Fixing Timber Cladding on a Ventilated Cavity

How far the board is going to move, and in which direction

Cross-grain movement is arithmetic, not weather lore. Below the fibre saturation point a board changes width in proportion to its width, to the change in its moisture content, and to a coefficient that belongs to the species and to how the board was sawn. The USDA Forest Products Laboratory Wood Handbook: Wood as an Engineering Material, General Technical Report FPL-GTR-190, tabulates those coefficients species by species in both directions, and gives the equilibrium moisture content a piece of wood reaches at a given temperature and relative humidity. Everything else in this section is those two tables applied to a wall.

Two numbers decide the answer and both of them are measurements, not assumptions. The first is the moisture content the boards are at when you fix them, which takes a minute with a resistance meter pushed into a freshly cut end away from the surface. The second is where they will settle, and for a board on the outside of a building in a temperate maritime climate that is somewhere in the mid-teens, swinging several points between a wet February and a dry August. BS EN 335, Durability of wood and wood-based products — Use classes, is the document that describes that exposure; Eurocode 5, BS EN 1995-1-1, describes the same thing from the structural side as service class 3. What matters on site is that the delivered figure and the in-service figure are rarely the same, and that the sign of the difference tells you which failure you are designing against.

Boards delivered dry — out of a heated store, off a kiln schedule aimed at joinery, or simply stacked indoors over a warm month — will swell. That is the case with no forgiveness in it, because swelling has nowhere to go except into compression, and compression comes out as cupping, as a split from a restrained fixing, or as a whole elevation that bows off the battens. Boards delivered wet — air-dried stock, a pack that sat uncovered in the yard, green oak or chestnut by intention — will shrink, and shrinkage is the visible failure rather than the structural one: joints open, tongues pull clear of grooves, a shiplap rebate that looked tight in March shows daylight and the black of the membrane behind it in July.

Sawing pattern is the other half of the coefficient and it is worth asking the supplier about. A flat-sawn board moves across its face at the tangential rate, the larger of the two; a quarter-sawn board moves at roughly two thirds of that and cups far less, because the growth rings run through the thickness rather than around the face. Dense hardwoods move considerably more than construction softwoods in both directions. Where the pack is mixed and nobody can tell you how it was cut, take the tangential figure — the price of being conservative is a slightly wider gap that nobody will ever notice.

How deep the cavity is, and how much of each end is actually open

The cavity behind timber cladding is doing something the same cavity behind a fibre cement or a masonry skin is not asked to do: it is drying the back face of a material that swells. A board coated on the front and open at the back, with still air behind it, dries asymmetrically and cups towards the drier face. Air movement in the cavity is what keeps the two faces closer together in moisture content, and it is also what carries away the water that ran down the back of the board after wind drove rain through an open joint. Depth and openness are therefore not a code box to tick; they are the mechanism by which the board stays flat.

The depth itself is a specification question with a real answer and no universal number. BS 8605-1, External timber cladding — Part 1: Method of specifying, is the document that sets out how to state it, and cladding system literature commonly asks for something around 25 mm behind vertical battens, deeper where a counter-batten grid is involved. Treat that as the figure to confirm against the system in front of you rather than the figure to carry from the last job — and note that whatever depth you choose has to survive the fixings, because a batten pulled tight over a bulge in the membrane or a proud sheathing edge has locally no cavity at all.

What actually decides the ventilation is not the gap you left but the free area remaining after the insect screen. Openings at the base and at the head both need mesh, both need it in a material that will not corrode into the cavity, and the aperture is a trade-off nobody escapes: coarse enough to pass water and debris, fine enough to keep insects out. A perforated vent strip or a mesh closure typically passes only a fraction of its gross area, and the gap that looked generous when the battens went on measures small once that percentage is applied. Do that arithmetic before you close the base, because the fix afterwards is taking boards off.

Every horizontal interruption starts a new cavity

A ventilated cavity is only continuous until something crosses it, and on a real elevation several things do: a closure at a window head, a cill flashing, a change of cladding direction or material, and — where the building's height, use or boundary position calls for it — a cavity barrier. Each of those is a new floor for water running down the back of the boards, and a floor with no outlet is a gutter with the ends welded shut. Every interruption needs its own discharge below it and its own inlet above it, counted along the run the same way a line of weeps is: an opening at centres, plus one at each end, plus an extra wherever the flashing stops or changes direction.

Two cavities get confused on extensions, and it is worth being explicit about which one you are counting. Where timber cladding on battens goes over an existing masonry cavity wall, the masonry cavity has its own trays and its own weeps discharging through the outer leaf, and the cladding cavity sits in front of that with its own separate set of outlets. Blocking one to serve the other is how a tray ends up discharging into a batten void. And the fire question is separate again: in England, Approved Document B requires concealed spaces to be closed at their edges and subdivided, which is what puts cavity barriers into a cladding cavity in the first place, while regulation 7(2) of the Building Regulations restricts combustible materials in the external walls of certain relevant buildings above eighteen metres — a different world from a single-storey garden room, but the reason the question is now asked on every job. Proximity to a relative boundary also changes the surface classification the cladding itself has to meet, and that one does catch garden buildings.

Vertical boarding, and the batten that covers the joint

Board and batten is the profile that solves the movement problem by design, and it is worth understanding why rather than copying the pattern. The wide under-boards are fixed with a single central row of fixings, so each one is pinned at its centreline and free to grow or shrink symmetrically about it. The cover batten is then fixed through the gap between two under-boards, into the framing behind, never through the under-boards themselves. Get that wrong — one nail through the cover batten into each of the boards it covers — and you have built exactly the clamp described at the top of this article, with the added indignity that the split is hidden under the batten until the batten falls off.

The gap between under-boards is a design dimension, not a fitting tolerance. It has to be wide enough that a swelling board does not close it and start lifting the batten, and the cover batten has to be wide enough that a shrinking board does not pull its edge out from under the batten and open a line straight through to the membrane. Board-on-board is the same logic with a wider overlap instead of a batten, and it is more forgiving of shrinkage for exactly that reason. Either way the arithmetic starts with how many boards fit across the wall and how many cover pieces that implies, and the answer is one more cover piece than boards, because the outer edges each need one as well as every seam between.

Vertical boards also change what the cavity needs. Battens for vertical boarding run horizontally, and a horizontal batten laid flat on the membrane dams the cavity: water runs down, meets the batten, and sits. The fix is a counter-batten grid — vertical counter-battens first, horizontal battens over them — which restores the drainage path and the vent path and costs a second layer of timber and a deeper cavity. Notching or shimming a horizontal batten is the shortcut, and it is the detail that gets skipped when the programme tightens, on the elevation where it matters most.

Buying it: the face width is not the cover width

Timber cladding is quoted by the linear metre or the square metre of a profile, and the trap is which square metre. A 145 mm shiplap board covers noticeably less than 145 mm once the rebate is engaged; a feather-edge board covers its width minus the lap you chose; tongue-and-groove loses the tongue. Ordering off the net wall area at the face width leaves a job short by whatever percentage the profile takes back, and it is a percentage large enough to matter over a whole building. Get the net area first, then divide by the cover width rather than the face width to get linear metres, and only then start adding for waste.

Waste does not behave like waste on a sheet product either. Random-length packs need a cutting plan or they generate offcuts too short for anything; fixed-length packs waste at every wall whose length is not a multiple of the board. Boards with a defect — a loose knot on a face that will be seen, a split end, a shake — get downgraded to short pieces rather than binned, which recovers material only if somebody keeps a rack of them on the scaffold. Take gables and dormers off separately, because angled offcuts on a vertical-boarded gable are almost never reusable.

One more line that gets forgotten: if the boards are being coated on all four faces and both ends, the coating quantity is against roughly double the area you would price from the elevation, and it has to be bought and applied before the boards go up rather than after.