How do oak pegs hold a stone roof together?

Look closely at an old stone slate lying on the ground and you will see a small hole near one end, usually neat and round, worn a little by a century or more of weather. That hole is where the peg went. Cotswold District Council's technical guidance sets out the method: a wooden peg, occasionally a pair, goes into a hole at the head of the slate, and the pegged slate is then hung over the batten.

Put plainly, the peg acts as a hook, and the slate hangs from the batten much as a coat hangs from a rail. The slate below overlaps the hole of the one above, and the weight of each course helps keep the others in place.

There is no nail through the face of the slate, which means no hole letting water in from above. It also means the whole covering depends on timber that the roofer cannot see once the roof is finished, and on the pegs doing their job for decades.

The same guidance says that oak was usually cleft to form a peg. Cleaving, or splitting along the grain, produces a peg that follows the natural run of the fibres. A sawn peg cuts across the grain in places and snaps more easily under load. A cleft one is tougher for its size.

Oak suits the job because it is dense and holds up well to damp. It is also a timber that would have been to hand in the country where these roofs were made. Where a particular building turns out to carry something different, we record what is actually there and match that.

For a roof that is being repaired in the traditional manner, matching the material of the pegs is part of keeping the roof consistent. A quote that speaks of fixings should say what the fixings are.

The usual failures are rot, splitting and loosening. A peg that sits in a roof which has leaked for years can soften until it is little more than pulp. A peg may also snap at the shoulder where the slate presses on it, especially if the slate has shifted. Over time a peg can work loose in an oversized hole and let the slate drop by a few millimetres each winter.

The first sign from the ground is often a slate riding below the line of its neighbours, or a gap that was not there last year. If a slate has slipped, its peg has almost always failed, even though the slate itself is fine. Slates that look sound can often be refixed rather than replaced, and that is cheaper than new stone.

It is worth knowing that the Stone Roofing Association describes peg holes as difficult to make in Forest Marble. So where a hole has broken out, a slate in that stone may be harder to save than one in the finer Stonesfield stone, and our page on the two Cotswold stones explains the difference.

The council's guide describes the traditional stone roof as a straight slope usually at 47 to 55 degrees. At that steepness gravity does a lot of the work of keeping water moving, and the pegs only need to hold each slate against sliding, with the weight of the courses above doing the rest. See our page on stone roof pitch for how the angle relates to the covering.

The covering is heavy. The guide gives about 1,000 kg per 10 square metres. All of that weight hangs from timber battens, which hang from the rafters, so the pegs and battens are a chain, and a chain is only as good as its weakest link. When a batten has softened, the pegs on it are no longer secure, whatever condition they appear to be in.

So we inspect the roof from inside as well as from outside. A loft inspection shows the backs of the slates, the pegs and any damp staining on the battens.

A good repair lifts the slates above the fault, renews the failed pegs, replaces any broken slates and rehangs the course. It sounds simple, and in the right hands it is, but it takes patience because stone slates are brittle and every slate has to go back in its own place in the diminishing courses.

Before you agree to work, ask what is being used to fix the slates and why. Ask also whether salvaged slates will return to your roof or be replaced with new ones. Our page on reclaimed and new stone sets out the trade-offs.

The slate names used on site, such as cussoms and followers, are explained on our page on traditional names.

Pegged stone is one of the older roofing systems still in use, and it has a very different character from a modern tile roof held by nails and clips. If you are weighing the two, our page on stone slate or tile compares them.

Slipped slates and a wish to know why? Photographs or a survey will tell us. The quote is written, and it arrives with photographs of the slope, and the repair we propose is the lightest one that works. Sometimes a dozen refixed slates are all a slope needs. Our Cotswold stone roofing page gives the wider picture.