Most roof coverings are laid one way. Horsham stone has two. The Stone Roofing Association records two methods: the standard double lap, and a second, single lapping, which it describes as peculiar to Horsham stone roofs. Anyone repairing, relaying or buying a house with a stone roof meets the distinction sooner or later.
The difference can pass unnoticed from the pavement. Both systems use slabs of random size, graded down the roof, and both weather into the same broken grey-brown surface. Close up they rest on different principles, and they ask different things of the timbers underneath.
The distinction matters most at the moment someone proposes to turn one into the other. The Association recommends keeping or returning to double lap wherever possible, and it says that converting from single lap to double lap means the structure must be reassessed first.
This guide takes each system in turn, then explains the advice, the loading question, the consents that may follow and how to read a roof that has been altered over the years.
How a double-lap stone roof sheds water
Double lap is the arrangement that comes to mind for an ordinary pitched roof. Each slab is laid so that it overlaps the courses below it, and its joints sit over the middle of the slabs underneath. Water that finds a joint meets a second thickness of stone before it can reach the underlay or the timbers.
The Association calls this the normal system, and on Horsham stone it still follows the local habit of grading. The slabs at the foot of the slope are the broadest and heaviest, and the courses shrink as they climb, so that the roof is a graded cover in which every joint is protected by what lies beneath it.
The overlap is why double lap is regarded as the more forgiving system for weatherproofing. The roof does not depend on any single joint holding, because each one is backed up by stone. It also gives a roofer a clear logic to follow when sorting slabs: pick for size, then lay with the overlap the pitch and the exposure require.
What this means for repair
A slipped or broken slab on a double-lap roof can often be replaced individually, by easing out the damaged piece and slipping a new one into the same position with a fixing. The surrounding courses stay as they are. Our guide to replacing one stone slate covers that small repair, which is a common reason for a call.
Because the system is so repairable, the Association's preference for it is easy to understand. It is the form that treats the roof most kindly over the long run, and the one a surveyor can read most readily when something goes wrong.
How single lapping works, and where the shadows come in
Single lapping is the system the Association describes as peculiar to Horsham stone. Instead of each slab overlapping the one below as a matter of course, the slabs are laid in a way that leaves joints between them, and those joints are covered from beneath by Welsh slate pieces called shadows, or shades, set in mortar.
Each shadow is a thin strip of slate that sits out of sight under a joint, carrying water across the gap and onto the stone below. The word describes how it hides beneath the slabs, doing the work of a second layer exactly where a gap would otherwise let water through. Our short answer on what shadow slates are goes into the detail.
The consequence is that a single-lap roof relies on a different set of details. The weathering is part stone and part slate and mortar, so the condition of the shadows and their fixing matters as much as the condition of the slabs. When the mortar fails or a shadow slips, the joint above it is the first place water gets in.
None of that makes a single-lap roof a poor one. Many have lasted a very long time, and a sound one is better left as it is. It means that repairs need a roofer who recognises the system and does not apply double-lap logic to a roof that was built differently.
In practice, the sign we look for from outside is a slab edge with a darker line of slate or mortar tucked under it. Inside the loft, the underside of the roof shows the same arrangement from the other direction, and photographs taken there become part of the survey record that goes to you alongside the quote, so the evidence is in your hands.
What the Stone Roofing Association advises
The Association's guide is direct. Wherever possible, it recommends, a roof should keep its double-lap form or be returned to it. That advice has three parts, and each one matters for an owner.
Keeping means that where a roof is already double lap, it should stay that way. The sensible repair respects the existing system, replaces what is broken and leaves the structure of the covering alone, even when a tidier-looking alternative is tempting.
Returning means that where a roof has been altered, for example by a past repair that converted part of it or covered it differently, the double-lap form is the one to go back to, if the roof can be taken back to it. A survey records which parts of a roof are original and which are not.
Wherever possible is the qualifier, and it is a real one. Some roofs were built single lap, with the timbers, pitch and details to suit, and the practical answer for them is to repair them as single lap. The Association is not telling owners to rebuild every single-lap roof. It is saying that where there is a genuine choice, double lap is the preferred form.
You can read the guidance for yourself in the Stone Roofing Association's Horsham stone roofs guide. It was published in 2009 as a second edition, describing the best practice of 2007, and we treat it as the reference on method.
Why changing system means looking at the loading
The Association says that converting a single-lap roof to double lap makes it necessary to reassess how much weight the frame can safely take. This is the part of the advice owners rarely expect.
A double-lap roof uses more stone for the same area, because each course overlaps the next. More stone is more weight. The frames that carry Horsham stone are already heavy-duty, since the council's appraisal for Horsham notes stone roofs on steep pitches with strong timbers under them, but strong is not the same as unlimited. A change in the covering is a change in the load.
We do not give a weight figure for Horsham stone, nor a minimum pitch, because neither is published in the guidance we work from and an invented number would be worse than none. What a survey can do is open the roof frame, look at rafters, purlins, wall plates and any past repairs, and record what is there. The assessment itself then belongs to someone qualified to carry it out, usually a structural engineer or the building control body.
What the survey looks for
On our visit we look for sagging between supports, rafters that have been cut or notched, signs of past movement at the wall plate and timber that has been weakened by damp. Our guide to sagging stone roof timbers explains what those signs mean.
If the roof is simply being repaired within its existing system, the loading does not change. The assessment matters when the system does, and it is far better done before stone is ordered than after it arrives.
Consent when the system changes
The Association also sets out what follows a conversion to double lap. Building control must approve the work, and a church needs a faculty on top. If the building is listed, listed building consent is required from the local authority.
This fits a wider pattern. Planning rules are gentler on repairs that copy what is already on the roof. Converting the lapping system is a change of covering, and the rules around listed buildings and Article 4 areas are stricter still. The practical step is to speak to the right council before anything is ordered. That is the Horsham District Council planning team for Steyning, Henfield and Storrington, the Adur & Worthing planning team for the coast, or the South Downs National Park Authority where the Park is the planning authority.
Churches are a separate case. The exemption that lets some denominations run their own consent system covers the Church of England, which works through the faculty process, and several other major churches. A church that is no longer in use for worship needs listed building consent in the ordinary way. Our guide to church stone roofs covers that in more detail.
For a typical house, the sequence is simple. A survey establishes the system and the condition. If the work is repair within that system, check with the council where listed status or an Article 4 direction applies. If the system is changing, loading, building control and consent all come first. We tell you which consents apply and deal with the Building Regulations notification when the job needs one.
Reading a roof that has been altered over the years
Many Horsham stone roofs are not purely one system. Council appraisals record roofs where repairs have left both methods side by side, with slate shadows appearing in places where the stone ran short. That describes a roof that has been mended in stages by people with different materials to hand.
The shadow slate in particular is a sign. It records a decision made in the past, which a new owner can see and weigh. Some of those decisions were sound and have lasted. Others were quick fixes that now need attention.
When we survey an altered roof, we map it. We photograph each slope, note which areas are double lap, which are single lap and which are patches, and record the condition of the stone, the shadows and their fixing. Then we explain which parts to leave alone, which to repair and which, if any, are worth restoring to the double-lap form the Association prefers.
For a roof that needs stone it does not have, the question becomes one of supply. Our guide to reclaimed Horsham stone and the page on Horsham stone supply explain the options. For the material itself, what Horsham stone is is the place to begin, and the Horsham stone roofing hub ties the topics together.
Send us photographs of your roof through the quote page and we will arrange the survey. Our quote is sent in writing after the survey.
Whichever system you have, the aim is the same: find where water gets in, fix that, and leave the rest of the covering as it was built.
