Question answered
Is Horsham stone a limestone or a sandstone?
The short answer
Horsham stone is a sandstone, not a limestone. In the Stone Roofing Association's words the slates are calcareous, flaggy sandstones: a sandstone base, with lime in the patches that yield roofing stone. Some writers still call it limestone, so descriptions vary. The difference is practical, because the SRA says a limestone slate does not belong on a sandstone roof.
People describe Horsham stone both ways, and the confusion is understandable. The stone is a sandstone with lime in it, and the distinction decides what can safely sit beside it on a roof.
A sandstone base with lime in the roofing patches
The Stone Roofing Association (SRA) guide to Horsham roofs opens with a plain description of the material. Its slates, also called flags, are calcareous, flaggy sandstones. Calcareous means it carries lime. Flaggy means it splits into flat, thin slabs, which is why it makes a roof covering at all.
The guide quotes a quarry manager writing in 1945. He said the base of the stone is sandstone, but that the patches which yield roofing stone contain lime, and that the lime hardens the rock into a granity stone. Roofing stone also came from only one layer, a stratum a few feet down that varied in depth from place to place.
So the short reading is this. The grains are sand, the cement holding them has some lime in it, and the geology puts it in the sandstone family. A stone can contain lime and still be a sandstone, in the same way a fruit cake can contain brandy and still be a cake.
Why the descriptions disagree
A 2019 report on the stone's supply noted that it is variously described, and you will meet both words in estate agents' particulars, old surveys and casual conversation. The lime content is the reason. A stone that reacts to acid, or looks pale and hard, gets called limestone by people who have no cause to be exact.
For a homeowner the label rarely matters in conversation. It matters when a quote, a listing description or a specification uses one word and a supplier uses the other. If a document calls your roof limestone, treat that as a prompt to ask what was actually inspected, not as a finding.
We describe the covering the way the SRA does, and we write the same wording on the survey notes. It keeps the paperwork consistent if the roof is ever discussed with a conservation officer, a surveyor or a stone supplier later on.
Why the label matters on a real roof
The SRA gives a specific warning about combining the two rocks on one slope. Its reasoning is that limestone can dissolve in rainwater and be absorbed into the sandstone beside it. As the stone dries out, crystal growth can then weaken the sandstone.
That is a repair rule with teeth. A roof that needs a few stones making up is often tempting to patch with whatever flat stone is nearest. If the patch is a true limestone, it is the wrong neighbour for Horsham slabs, however similar the colour looks from the drive.
The same guide says random metamorphic slates are too different in colour, thickness and texture to stand in for Horsham stone, and that sandstones will always be the better substitute. It names certain Carboniferous sandstones as the most promising alternatives, but warns that most are much smoother than Horsham slabs, and suggests using them only for a few slates, ideally on the less visible slopes.
How the stone becomes a roof
Horsham stone cannot be cut to a pattern the way Welsh slate can, and no two slabs are quite alike. The natural cleave has to be there, and a stone with a tight cleave is traditionally left standing in wind and frost for months so the weather helps it split. That explains the uneven surface and the thickness that varies from slab to slab.
The largest slabs go along the eaves and the courses shrink towards the ridge, a pattern covered under diminishing courses. Horsham roofs also use a single lap system in which Welsh slate pieces, known as shadows, weather the joints; see shadow slates for how they work.
Because supply of the thin roofing slabs is limited, the cost of a job depends heavily on how much stone can be reused and how much has to be found, which is why two similar roofs can price very differently. The page on stone slate roof cost goes through what moves the figure.
What we check when stone is offered for a repair
Before a slab goes on, we look at three things. First, whether it is a sandstone of the right character, using the existing roof as the reference. Second, whether it is thick, flat and sound enough to take its place in the course. Third, where it has come from, because the SRA says the source of any reclaimed stone should be checked to make sure it was obtained legitimately.
The first choice is always the roof's own stone, cleaned and re-laid. Only when that falls short do we look at new Horsham slabs, and then at carefully chosen alternatives. If you have been told a stone is limestone, send us the description and a photograph. Hips and other awkward corners, covered on our page about hipped stone roofs, are where odd patch stones tend to turn up.
You get a written quote once we have surveyed, giving one total price, alongside pictures of your own slopes. All of the stone work sits under our Horsham stone roofing service.
Stone type: two quick checks
Can I test my roof stone with vinegar to see if it is limestone?
It is better not to. Horsham stone contains lime, so it may fizz even though it is a sandstone, and acid will mark an old roof for no benefit. A close look at the grain and a comparison with the rest of the roof tells a surveyor far more.
Does it matter if my roof is called limestone in the sale details?
It matters only if the wrong label leads to the wrong repair. Describe the roof to your roofer as Horsham stone slabs, and ask for any patch stone to be a matching sandstone. Mortared joints and shadow slates are worth mentioning too, as they affect how repairs are made.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.