Decking Height and the Damp Proof Course: Getting the Level Right

Why Decking Causes This More Than Paving

Paving stays visible for as long as it's down. You can look at a patio and the wall behind it together, any day you like, and see how the two relate. Decking hides that relationship the moment the last board goes on: the frame, the joists, the gap, or lack of one, at the wall all disappear behind a neat run of timber. From that point on, nobody's looking at what's happening behind it unless something has already gone wrong.

Decking is also simply easier to build up than paving is to build down. A frame gains height with another packer or another course of joist, almost without anyone deciding to raise it on purpose, creeping upward toward the door rather than being set deliberately against a line on the wall. The door, not the damp proof course, is what people end up unconsciously building towards.

Two Different Problems, Often Confused

The Deck Surface Bridging the DPC

The first problem is the one people have usually heard of, if only vaguely: build the deck surface up to or above the damp proof course line, and you've given moisture a route straight past the barrier and into the brickwork above. It's the same bridging problem I've written about with raised patios, in Raised Patio Against the House: Avoiding a Bridged Damp Course. The material changes; the physics doesn't.

The Deck Trapping Damp Against the Wall

The second problem is less well known, and just as damaging: a deck built comfortably below the damp proof course can still cause trouble if it's built hard against the brickwork with no gap. Timber laid tight to a wall stops air moving through that strip, collects leaf litter nobody can reach, and keeps the wall permanently damp, whether or not the DPC line has technically been crossed.

Bridging is about height. This second problem is about contact and airflow, and a deck can get the height right and still get this wrong. I see it more often than straightforward bridging, because it's the one nobody's warned about.

Decking Across Air Bricks

The same blind spot catches air bricks. A run of decking built across the wall at the wrong point can cover one entirely, cutting off ventilation to the floor void beneath the house, particularly on older Kent and Sussex properties with a suspended timber ground floor. I've covered what that ventilation actually does, and how patios and decking end up blocking it, in Air Bricks at Ground Level: Why Your Patio Must Leave Them Clear, and every word of it applies just as much to decking as to paving. If there's an air brick anywhere near where a deck is going, it needs a place in the design from the first sketch.

Getting the Level Right: Principles, Not Numbers

I won't hand you a measurement here, because I don't have one that's honestly yours; the right clearance depends on your wall, its construction and your ground conditions, and needs confirming for your specific house. What I can give you are the principles I design to every time.

Set the deck level from the DPC, not the door. The threshold tells you where the floor is inside. It tells you nothing about where the damp proof course sits. Find the DPC first and let that decide the deck height; work back from the door and you're designing against the wrong line entirely.

Leave a gap. Keep the deck structure, not just the surface, clear of the brickwork, with a genuine ventilation gap running along the wall. That gap stops the deck bridging the DPC, and keeps air moving so the wall doesn't sit permanently wet behind the boards.

If a level threshold is genuinely what you want, that's achievable, but it's a designed detail with drainage built in, not a decking decision alone. I've set out what that involves in Level Threshold Patio Doors: Designing a Step-Free Garden Entrance.

Ventilate under the deck itself. A deck is a void with a floor on top, not a solid surface, and that void needs air moving through it, just as the space under a suspended timber floor does. Seal it in on all sides and you've built a damp problem with a lid on it.

What Goes Wrong Underneath

This is the part nobody looks at, not without getting down and lifting a board. No airflow and often no light means the ground underneath a badly detailed deck stays damp long after the surface above looks bone dry. Leaf litter gathers where nobody sweeps it, and the joists and bearers doing the structural work sit in that damp, still air for years at a time.

The boards you walk on are usually the last thing to fail, not the first. They're exposed, they dry between rainfall, and they're the part anyone would notice if something looked wrong. The joists underneath get none of that: they sit in shadow, in still air, often against damp ground, and by the time a board feels soft or springy underfoot, the structure beneath it has usually been struggling for a good while longer.

A Word on Timber

Softwood, hardwood and composite decking all behave differently in wet, shaded conditions, and I won't quote you a lifespan for any of them; too much depends on the specific timber, the detailing, and how the site is maintained. What I will say plainly is that composite not rotting doesn't mean the problem goes away. The boards may be largely immune to what rots softwood, but the frame underneath a composite deck is, in most installations, still ordinary timber. Choosing composite for the surface and ignoring ventilation underneath solves half the problem.

Conclusion

Decking's real advantage, the reason I still design it into gardens regularly, is that it's built rather than dug, which makes it flexible over uneven ground and forgiving of a site paving can't always work with. That's exactly why it's the easiest way to get a damp proof course wrong without anyone intending to. Set the height from the DPC, not the door, leave a genuine gap at the wall, and give the void underneath somewhere for the air to go, and decking does its job well for a very long time.

If you're planning decking anywhere near a house wall, it's worth having the levels looked at properly before the frame goes up, not after.