A deck is a floor with weather under it as well as on it. In Durham that underside stays wet for much of the year: shade from the bank, trees, and air that does not dry the way a breezy coast sometimes does. Our team builds the void on purpose. Joists that cannot dry will not be saved by a better board on top. This is the detail behind Newton Hall and behind every sloped garden we frame.
Why the void matters
Timber joists and bearers lose strength when they stay damp. Metal fixings fret in a closed, wet space. Even composite boards, which do not rot, are held by a frame that can. The void is there so air moves across that frame after rain. If the deck is laid tight to the lawn, there is no void, only a damp sandwich, and the garden smells it by autumn.
Durham gives the void extra work. Leaves from the woods on the banks drift under the edge and hold moisture like a sponge. A north-facing plot behind a tall house may not see sun on the soil at all. The Wear valley holds damp air. None of that is solved by a coating on the top of the board. It is solved by a gap that stays a gap.
A modest void may need a step at the door. That step is a fair trade. Levels are covered in the Wear-bank guide. The air is handled here.
How the frame is built
Posts and footings lift the bearers off the soil. Joists run above that, at centres the board requires, with enough depth that the underside is not brushing the ground at mid-span. On a slope the downhill edge is naturally higher. We use that opening. On a flat estate garden we create the opening ourselves and we refuse to lose it by packing soil back against the joists when the job is finished.
A membrane and a layer of clean gravel stop weeds and stop soil splashing up. They are not a sealed tray. Water that lands under the deck should soak or run, not sit in a liner with no outlet. Edges stay open on at least two sides. Where a deck must sit close to a wall, the far edge and the ends do the breathing. Air bricks in the house wall stay in front of the frame, never buried behind a fascia.
Skirts are the usual mistake. A board run around the perimeter looks finished and stops the wind. In this valley that is the wrong finish. If a skirt is wanted for appearance, it is detailed with gaps, not as a sealed plinth. Stored boxes, bags and toys pushed into the void are the same problem after we leave. We leave the void reachable so it can be cleared of leaves once a season.
Shade, leaves and which bank
A shady north bank dries slower than a south-facing deck above the Wear. We still ventilate both, with a taller void on the shady one if the door allows. Composite is easier there. The frame is ventilated either way.
Leaves are a maintenance fact, not a reason to close the deck. We keep a gap you can rake or brush from the open edge. We do not design a void that can only be cleaned by lifting boards. On Wear banks the uphill edge is where soil and leaves migrate. That edge gets a stop. The downhill edge stays open.
The BGS viewer tells us the mapped ground, which affects footings, not the need for air. The flood map tells us about the postcode near the river. A deck above a flood-prone corner still needs to dry between ordinary rains. Ventilation is not a flood defence and it is not optional on high, dry ground either.
What this does not change
Air under the boards does not change planning. For a house, the deck is still normally permitted development only within 30 cm of the ground, within half the garden, and behind the front wall. A taller void that pushes the surface over 30 cm is a planning question, and we will say so rather than quietly building it. Conservation areas restrict decking at the side. Listed buildings have no permitted development for decking in the garden. The peninsula adds World Heritage limits, covered in the peninsula guide. We check the address.
The quote includes the void as a described part of the job: post height, open edges, membrane and gravel. It is not an extra muttered at the end. Ask for a quote with the postcode and whether the garden is shaded. If you already have a deck that smells damp, say that too. Often the repair is to open the void, not only to change the boards.
FAQ
Can the deck sit straight on the ground?
We do not build it that way in Durham. The boards need a frame and a void that air can cross. Sitting them on the soil stores damp.
Will a skirt make it look neater?
A sealed skirt stops the drying. If you want the edge covered, we leave deliberate gaps. Appearance follows the airflow, not the other way round.
Does composite remove the need for air?
No. Composite does not rot, and the frame holding it can. The void is for the structure.
Is a higher void always better?
Only until it crosses 30 cm above the ground, or until the step at the door becomes awkward. We balance drying against the height rule and the sill.