Leaking stone slate roof: common causes

1Slabs that have slipped or dropped

Every stone slate hangs from a single peg or nail. When that fixing rots, rusts or works loose, the slab slides down and leaves a gap above it. Rain finds the gap, runs onto the batten below and finds its way to the ceiling.

A slipped slab is often visible from the garden as a tail sticking out lower than its neighbours, or a dark stripe where the course above no longer covers properly. After a gale it is worth looking for fragments on the ground.

The Stone Roofing Association (SRA) notes that the usual modern way of fixing is to nail to sawn battens, and that traditionally pegs on split laths were used. Either way the fixing is small compared with the weight of the stone, so it is a frequent culprit. The fixing details a repair should follow are set out under stone slate fixings.

2Single lap joints that have lost their seal

Horsham stone has a laying system found nowhere else. In single lapping the stones reach down over just one course, and the upright joints are sealed with Welsh slate shadows and bedding mortar. According to the SRA, keeping water out depends on both, and that problems including roof failure due to leakage have arisen from uncertainty about the details of the two systems and confusion between them.

That is the central risk. If a shadow cracks, slips or was cut too narrow, or the bedding mortar has crumbled away, the joint has nothing else to rely on. The SRA specifies a side lap of the stone over the shadows of between 100 and 150 mm.

A roof patched over the years may mix both systems, which can leave awkward weak spots where one meets the other, and some slopes may need more than a quick fix. On survey we identify which method each slope uses before deciding on a fix, and the repair must match it.

3Mortar details at walls, verges and ridges

Older stone roofs often meet a gable or chimney wall with a mortar fillet instead of lead flashing. The SRA says these fillets are particularly prone to cracking as the wall and the slope behind it shift by different amounts, and suggests backing them with lead soakers, one per course.

Verges need attention too. They are commonly bedded onto a slate undercloak, and the guide observes that the mortar often falls out, leaving the edge open to a crosswind. Ridges depend on bedding in much the same way, and ridges on Horsham stone roofs have their own page.

Where roofs have valleys, the SRA says they need to include lead sheet to be watertight, and that Lead Sheet Association recommendations apply. A split in that lead, or debris packed into the channel, sends water sideways under the stone.

None of these faults sits above the stain. Water that enters at a fillet can track down a rafter and appear in a bedroom several feet away, which is why the search starts indoors.

4Blocked gutters, hidden channels and trapped damp

Some stone roofs use a hidden gutter at verges and abutments instead of soakers. The SRA notes that these need regular cleaning, and a blocked one overflows into the masonry or behind the edge of the roof without any sign from the street.

Moisture can also build up from inside. The guide explains that a roof sealed with mortar, or one where insulation has been added, needs ventilation planned in to avoid condensation, and that the gappy nature of an unsealed stone roof is what normally lets it breathe. A damp patch in the loft is not always a leak from outside, so the diagnosis matters.

Moss and leaf litter make every one of these worse; we cover gentle clearing in moss on a stone roof. If water is coming through, catch it safely indoors and keep off the roof yourself.

5How we trace it and what you get

Our survey starts under the roof, with a torch, tracing stained and damp timber back along the slope to where the water gets in. We then match that to the outside, looking at the slabs, the shadows, the bedding and the details at walls and edges.

We recommend the least work that removes the cause. That might be refixing a few slabs, replacing a shadow, renewing a fillet or relining a valley. Where the roof is failing across many courses, we tell you plainly and explain what relaying involves; see Horsham stone repairs for the range.

We send photographs of the faults, followed by a written quote giving one price. Leaks are a priority, and where rain is entering the house we deal with making it safe ahead of anything else. For the wider service, visit our Horsham stone roofing page.