How are stone slates fixed?

1Top hung on pegs or nails

A stone slate does not sit in a bed of cement. It hangs. A hole is made near the head of the slab, and a peg or nail passed through it holds it to the timber behind.

The Stone Roofing Association (SRA) sets out in its Horsham guide that traditionally each stone-slate was top hung with wooden pegs on split wooden laths. That top hanging was essential when the laths were thin and split from the log. The usual modern practice is to nail to substantial sawn battens instead.

Because the slab pivots on that single point, everything depends on the hole, the fixing and the timber behind it. A peg that has rotted, a nail that has corroded or a batten that has softened lets the stone slip, and that is a common start to a leak. It is also why a slipped stone should be reported promptly: the slab itself may be sound, and only the fixing has failed.

2Which metals the guide prefers

For nailing stone-slates the SRA says to specify non-corroding nails, preferably copper, with a shank diameter of 4.5 mm or larger. Length matters too. The shank must go deep enough into the batten to hold, but not so far that it touches or pierces any underlay or vapour permeable membrane.

Pegs follow the same rules. They can be wooden or a non-corroding metal. Copper is the preferred metal for pegs and nails, while stainless steel and aluminium are also acceptable. The guide adds that aluminium should not be used in contact with mortar.

Nails for the shadow slates beneath, covered in our page on shadow slates, can be thinner, at about 3.0 mm.

We work to this guidance on Horsham roofs, and if a roof calls for something different we say why before the work starts.

3Holes, wooden pegs and worn fittings

Old roofs bring their own problems. The holes in reclaimed or ageing slabs are often larger than the peg that was first used, and a small modern fixing can tilt or work loose in them.

The SRA has a neat answer. For metal pegs, new holes should be slightly larger than the shank, and in an old, wide hole a nail or peg with a large head can be used, because the head locks against the surface of the slate. With wooden pegs, small amounts of mortar around the peg help hold it in place.

The guide also warns that the shank must reach to the back of the batten without touching any underlay. Where an underlay or membrane is present, counter battens may be needed to give clearance, which changes the build-up of the roof.

When a stone has to be reset, we check the peg hole first and the fixing second. Reusing stone from other roofs raises the same questions, and our page on reclaimed Horsham stone goes into where it comes from.

4The battens behind the stone

A fixing is only as good as the timber holding it. The SRA describes battens as sawn wooden supports for hanging or nailing, typically 50 x 25 mm, with 75 x 25 mm preferable for larger slates. Stones laid single lap span a bigger unsupported distance than double lap, so for the larger sizes extra battens can add intermediate support.

Batten fixings are a different matter from slate fixings. For fixing the battens themselves the guide suggests galvanised or sherardised nails with round or ring shanks, or stainless steel in some cases, typically 65 x 3.35 mm for battens up to 25 mm thick.

Damp battens are a weak point in any slate or stone roof, with consequences for the fixings in the course above. If bedding mortar reaches the batten, moisture is drawn to the timber and rots it, which in turn loosens every fixing along that course. That link between mortar and rot is covered in our answer on a leaking stone roof.

5What we look for on a survey

From the loft we check the battens for soft or stained timber and for pegs and nails that have worked out. From the roof we look for stones that have dropped at the tail, and for fixings showing rust or green staining where copper has weathered.

A repair may need nothing more than one slab refixed in its proper place. Where several courses have lost their hold, the roof may need lifting and relaying, with the stones sorted, holes checked and the fixings renewed to the specification above. Which of the two applies depends on what the battens are like and how many slabs are loose.

After surveying, we send photographs of the loose or rusting areas along with one written total for the job. Our Horsham stone roofing page explains the wider service, and we always say which repair will deal with the cause.