Composite decking on grass or soil: how to build the base and frame

Most decks in Britain go where a lawn used to be, and the question people search for is whether the boards can simply go down on the grass. They cannot, and a deck laid on the ground is the one that fails first. This guide explains what has to happen between the soil and the boards, the three ways to build it, what each costs for a typical deck, and the mistakes that put a composite deck in the skip in five years.

The short answer

Composite decking never sits on grass or soil. It sits on a frame of joists that is itself held off the ground on pads, blocks or posts, with weed membrane and gravel underneath and 40mm of air between the frame and the earth. On soil, the right frame is recycled plastic joists on plastic posts or concrete pads: rot-proof, so the frame lasts as long as the 25-year boards. A 3.6m by 3m deck on a lawn costs about £800 in boards, clips and trims plus £350 to £600 for the frame and base, and takes two weekends.

Shop the parts: recycled plastic joists and posts, 3.6m composite decking boards and 4.8m composite decking boards, clips and screws.

Why not straight on the ground

Three reasons. Composite boards are a surface, not a structure; they need support every 350mm or they bounce and creak. Ground moves, holds water and grows things; a frame on soil rots if timber and shifts whatever it is made of. And airflow under the deck is what dries the boards after rain and keeps algae off; a deck sitting on the ground stays damp underneath permanently. Even the toughest board on the market fails as a deck if it is laid on grass.

Preparing the ground

1. Strip the turf across the deck footprint plus 100mm each side. Take off 50 to 80mm of soil so the finished deck sits at the level you want.
2. Level roughly with a fall of about 1 in 100 away from the house, so water under the deck drains away from the wall.
3. Weed membrane across the whole footprint, overlapped 150mm, pinned. Nothing grows up through the deck.
4. Gravel, 40 to 50mm of 20mm stone over the membrane. It keeps the ground under the deck dry and stops the membrane lifting.

Half a day for a 3.6m by 3m deck with two people and a wheelbarrow. This is also the part a fitter is most worth paying for on a sloping or waterlogged garden.

Three ways to build the frame on soil

1. Recycled plastic joists on concrete pads. Set 450mm by 450mm paving slabs or concrete pads on compacted gravel at 1.2m centres, and lay 50 by 100mm recycled plastic joists across them at 350mm centres, fixed with brackets. Rot-proof, cheap to set out, and the deck sits 150 to 200mm above the ground. The frame for a 3.6m by 3m deck is about £450 in joists and pads.

2. Plastic posts in the ground for a raised deck. Where the deck has to meet a door threshold or the garden slopes, 80 by 80mm plastic posts concreted into the ground carry bearers, and the joists sit on the bearers. Never rots, never needs painting. About £900 to £1,000 for a 4.8m by 4m deck. The subframe guide covers spans and post spacing.

3. Treated timber on pads. C16 or C24 joists with ground-contact treatment, kept off the soil on pads, joist tape along the top edge. The cheapest frame at £150 to £200 for a small deck, and the one with a life: 10 to 15 years done well, 5 to 8 done badly. Timber directly on soil is the most common reason a composite deck is replaced.

Frame rules that do not change

• Joists at 350mm centres, 300mm if the boards run diagonally.
• A double joist under every board end and under a picture-frame border.
• 40mm of clear air under the frame, everywhere.
• Frame level along every joist and square across the diagonals before a board goes on.
• Outer joist set so the fascia board finishes flush with the board ends.

Then the boards

From here it is the standard build: starter clips on the outer joist, boards clipped and screwed one at a time with the expansion gap for the day's temperature, fascia on the visible sides, end caps on any open end. The installation guide has every figure, and the clips guide the clip count.

What it costs, 3.6m by 3m on a lawn

Item	Cost
Membrane, gravel, pads	£120 to £160
Recycled plastic joists, brackets	£350 to £450
20 boards at £25	£500
Clips and screws, 3 packs each	£85
Fascia, 3 boards, and end caps	£75
Materials total	£1,130 to £1,270
Fitted by a landscaper instead	add £750 to £1,000 labour

Against the same deck on an existing patio, which needs only battens, soil adds about £400 in frame and base. It is the price of a deck that does not move, and it is what the installation cost guide assumes for a lawn.

The mistakes we see

• Boards screwed to timber bearers laid flat on the grass. Rotten in three years, boards buckled from being face-screwed.
• No membrane; bindweed through the gaps by the second summer.
• Frame built level with no fall, so water sits against the house wall.
• Joists at 600mm centres because that is what the old timber deck had. Composite bounces at 600mm.

Send us the deck size and a photo of the ground and we will list the frame, boards, clips and trims as one order. Free samples first if you have not chosen the colour.

Frequently asked questions

Can you lay composite decking directly on grass?

No. Composite boards are a surface, not a structure, and need support every 350mm; laid on the ground they bounce, stay damp underneath and fail first. Strip the turf, lay membrane and gravel, and build a frame held 40mm off the ground on pads, blocks or posts.

What goes under a composite deck on soil?

Strip the turf and 50 to 80mm of soil, level with a slight fall away from the house, lay weed membrane overlapped 150mm, then 40 to 50mm of 20mm gravel. The frame sits on concrete pads or plastic posts above that, never on the soil.

How much does a composite deck on a lawn cost?

About £800 in boards, clips and trims for 3.6m by 3m, plus £350 to £600 for a plastic joist frame and the base materials: £1,130 to £1,270 in total, or add £750 to £1,000 for a landscaper to fit it. Two weekends to build yourself.

Which frame is best on soil: timber, composite or plastic?

Recycled plastic joists on concrete pads or plastic posts. They cost about twice treated timber and never rot, so the frame lasts as long as the boards. Timber on soil is the most common reason a composite deck is replaced early.

Boards, trims and frame parts are priced on the composite decking price list; free samples in every colour.