Why do stone slates slip?

A Horsham stone slab is thick and heavy, so it takes real force to shift one. Nothing moves it unless the fixing has given up. Each slab hangs from a peg or nail near its head, which in turn rests on a batten, the timber strip that crosses the rafters. If that chain breaks anywhere, the slab creeps down the slope until it wedges against the course beneath.

A slipped slab is therefore a symptom, a sign that something underneath has failed. The question worth asking is not how to put the slab back but why it came away, because a slab hung again on the same perished peg or soft batten will go again at the next blow. Treat the visible slip as a clue, and read it the way a doctor reads a rash: the cause lies somewhere under the surface, and the repair has to reach it. Skipping that step is the commonest reason a stone roof repair fails within a year or two.

The first is the peg. Oak softens over the decades and nails rust through where damp lingers; our page on oak pegs on a stone roof shows how they work. The second is the batten: a split or soft one cannot hold any fixing, however good. The third is the stone itself, because water and frost can flake the slab around its hole until the fixing pulls through, a problem our guide to frost damage on stone roofs describes.

The fourth is movement. A roof frame that has spread or sagged alters the angle the slabs lie at, and the fixings are asked to pull in directions they were never made for. A run of slipped slabs all going the same way is a strong hint of that, and it deserves a look at the timbers before any stone is touched. Rafters that have spread at the foot, or a ridge that has dipped, will show up when the roof is sighted along its length.

From the garden, look for a slab sitting lower than its row, a dark gap above it or a slab tilting outwards. Binoculars on a calm day beat guesswork, and a phone picture zoomed in from the ground can be sent to us for a first opinion. Stains on a ceiling or daylight in the loft point to the same fault from inside the house.

Please do not go up to look, and do not try to shove a slab back. Stone is heavy, and one that has already slipped can drop further without warning. Check the ground below before anyone walks there, and keep children and pets clear if a slab hangs over a path or doorway. A slipped slab beside a gutter or valley matters more than one in the middle of a slope, since water runs straight into the gap. When rain is already getting in, stopping it comes first, and the lasting repair is planned afterwards. Our page on scaffolding for stone roofs covers how the roof is reached.

First the slipped slab comes off, along with its neighbours, which uncovers the battens and fixings. Any batten that has gone soft is replaced, a fresh fixing goes in and the slab is laid back into its own course. Where the slab itself has cracked or lost the edge of its hole, it is exchanged for a matched slab, ideally reclaimed. A lead tingle, a small strip of lead folded over a batten to hold a slab up, is sometimes used as a discreet extra grip where a hole has broken out.

The survey records each slab that has shifted, including those well away from the stain, so you can see the full run of the problem in pictures. Our answer on cracked stone slates covers what happens when the slab itself has failed rather than its fixing.

A single slipped slab on an otherwise sound roof is a repair. Several in a line, or a pattern across a slope, means the fixings or battens are failing over a wider area, and patching them one at a time only chases trouble around the roof. That is the moment to talk about a re-lay, which strips the slope, mends what lies beneath and lays the sound stone back in order. Our guide to repair or re-lay for a Horsham stone roof sets out the choice.

Either way, what you receive is a plain recommendation, pictures from the roof and a single written total. For the wider range of work on these roofs, see our Horsham stone roofing page.