The Breathing Room Behind the Boards: Getting Ventilation Right on External Timber

Most timber that fails on a garden or trade job doesn’t fail because it got wet once. It fails because it stayed wet. A board can shrug off a soaking, but put it tight against a wall, a rail or another board with no way for air to move, and you’ve given moisture somewhere to sit. The fix is rarely complicated: give the timber room to breathe.

Why the gap matters more than the finish

A ventilation gap behind external timber does two jobs. It lets water drain away, and it lets air move through so the timber can dry. Those are separate jobs, and both matter in a UK climate where a west-facing wall can take driving rain on Tuesday and sit in damp, still air for the rest of the week.

Tight-fitting timber also creates a narrow, sheltered layer where condensation can form. Warm, moist air from inside a shed or conservatory meets a cold board, and the water has nowhere to go. A drained and ventilated cavity breaks that cycle. It is not a sealed void and it is not a magic cure — it only works if air can actually enter at the bottom, travel up, and leave at the top.

How wide should the gap be?

For most external cladding and linings, 25mm is the sensible default. That is enough to drain and ventilate without creating a wind tunnel. In sheltered spots — under a deep eaves, say — you can sometimes work closer to 15mm, but 25mm is easier to build consistently and easier to keep clear.

Going much wider is not automatically better. A 100mm cavity behind featheredge cladding may look generous, but it can catch wind-driven rain and turn into a sheltered ledge for debris. Stick to a designed gap rather than a random one. If you are unsure, match the batten thickness to the exposure: 25mm minimum for exposed elevations, 25mm as standard on garden buildings, and never less than 10mm even in the most protected position.

Battens, spacers and a fixing chain that doesn’t crush the gap

The usual method is treated timber battens fixed over a breather membrane, with the cladding fixed to the battens. The battens should be vertical where water needs to run down, and spaced at 400mm to 600mm centres depending on the cladding and the wind loading. Use stainless or properly coated screws — not bright nails that will rust and stain the boards.

Fix the battens through to the studs or frame, not just to the sheathing. And watch the fixings: overdriving a screw can pull the batten tight and crush the gap, especially with softwood. A small spacer block held behind the batten while you fix it is a simple way to keep the thickness honest.

On sheds and fence work, the same principle applies at a smaller scale. Fence rails hold the boards off the posts, but the boards still need a little room between them for air to move. Shed cladding fixed directly to the frame with no batten has nowhere to drain. If you are lining a conservatory wall with timber, battens over the masonry — with vents at the top and bottom — stop the lining becoming a damp trap against the wall.

The two ends people forget

A gap with no openings is just a sealed box. At the bottom, keep the ventilation path open and cover it with insect mesh to stop wasps and mice moving in. Don’t block it with paving, gravel boards or a bead of sealant. Keep the bottom of the timber at least 150mm above ground level and respect any damp-proof course.

At the top, leave a vented closure rather than sealing the cavity shut. Some systems use a proprietary vent strip; a simple mesh-covered gap works on plenty of garden buildings. What matters is that air can pass from bottom to top without a dead end.

And please, don’t fill the gap with expanding foam. It feels like a draught-proofing shortcut, but it blocks the drainage path and holds water against the timber. If you need insulation, put it in the frame, not in the ventilation layer.

Job-by-job checks

On featheredge cladding, check the battens are straight, the membrane laps correctly, and the bottom vent is clear before the first board goes on. On a shed, make sure the cladding is not fixed tight to the frame and that the roof space can vent. On fence rails, leave a small, consistent gap between boards and keep the bottom rail clear of soil. On a conservatory lining, confirm there is a vent path above and below, and that any insulation sits behind the ventilated cavity rather than filling it.

Before you cover the gap, run a quick test: put your hand behind the batten, look for light at the top and bottom, and check the cavity runs continuously past corners and openings. Take a photo before the cladding goes on. It is cheap evidence if a question comes up later.

The gap is one of the least expensive parts of an external timber job, and one of the easiest to get wrong. Build it once, keep it clear, and the boards will dry out instead of quietly rotting from the back.