Horsham stone slabs are heavy, rough-edged and laid by hand, and a roof of them can stand for centuries on strong timbers. Its faults are therefore local. One slab drops a few millimetres, a joint beneath it opens, and water finds a way down the face of the stone and into the roof space. The repair has to follow the same logic: find the single place where the roof has moved, and put back only what has failed.
That sounds simple, but a stone roof is easy to damage in the name of fixing it. Cement poured into joints, bitumen strips under slabs and whole slopes taken apart to cure one drip all leave a roof worse off than before. A stone roof also has history written into it, in the sizes of its courses, the slate shadows under its joints and the patches of earlier owners.
This guide covers what goes wrong, what a proper repair involves and what to avoid. It sits within our wider Horsham stone roofing work, and it is written for owners of houses, barns and outbuildings in and around Steyning, Storrington, Henfield, Lancing and the Weald villages.
Where a stone roof starts to let water in
Stone slabs shed water by overlapping, and by the weight and roughness of their surface, not by any membrane. That arrangement fails at particular spots, and they are worth knowing. The first is a slipped slab. When a stone shifts down its slope, it uncovers the joint beneath and exposes the underside of the course above, and rain runs straight in. The second is a cracked slab, which splits across and lets the lower half drop. The third is the junction with something else: a chimney, a valley, a wall, a dormer cheek, where lead or mortar has to bridge the gap.
Look out for a few signs from ground level. Slabs that seem to sit at a different angle from their neighbours, gaps of daylight along a course, a dark stain under one particular stone, and fallen fragments in the gutter or on a path. Inside the loft, daylight at the joints, damp patches on timber, or dust and grit that has fallen from above all point to movement.
Sagging and spreading
A slope that dips between the rafters, or a ridge that has lost its straight line, is a structural question as much as a stone one. These roofs rely on strong timbers to carry the weight of the stone, as the Horsham conservation area appraisal notes. If a roof has dropped, the cause may be rot or movement in the frame, and no amount of resetting slabs will cure it. We look at the timbers from the loft before suggesting anything on the slope.
Damp that comes and goes with the weather is the clue to follow. Our guide to a leaking stone roof goes through how we trace it.
Slipped and cracked stones: reset, replace or relay
The decision about a failed slab is one of three: put it back, swap it for another, or rebuild the area around it. The least invasive answer that works is nearly always right.
A slab that has slipped but is sound can be lifted, the bed beneath it checked, and the stone set back in position and secured so it cannot move again. The cause matters here. If the fixing that held it has corroded or the timber it was hung from has decayed, putting the slab back in the same place without dealing with that simply repeats the fault. A cracked slab is different. A crack running across usually means the stone has reached the end of its useful life, though a small chip at a corner can be left alone if the overlap still does its job.
Where a stone has to be replaced, the first source is the roof itself. A stone from a hidden area, such as under a chimney flashing or from the back of a lean-to, can be swapped into a visible position, and a reasonable stone replaces the poorer one in the hidden place. If that is not enough, the stone has to come from elsewhere, and the supply position is covered in our guide to the Horsham stone shortage.
When a whole area needs relaying
Relaying means stripping a section, sorting every stone by size, checking the battens or laths and the timbers beneath, and laying the stone back in order. It is justified when several neighbouring slabs have failed, when the structure below is unsound, or when earlier repairs have left an area that is leaking at multiple points. It is heavier work, and it needs scaffolding, so we only recommend it when the survey shows a smaller repair would not hold.
Shadow slates and where they belong
Shadow slates are small, thin pieces of Welsh slate set under the joints between stones. On a single lap Horsham stone roof, the Stone Roofing Association describes them as the means by which joints are weathered, bedded in mortar. In that setting they are part of the original build and they are meant to be there.
The detail causes confusion because shadow slates also appear on roofs where they do not belong. The Horsham conservation area appraisal observes that past repairs have mixed double and single lap, adding shadow slates wherever stone was in short supply. A roofer who understands the roof can tell the original from the improvised, and the difference changes the repair.
Where shadow slates are part of the original single lap system, a repair keeps them, replacing any that have slipped or broken with the same kind of material, bedded the same way. Where they have been added to make up for missing stone, the better repair may be to restore the proper lap if stone can be found, because the association's preference is double lap, kept or brought back where it can be. That is a bigger piece of work, and it has consequences for loading and consent.
What not to do with them
Do not allow anyone to slide a strip of slate or felt under every joint as a quick fix for a leaking stone roof. It may stop a drip for a season and then trap damp against the stone and the timber. A shadow slate in the wrong place hides a problem rather than solving it. The overall pattern of courses matters too, which is why the way stones are graded is explained in our guide to diminishing courses.
Why mortared joints are the wrong repair
A cement-based mortar fillet across a stone joint looks like a neat repair, and for a short time it sometimes works. It is the wrong answer on a Horsham stone roof, and the council appraisal for Steyning says so directly. It lists mortared joints on relaid Horsham slab roofs among the repairs it considers inappropriate.
The reasons follow from how the roof works. A stone roof is meant to shed water by overlap, not to be sealed like a bathroom. Mortar locks the slabs together in a rigid layer. When the timbers shift, a hard joint tends to crack, and once it has cracked it lets water behind itself and holds that water against the stone, so the repair has made the damage worse.
Mortar can also hide movement. A joint filled in mortar looks tight from the ground while the slab beneath it is still slipping. And it is hard to take out later without chipping the stone, which is a real loss when stone is short.
What proper joint repair looks like
If a joint is open, the repair is to set the slab correctly so the overlap does the work, and to use the detail the roof was built with, whether that is a slate shadow bedded in the right material or a proper double lap. Where mortar was part of the original build, as with the bedding of a shadow slate, it is used as the original builders used it, in the place it belongs, not smeared across the face.
If a previous owner or contractor has already filled the joints with cement, we show you where on the survey photographs, and we say what removal would involve.
Weight, structure and what a repair can change
Stone is heavy, and the timbers under it were built for that. The Horsham conservation area appraisal describes steep roofs carried on strong timbers built for substantial weight. We do not quote a weight or a minimum pitch for the stone, because each roof is judged on its own frame.
That matters because any repair that changes the covering can change the load. Swapping to a lighter or heavier material alters what the timbers carry. Moving from single lap to double lap, which the Stone Roofing Association favours wherever possible, means reviewing the loading and the stability of the structure, and building control approval will be needed, along with a church faculty if the building is a church.
Building control is also a point to raise if the repair becomes a re-cover. The Planning Portal sets out when re-roofing needs an application, and heavier coverings are covered by a 15 per cent test, which we explain in the survey where it applies. A plain repair in matching stone does not usually raise these questions, but a roof where large areas are renewed might.
Consent on listed and protected buildings
Many stone roofs belong to listed houses, churches or buildings in conservation areas. The rule of thumb is that repairing like with like, in place, is generally acceptable, while changing the character of a listed roof needs the council's consent. Our guide to listed building consent for stone slate roofs sets out the legal position, and consent is explained at the survey, with the Building Regulations notification dealt with by us where work is notifiable.
If you are not sure whether your building is listed, the Adur & Worthing planning team or your own council can confirm it.
What we show you and how the work is priced
The survey for a stone roof takes in the slopes from a ladder or access tower, the loft from inside and the details at chimneys, valleys and abutments. We look for the cause of any water, not only for the stain, because on a stone roof the damp patch often sits a long way from the opening.
The survey gives you pictures of the faults on your own slope, and our written quote afterwards covers the whole job in one figure. The photographs show the slipped stones, the cracked ones, the joints that have been filled with cement and any timbers we are worried about. The quote names the repair we recommend and, where there is a choice, explains the alternatives. Our aim is a repair sized to the fault, not the biggest one that can be sold.
What moves the price is the access needed, how many slabs need resetting rather than replacing, what stone has to be sourced, what is found under the surface and whether the work is listed. Pricing follows the survey, and we set it at a good, fair level.
Finishing the repair and clearing the site
When the job is finished, the roof and site are left clear, and the rubbish goes with us. Any sound stone that has come off is kept for you where it can be reused. Slabs too far gone to reuse are set aside so the reason they failed is plain to see.
Where water is coming in at this moment, we deal with the active leak first by making the roof safe, then plan the lasting repair once the weather allows. Where you are unsure whether the problem is the roof or something else, tracing a stone roof leak sets out the steps we follow before any repair is priced.
