Laying decking on soil, grass, concrete or uneven ground

How do you lay decking on soil?

Never lay the frame directly on soil. Clear the vegetation, level the ground with a slight fall, lay a geotextile weed membrane, and build the frame on supports so no timber touches the ground. A UK builder says the membrane keeps weeds from pushing up through the board gaps six weeks after you finish.

Support options:

• Concrete pads with fence post stilts. One DIYer used twelve for a 23.5 m² deck, at about £191 for groundworks.
• Posts set in concrete. Slope the top of the concrete away from the post so water runs off rather than collecting at the rot-prone ground line, and brace the posts until it sets.
• Concrete decking posts. Cladco sells 100 by 100 mm posts in 600 and 1,200 mm at £23.95 including VAT, at one post per m², that can go straight in the ground or on a decking block (the listing showed it out of stock).
• Adjustable pedestals on a firm pad. See supports and posts.

How do you lay decking on grass?

The same way as soil, after lifting the turf. Don’t build onto turf, because the grass and soil hold moisture against the frame.

How do you lay decking on concrete?

A slab gives a firm base, but water sits on it. Ecoscape says its boards can’t be laid on a flat surface and need at least 25 mm of clearance. Raise the frame on pedestals or packers so the joists stay clear of standing water, and keep the slab’s fall. Millboard’s DuoLift pedestals need a solid, stable base. An installer building over an uneven slab used window packers to shim joists level, waterproofed any timber that sat on the concrete and thought about a moisture barrier between sleepers and frame. Run any cables for deck lights before the boards go down.

How much clearance does the deck need?

How do you deal with uneven ground or a slope?

Adjustable pedestals or posts of different heights, built level. London Stone’s pedestals run from 40 to 65 mm at £7.20 up to 160 to 260 mm at £13.80 each (including VAT as shown), and its 20 mm self-levelling slope corrector at £3.60 corrects slopes up to 5 percent. Millboard’s DuoLift raises from 15 mm up to 350 mm and its self-levelling joint corrects up to 5 degrees.

If the high side ends up more than 30 cm above the ground, you’re in planning territory in England; see planning permission. Above 600 mm, guarding is expected.

What if there’s a drain or pond liner underneath?

Leave an access hatch rather than boarding over a drain cover. A UK owner with a deck raised over a pond liner used the liner for waterproofing and built brick pillars to support the frame. See how to build decking for the frame.