Tongue and Groove Roof Boards: Which Way Up and How to Lay Them Tight
For the roof of our log cabin we used 22 mm (7/8 in) rough-sawn tongue and groove boards, planed on one side. Unfortunately, it is not enough to simply slot the boards together and nail them down: timber shrinks as it dries. If the boards are laid too loosely, the tongue pulls out of the groove of the next board and gaps open up. That does not just look bad – it also weakens the roof deck, which matters especially on a low-pitch pent roof. Here is how we laid our roof boards so that they stay tight.
Quick answer:
- Which way: start at the eaves with the groove facing down the slope; the tongue points up towards the high side.
- Which side up: if the underside stays visible inside, lay the planed face down.
- Tight joints: use dry boards and press them together hard with clamps and wedges before nailing.
- Nailing: two nails per board at every rafter – we used 60 mm (2 3/8 in) lost-head nails.
- Not too many at once: wedge no more than four boards together, otherwise they bow upwards.
Why the boards must be dry and pressed tight
Roof boards may only have a certain residual moisture when they go on, and they must be pressed hard against each other when you fix them. Otherwise shrinkage opens gaps between the boards over the following months – the tongue literally pulls out of the groove. You cannot fix that later without taking the roof deck apart.
An alternative to boards is OSB sheathing: it is quicker to lay and has held up pretty well on our carport roof (guide in German).
What you need
- Board clamps that hook onto the rafters (two of them, one each on two rafters)
- Timber wedges – we cut ours from roofing battens
- An offcut of the same board with the tongue cut off: it sits on the tongue of the last board and protects it while you wedge
- A hammer and 60 mm (2 3/8 in) lost-head nails
Laying the boards step by step
- First board at the eaves. Align the first board exactly along the eaves side and nail it to every rafter with two nails. The groove faces down, towards the low side of the pent roof; the tongue points up the slope.
- Add the next boards. Lay the next few boards on the rafters and push groove onto tongue.
- Fit the protective offcut. Slide the offcut – tongue removed – onto the tongue of the last board. It stops the tongue from being crushed when you drive the wedges, and without its own tongue the offcut lets the wedges apply more pressure.
- Drive the wedges. Place two wedges between the clamp and the offcut, as in the photo, and tap them against each other with light hammer blows until they put strong pressure on the boards. Do this at two rafters at the same time.
- Nail. With the boards pressed tightly together, nail every board to every rafter with two 60 mm lost-head nails.
- Repeat with the next three or four boards – never more.
Don't press too many boards at once
It is tempting to lay six or more boards and wedge them all together in one go. Don't: the pressure can make the boards in the middle bow upwards. In our experience, four boards at a time is the maximum.
We also nailed the boards to the top of the log walls wherever they cross them. That way the roof deck acts as a stiff horizontal plate that adds to the stability of the whole cabin.
With the boarding finished, the roof edges were trimmed and we could cover the roof with two layers of bitumen felt.
Frequently asked questions
Which way should the groove face on tongue and groove roof boards?
Start at the eaves, the low side of the roof, with the groove facing down towards the eaves and the tongue pointing up the slope. Each following board is pushed with its groove onto the tongue of the board below.
Which side of planed-one-side roof boards goes up?
The roof covering goes on the upper side, so the face you see later is the underside. If the underside stays visible inside the cabin, lay the planed face down. If you line the ceiling anyway, it makes little difference.
Why do gaps appear between tongue and groove boards?
Timber shrinks as it dries. If the boards are too wet or are only pushed together loosely, the tongue pulls out of the groove as they shrink and gaps open up. Use dry boards and press them together hard before nailing – we used clamps and wedges.
How many tongue and groove boards can you press together at once?
No more than about four. If you wedge six or more boards together, the pressure can make the boards in the middle bow upwards.
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