? Spread and compact a 100mm layer of MOT Type 1 to prevent deck feet sinking into soft ground. For raised builds, dig footings to at least 400mm, pour post-mix concrete, and allow 48 hours to cure before loading.
2. Set out the frame
Mark your deck area with string lines and pegs. Check diagonals are equal to confirm a true rectangle. Mark 50mm beyond your deck edge so you have room to work.
3. Level across uneven ground
Ground Level: Place rim joists on adjustable feet at each corner. Set your datum at the highest point, then use a spirit level or laser level to bring each foot up to match. Add intermediate feet at 400mm centres and adjust to suit. Fix once the level is confirmed.
Raised or Sloped: Fix a ledger board to the house as your datum (with a DPC behind it). Run a string line at deck height across the slope. Plumb and brace each post, fix bearers at the string line height using joist hangers and coach screws; finally, trim post tops flush.
4. Fix the joists
Run joists perpendicular to your boards at 400mm centres (300mm for composite). Hang each end with joist hangers; stronger and more accurate than toe-screwing. Seal all cut ends with end-grain sealant before fixing. Check that the frame is square diagonally before moving on.
5. Lay the boards
Start from the straightest, most visible edge. Leave 5 to 6mm gaps between boards using offcuts as spacers. Fix with decking clips for a hidden finish or pre-drilled stainless screws. Don't cut boards to length yet. Overhang them, then snap a chalk line and cut in one pass once all boards are fixed.
6. Finish the edges
Fix fascia boards to cover exposed joist ends. Sand any rough edges, apply decking oil or stain within a week of laying, and seal around any ledger connection to the house wall.
Find Decking at Joseph Parr Bradford
So, laying decking on uneven ground is completely feasible, provided you use the correct approach.
We stock various decking materials, including boards, structural timber, baserail, joist hangers, and screws.
Order online or get in touch if you need assistance with your project.
FAQs
Do I need planning permission for a raised deck?
Not usually, but under Permitted Development, the deck must sit no more than 300mm above ground, cover less than 50% of the garden, and not be within 20 metres of a highway. Listed buildings and conservation areas are excluded. Always check with your local planning authority first.
How do I stop decking from moving on soft ground?
Never place deck feet directly on soil. A compacted 100mm MOT sub-base spreads the load and prevents sinking. On very soft or waterlogged ground, add concrete pads (minimum 450×450mm) beneath each foot position.
What depth should decking joists be?
Match joist depth to span: 47×100mm up to 1.8m, 47×125mm up to 2.1m, 47×150mm up to 2.4m. Always use C16 or C24 graded structural timber not ungraded DIY-store timber.