A Horsham stone roof is not replaced the way a tiled one is. The covering is too scarce and too valuable to send to a skip, so when it fails the usual remedy is to strip it, sort every slab, repair what lies beneath and lay the sound stone back. The trade calls that relaying, or strip and relay.
Done well, it returns the roof to service for another long stretch and keeps the building's original material. Done badly, it can break slabs by the dozen, lose the course pattern that took generations to build up, and leave a roof that leaks at the first gale. The difference lies almost wholly in the preparation and the sorting, long before the first slab is laid back.
This guide follows the job from the scaffold to the ridge. It sits under our Horsham stone roofing service, and it assumes you have a roof that is in trouble but still largely made of its own stone. If the stone has mostly gone, read about the alternatives when Horsham stone is unavailable first.
Why relay instead of replace
The Stone Roofing Association's guide to Horsham roofs puts reuse of existing stone first, and the rest of the craft follows from that. The stone has already survived centuries of weather, and the roof's colour, texture and course pattern are tied up in the original slabs. New or reclaimed stone is harder to find, and reclaimed stone from other buildings is something the association says to avoid wherever possible.
There is a practical argument too. A stone roof that leaks is often leaking for reasons other than the stone itself. Rotted battens, a failed underlay or a dropped hip can let water in through a covering that is otherwise sound. Stripping the roof lets us cure those faults while keeping the slabs, which is usually a better outcome than a new covering over the same weaknesses.
When relaying is the wrong answer
Relaying does not suit every roof. If the slabs are delaminating, flaking in layers, or so fractured that few would survive handling, there may be too little sound stone to relay. And if the roof timbers have failed, stripping reveals a bigger job than the owner expected. These problems show up at the survey, and you hear about them before the quote is written.
The scale of the work also affects consent. On a listed building, repair that matches the original is treated more lightly than alteration, and a roof relaid in its own stone to its own pattern is usually in the first category. Our church stone roofs guide shows how a different approval route can apply to a place of worship.
Recording the roof before it comes apart
The first job on a stone roof happens before any slab moves. We photograph each slope in detail and make a drawing or a note of how the courses run. A stone roof is not laid in tidy, even rows, and once the slabs are down, the pattern cannot be rebuilt from memory.
The slabs on a Horsham roof come in random sizes. The giants sit low down along the eaves, and they shrink steadily as the roof climbs towards the ridge at the peak. That arrangement is called diminishing courses, and the height of each course changes gradually over the length of the slope. We record where the changes happen, how the hips and valleys are shaped and where any shadow slates sit.
Single lap or double lap
The roof is laid one of two ways. In the double lap system, each course overlaps the course beneath it, as on most tiled and slated roofs. Single lapping is the Horsham speciality: slabs butt together side by side, and slips of slate bedded in mortar protect each joint. Many old roofs combine the two after generations of repair, and we identify which parts are which.
The Stone Roofing Association would like double lap kept or restored wherever it can be. Converting a roof from single lap involves a fresh look at loading and stability, which in turn needs building control approval. On a listed building, that change needs listed building consent, while a Church of England church in use needs a faculty instead. For that reason we record the existing laying method and keep to it unless there is an agreed reason to change.
The record also protects you. If a question comes up later about whether the roof was returned as it was, you have photographs of the before and after.
Stripping, stacking and sorting the stone
Stripping starts at the ridge and works down, slope by slope. Stone slabs are heavy, uneven and brittle at the edges, so each one is lifted by hand, checked as it comes off and carried to a sorting area rather than thrown. The roof is protected from rain while it is open, with sheeting fixed over the areas stripped that day.
The sorting is where the roof is won or lost. Each slab is judged on three things: size, condition and which course it came from. Sound slabs are stacked in groups by size, so the right piece for each course can be found quickly once laying starts. Slabs that are cracked right through, or breaking up in layers, are put aside as waste. Chipped corners may not matter if the damaged part will sit under the course above.
Keeping the roof's own stone together
The association ranks the roof's own slabs ahead of every other source, and a good team goes out of its way to follow that. A cracked large slab can sometimes be trimmed and used higher up the roof, where smaller pieces belong. That is a different thing from bringing in slabs from elsewhere, and the roof keeps its own character.
We keep a count of slabs by size group, so that we know early whether the roof will have enough. If it will not, the shortfall is raised before the roof is closed up again. The association says new Horsham slates are to be preferred where they are available, and that the source of reclaimed stone should always be checked if it cannot be avoided. Limestone slates should not be mixed with the sandstones.
Slab size groups also feed our guide to stone slate sizes, which explains the range you can expect.
Battens, underlay and the structure beneath
With the stone off, the roof is open and the structure can be read properly. The Horsham conservation area appraisal calls for steep pitches and stout framing under Horsham stone, because of its considerable weight, and the framing are what a stone roof stands or falls on.
We check the rafters, the sloping timbers that run from ridge to eaves, for rot, insect damage and movement. We look at the purlins, the horizontal beams that carry them, and at the wall plate where the roof sits on the wall. A sagging line in the old roof is a warning. If the timbers have moved, laying stone back onto them simply repeats the problem, so repairs to the frame come first. Our guide to sagging stone roof timbers sets out the signs we look for.
Battens and what goes beneath them
Battens are the thin timber strips fixed across the rafters, and the slabs are hung from them. Old battens are often split, rotted or fixed at the wrong spacing for the course heights, and since the slabs vary in size, the spacing is set to suit each course. We replace them where they are in poor condition, and we set the new lines out from the course record made before stripping.
The layer beneath the battens is the underlay, a membrane laid under the covering as a second line against water. Whether a roof has one, and which type suits it, depends on the building and on any conservation requirements, so we agree it with you at the quote stage. If the work changes the roof's loading significantly, building control approval may be needed, and we explain what applies.
Timber repair and batten replacement are not glamorous, but they decide how long the relaid roof will last.
Laying the stone back
Laying begins at the eaves, with the largest slabs, and rises course by course to the ridge. Each course is chosen from the sorted stacks, matching the slab size to the height the course needs. The aim is a roof that looks as the old one did, with the same rhythm of sizes and the same gradual shrinking towards the top.
The slabs are fixed to the battens, and the joints are made weathertight by the lap in a double lap roof, or by slate shadows in a single lap one. A shadow slate is a slip of Welsh slate bedded beneath a joint, hidden from view. The old repairs recorded in the conservation area appraisal left some roofs partly in each system, with shadow slates where stone was short.
Using mortar sparingly
Where mortar is used, it should be sparing. The Steyning conservation area appraisal lists mortar-jointed relaying of slab roofs as an unsuitable repair, and the same attitude applies elsewhere. A roof glued together with cement cannot move or breathe, and it is hard to repair. See our short answer on mortar on stone slates for the detail.
Hips, valleys and the ridge are the last stages, and the most skilled. Hips need slabs cut to turn the corner, valleys need lead or stone shaped to carry water between two slopes, and the ridge needs a covering that is firmly bedded and fixed. We do not guess these details: they follow the record taken at the start.
When the roof is finished, we check it from inside and out, and we photograph the completed slopes to compare with the originals.
Cost, consent and what to expect
The price of relaying depends on factors that only a survey can weigh: the size and height of the roof, how easy it is to reach, how much of the stone is sound, the state of the timbers and whether the building is protected. A national cost guide quoted on our stone slate roof cost page gives a typical UK range for stone slate roofs, though the figure covers stone slate in general and not Horsham stone alone.
After the survey you receive the quote in writing, as one overall price. Wherever a price comes from, find out if VAT and scaffolding are inside it, as both alter what you pay. A scaffold standing on the public footway needs a licence, which the scaffold firm obtains from the highway authority.
Consent and building control
A like-for-like repair done in place does not normally need listed building consent, according to the South Downs National Park Authority, but alterations or the complete replacement of historic material that affects a listed building's character do. A change from single lap to double lap, or the swap of stone for another covering, counts as alteration and needs consent. In conservation areas and under an Article 4 direction, ask the planners before you start.
Roof work that alters the structure or its loading may need building control approval, and we talk you through what applies, then deal with the paperwork if the job is notifiable.
Plan on the roof being open for some days at a time, with weather protection in place. You will see the stone sorted and stacked on site. At the end the rubbish goes with us, and any sound slates left over are stacked for you to keep or taken away, whichever you prefer.
