Building a ground-level composite deck: ventilation, drainage and height

I think a deck that floats a hand's width above the lawn is the easiest kind to live with. It doesn't need stairs or a balustrade, and it gives you a flat place to put the table. It's also the deck I'd worry about most for the frame, because there is little air under it and the ground stays damp. Composite boards suit a low deck better than timber because they're made not to rot, but I'd build the frame under them as if it will spend its life in a wet cellar.

Why ground-level decks are hard on timber and easy on composite

On a timber deck at ground level, the trouble starts underneath. The joists sit in air that barely moves, and the undersides of the boards stay damp. One Whirlpool member warned in 2016 that H3 pine under a very low deck with next to no ventilation "will last a while but 5-10 years" before it needs replacing¹¹. One maker sells its capped composite board as one that won't split, rot or warp², and that is the whole case for composite on a low deck. It isn't a case for ignoring the frame.

The makers' ground clearance figures are written for that damp frame, and they're worth comparing. One fixing chart gives a minimum ground clearance of 300 mm for its boards¹. Another maker asks for 100 mm over well-drained ground and says 300 mm between the boards and any permeable surface is ideal⁵. A third gives 100 mm over grass or dirt⁴. The lowest of the four is 40 mm, and only where there is adequate drainage beneath the deck, with 90 mm asked for otherwise³. My reading of the spread is simple: the less air you can give the deck, the better the ground under it has to drain.

How to frame a ground level deck

The usual frame is bearers on stirrups or pads with joists on top, which is at least two timber depths before you reach the boards. On a low deck you often don't have that height. The frame has to flatten out, and the ground decides how.

The first option drops the bearers. In a 2016 Whirlpool thread on low-profile decks, one reply suggested setting concrete pads along the length of each joist so the pads do the bearers' job¹⁰. In the same thread, adavis described a variation built twice: 90 x 90 treated pine bearers buried on concreted stirrups with the joists on top, which put the deck about 110 mm above the ground¹⁰. That post warned that with so little ventilation the decking had to be treated pine or one of the hardest hardwoods. Composite removes that worry for the boards, but I'd still want every piece of the frame held clear of the soil.

The second is a frame on pedestals, which I'd use over any hard surface. A small pedestal foot adjusts from 25 mm to 40 mm and is rated for 2.2 kN⁸, and the joists sit on the feet with air under everything. Over soil, one maker's pedestal system can sit on concrete pads set in the ground, and over bare earth it asks for compacted, well-drained ground under a membrane⁵. The detail is in the guides on pedestal frames and composite decking over concrete.

The third is a steel or aluminium frame, which is the one I'd use beside a house in a termite area. The NCC lists steel, aluminium and concrete among the building elements that are not subject to termite attack, alongside timber that is naturally resistant or preservative-treated to the termite standard⁶. The trade-offs are in the steel framing page and the guide on steel versus timber frames.

Whichever frame you use, keep the joist centres at the figure the board maker gives, and the joist spacing guide explains why. One maker's guide also asks that every board run across at least two spans, so three joists, with no cantilever past the last joist of more than 50 mm³. There's no height to be saved with wider centres, only a bouncier deck.

Ventilation: leave the ends open

The mistake I'd guard against first on a low deck is the fascia. If a fascia board runs around the perimeter and down to the mulch, it seals the deck into a box. One maker's instructions say plainly not to seal off deck ends and so prevent airflow, and they name garden beds, trees and solid objects as the things that do it¹. A low deck needs air in at one side and out the other. If you want a fascia for the look, I'd stop it short of the ground or use a slotted board, and keep the garden beds back from the edge.

A Whirlpool thread from December 2014 is a good illustration. The owner, Ali Some Boy, said the clearance from the ground was very low, maybe 150 mm, and was weighing up a timber frame against laying a slab and decking over it⁹. One reply, from Teddles, said a low-profile timber frame would do the job but that the perimeter "shouldn't be closed off", and suggested an agricultural drain if water collected there⁹. Another member, SnakeHed, recommended stirrups concreted into the ground with a gap left for termite inspection and moisture, and weed matting underneath so nothing grew up through the deck. The owner came back in June 2015 to say they had poured a slab in the end, and was working out how to batten over it with the slab falling away from the house.

That thread also has the one piece of advice I'd push back on. A member suggested that with only 40 mm to play with, a composite deck could be fixed straight down onto the slab⁹. The makers don't allow that. Two of the install guides cited here rule out fixing the boards directly to a solid surface such as concrete³.

Drainage: grade, mat and gravel

Water under a low deck has to go somewhere. One maker wants the area under the deck dry and clear of vegetation, with drainage put in where water usually lies under the deck⁵. I'd grade the ground so it falls away from the house, lay weed mat, then spread a layer of gravel over it. The mat stops weeds and keeps the gravel out of the soil, and the gravel keeps the surface draining. Where the ground is a natural low spot, I'd put in an agricultural drain to the storm water line rather than build a frame that sits in water.

This matters more for composite than you'd think, because of the clearance figures above. Two of the makers tie their lower figure to good drainage, so drainage is what lets a ground-level deck use the lower number.

Termites and the house wall

The NCC treats a deck as an attachment to the building, and the termite options it lists for an attachment include an inspection zone between the attachment and the building⁶. Where the house has parts that termites can attack, in an area at risk, the provisions call for a termite management system and a durable notice recording what was installed and how often it should be inspected⁶. I wouldn't let the deck cover that inspection zone. I'd keep a gap between the deck frame and the wall so the slab edge stays visible, or at least make the first board beside the wall easy to lift. A steel or aluminium frame gives termites less to eat on the deck itself, but the house wall still needs its inspection zone.

Treated pine can work under a low deck if it's the right hazard class for where it sits. The H3 or H4 guide explains which goes where. With so little air under a low deck, my advice keeps coming back to pedestals, steel and open ends.

Mould on a damp low deck

A deck that stays damp can also grow mould on the boards. A ModWood owner in Victoria, Peter jaspers, wrote on ProductReview in August 2026 that the decking "turns green and extremely slippery every winter" despite regular cleaning¹². I can't tell from the review how low or shaded that deck is, but it's the risk I'd plan for on any low deck. The fixes are airflow underneath and a regular wash. The guide on cleaning composite decking covers the method.

Before you build

Check the finished deck height against the door sill, and look at the ground after rain rather than on a dry afternoon. Read the clearance figure in the install guide for the boards you've chosen, then look at whether the ground drains well enough to use the lower number. Choose between stirrups, pedestals and a steel frame based on what's underneath and how close the house is. Leave the perimeter open. If every edge of the deck stays less than 1 m above the ground below it, the NCC doesn't call for a barrier⁷, which on a ground-level deck is the easy part. The deck designer will set out the joists at the maker's centres and give you a materials list as a concept plan to take to your builder.