Question answered
Stone slate sizes
The short answer
Stone slate sizes are random, because the slabs are split from natural stone and not made to a pattern. On a Horsham stone roof the largest slates are laid at the eaves and the sizes diminish gradually to the smallest at the ridge. That order, called diminishing courses, keeps the roof weathertight, so any repair or relay must keep each slab at its proper level.
Stone slates are not cut to a standard size. A Horsham roof is built from slabs of every length, sorted and laid in order, which shapes how it is repaired and what any replacement has to match.
Why no two slabs match
The Stone Roofing Association describes Horsham stone-slates, or flags, as calcareous, flaggy sandstones. They are split from the rock along its natural beds, so the thickness and outline of each piece follows the stone and not a template. A slater working in stone takes what the quarry gave, then sorts it.
That is very unlike machine-made tile, where every unit repeats the last. A stone roof is closer to a dry-stone wall that happens to keep the rain out. Each slab has a face, an edge and a weight of its own, and a good slater learns them by handling.
The result is the quiet, uneven character people value in a Sussex village. It also means there is no catalogue to order from. When a slab breaks, its replacement has to be found by measure and by eye, with length, thickness and colour all weighed against the courses on either side.
Sorting by length and laying in diminishing courses
Before a stone roof is laid, or relaid, the slates are graded by length. The Association's guide describes random-sized slates, the largest placed at the eaves and gradually diminishing to the smallest at the ridge. A course is one horizontal row of slabs, so each row up the slope is a little shorter than the one beneath it.
The big stones go low for sound reasons. They are heavy, and the eaves is where a roof needs its firmest edge. Water also arrives there in its greatest volume, so a long slab gives generous cover. Smaller stones near the top keep the load there lighter.
We cover the principle at more length in diminishing courses. When we relay, the sort is done carefully on the ground first and the slates laid out in their rows, so nothing is guessed on the scaffold.
Double lap and single lap change the sorting
Two systems exist. The Association calls them the normal double lap system and single lapping, which it describes as unique to Horsham stone roofs. In single lapping the joints between slabs are weathered with Welsh slate shadows, also called shades, bedded in mortar. In double lap each slate overlaps the courses below in the familiar way.
Both use random-sized slates, but the way the gaps are covered differs, and so does the right approach to repair. The Association's preference is to keep or restore double lap wherever that can be done. Single lap Horsham stone shows how the shadows work.
If you are unsure which system you have, check the underside from the loft or look closely at the joints from a safe spot. The survey pictures we send show it plainly, and the written quote follows. For the full range of work on these roofs, start at Horsham stone roofing.
Matching a replacement slab
A single broken slab is replaced with one of the right length and similar thickness, set at the right level so the course stays even. Too short and it leaves a gap in the cover. Too long and it lifts its neighbours, which invites wind and water under the joint.
Matching also concerns the stone itself. The Association warns against combining limestone slates with sandstones, since the two weather differently and can fail side by side. That is one reason the origin of a replacement matters; checking the provenance of reclaimed stone slates goes into it.
Where a roof is repaired, we keep the sorted stock from your own roof for the purpose rather than treating it as waste. Slabs set aside during a strip often turn out to be exactly the length a later repair needs, and a pile sorted by size on the day saves a long search afterwards.
Sizes and the load on the frame
Because the largest and heaviest slabs lie low, the load on a stone roof is not even. The strong timbers beneath carry it, and where a sag has developed it often shows below the band of biggest stones. Sagging stone roof timbers deals with that.
It affects handling as well. Large slabs need more than one pair of hands and careful lowering, which is partly why stone work costs more and takes longer than tiling. Weights vary with the stone, so any figure for a whole roof has to come from inspection and not from a table.
Two things you can do before a survey: photograph the roof straight on in good light, and note anywhere the courses look uneven or a run of small slates sits low on the slope. Pictures like that tell us where past repairs have disturbed the original order, and they make the later conversation about cost far more concrete.
Stone slate sizing: two more questions
Can I order stone slates by size from a supplier?
Not in the way you would order tiles. Stone is sorted by length from whatever stock is available, so a supplier talks in lengths and quantities, and a roofer then selects slabs to suit the courses being repaired. Expect to supply a sample or a clear photograph.
Do the smallest slates always sit at the ridge?
On a properly laid Horsham stone roof, yes: the Stone Roofing Association describes the sizes diminishing from eaves to ridge. Past repairs sometimes break that order, which is one of the things we look for when tracing a leak.
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