How do I build a deck on uneven ground?

Adjustable pedestals, post-and-bearer frames and stepped decks — the three ways to get a level surface over sloping or lumpy ground.

Difficulty: intermediate
Time: 12 hr
Read: 4 min
Safety: warning

Short answer

For a small level difference, adjustable decking pedestals under the joists are the quickest answer. For a real slope, set posts into concrete at varying heights and carry bearers across them. For a steep slope, step the deck into two or three levels rather than building one high platform.

Ground is never as flat as it looks and levelling it is usually the wrong instinct. The frame is what makes the deck level; the ground only has to be stable, well drained and clear of vegetation.

Safety

A deck surface more than 600mm above the ground needs a balustrade, and a raised deck is a structural job that should be designed rather than improvised — posts, bracing and foundations all have to be sized for the load. Check for buried services before digging or driving any post. This guide covers low decks only; get a raised deck designed by someone competent.

Joist centres, board gap and fixing positions

Step by step

Survey the ground properly first.Pegs at the corners, a string line and a level, or a long straightedge and a spirit level. Measure the fall corner to corner and along both axes. The numbers determine which method to use.
Choose the method by the fall.Under about 100mm, adjustable pedestals or packers under the joists. From 100mm to 600mm, posts in concrete carrying bearers. More than 600mm, step the deck into levels — a high platform is a different, structural job.
Prepare the ground regardless.Strip vegetation, lay a permeable membrane, and gravel over it. Water must be able to drain away under the deck, and nothing should be growing up into it.
Use adjustable pedestals for small variation.Screw-adjustable plastic pedestals sit on slabs or a firm base and support the joists, and are levelled by turning the collar. Set them at no more than the joist span allows and on a solid bearing surface.
Set posts to a string line for a real slope.Post positions marked from the frame layout, holes dug to 600mm minimum, posts set in concrete and left proud. Once the concrete is cured, cut every post to a level line marked from a datum with a laser or a water level.
Carry bearers across the posts.Bearers bolted to the posts with coach bolts and washers, not skew-screwed. Joists then sit on or hang from the bearers, all at one level regardless of the ground below.
Brace a raised frame.Any post over about 600mm needs diagonal bracing in both directions. An unbraced post frame racks sideways under use.
Step the deck on a steep slope.Two or three separate low platforms with steps between them are safer, cheaper and better looking than one tall deck, and stay within low-deck territory.
Keep the ventilation and the DPC clearance.50mm minimum air gap under the joists, an open or vented perimeter, and the deck surface at least 150mm below the damp-proof course of any wall it adjoins.

Tips

A laser level pays for itself on this job. Transferring a level round a sloping site with a spirit level and a plank is slow and error-prone.
Set the posts long and cut them to the line afterwards. Trying to set each post at exactly the right height in wet concrete is unnecessarily hard.
On clay that moves seasonally, wider concrete pads under fewer posts behave better than many small ones.

Common mistakes

Levelling the ground instead of the frame — Cutting into a slope creates a bank that needs retaining and a hollow that collects water. The frame is what needs to be level; the ground just needs to drain.
Standing posts on the surface without a footing — They sink unevenly and the deck moves. Posts need a concrete footing below the soft topsoil layer.

If it doesn't work

Deck settled unevenly after a wet winter

Cause: Footings too shallow or on soft ground — Fix: Jack the affected bay, dig out and pour a proper footing below the topsoil. Packing under a pedestal is a temporary fix only.

Raised frame moving sideways

Cause: No diagonal bracing — Fix: Add braces in both directions between posts and bearers. This is not optional above about 600mm.

Water collecting under the deck

Cause: Ground falls towards the middle or the house — Fix: Regrade the ground under the deck to fall away, add gravel, and check nothing is discharging there.

Pedestals sinking into the ground

Cause: Sitting on soil rather than a firm base — Fix: Each pedestal needs a slab or a concrete pad under it, on compacted ground.

Questions people ask

How high can a DIY deck be?

Above 600mm above ground it needs a balustrade and becomes a structural job worth having designed, and in some circumstances it may need building regulations approval. Keep DIY decks low, and step them on slopes rather than building tall.

Are adjustable decking pedestals any good?

For small level corrections on a firm base, yes — they are quick and adjustable later. They are not a substitute for footings on soft ground or a real slope.