Look up at an old stone roof in Painswick or Bisley. The slates along the bottom edge are broad and heavy; by the time the eye reaches the ridge they are barely the length of a hand. That grading is called laying in diminishing courses. A course is one horizontal row of slates, and on a stone roof each row is a little shorter than the one below it. It is a working method, not a decorative finish: every course is set out to suit the slates actually sitting in the stack, and the battens (the timber strips the slates hang from) are spaced to match them.
This guide covers why stone was graded this way, how a pile of mixed slates is sorted into courses, how the batten spacing is worked out as the slates shorten, and what goes wrong when the system is ignored. Figures come from Cotswold District Council's 2000 technical guidance on stone slate roofing, which records traditional practice. For repairs, costs and the rest of the service, start at our Cotswold stone roofing page.
Why the quarry decided the shape of the roof
Cotswold District Council's guidance names two oolitic limestones as where Cotswold stone slates come from, and they are Stonesfield Slate and Forest Marble. Forest Marble was worked into slates by hand, and those hand-split pieces went by the name presents. Stonesfield Slate was left spread on the ground through the cold months so frost could open its beds, which is why the guidance calls it frost-split. Neither stone gives a neat, even product. The guidance says the stone breaks naturally into far more small slates than large ones.
That one fact explains the whole roof. If a builder had insisted on a single size, most of every quarry load would have been thrown away. Instead the old roofers used everything. The scarce long slates were kept for the bottom of the roof, the plentiful short ones were saved for the top, and everything in between was graded to fill the slope from one to the other.
Where the water goes
There is a weathering logic as well. Rain gathers as it runs down a slope, so the lowest courses carry the most water and need the longest slates and the deepest cover. Near the ridge there is far less water to shed, and a short slate does the job. A steep pitch helps the rough, uneven stone throw water off quickly; the plainest roofs in the guidance are straight-pitched, typically at 47° to 55°. That is a traditional range, not a minimum pitch, and no single roof is bound to follow it.
Tile or slate
You will hear both words. The guidance notes that stone tile is the usual term in the south of Cotswold District and stone slate in the north. They describe the same material, found on roofs from Chipping Campden down to the edge of Bath.
Cussoms at the eaves, short cocks at the top
The courses have names, and the names tell you where each slate belongs. Our answer on Cotswold slate names runs through the full vocabulary. For laying, three positions matter most.
| Course | Position on the slope | Length given in the 2000 guidance |
|---|---|---|
| Cussoms | A double course along the eaves | Normally about 600 mm, some up to 750 mm |
| Followers | The first course laid above the cussoms | Shorter than the cussoms, longer than the courses above |
| Short cocks | The highest courses, just under the ridge | Only 150 mm |
Why the eaves course is doubled
The eaves (the lower edge of the roof, where it overhangs the wall) is laid two slates thick. The first layer is an under-course whose job is to sit beneath the joints of the layer on top. Without it, the gaps between the bottom slates would open straight onto the wall head, and the eaves is exactly where the full run of water off the slope arrives.
The courses in between
Between the followers and the short cocks there is no fixed number of courses. A long slope with a generous stack of big slates might carry many courses of gradually falling length; a small cottage roof might drop from long to short much faster. What stays constant is the direction of travel: each course is the same length as the one below it or shorter, never longer.
Sorting a stack of mixed slates into lengths
Before a single batten goes on, the slates have to be sorted. On a re-lay this starts as the old roof comes off. Each slate is lifted, checked for cracks, delamination and a sound peg hole, and either set aside for reuse or rejected. The guidance says that on a stripped stone roof roughly half the slates are commonly found to be perished, so the sort also tells us how much new stone the job really needs.
Measuring and stacking
Sound slates are measured from the peg hole down to the tail, the bottom edge that shows on the finished roof. That is the length that counts, because it decides how far the slate can reach down over the course below. Slates are then stacked by length in bands, longest to shortest, so the roof can be built from the bottom band upward. Width is far less important; stone slates come in random widths and the courses are made up by mixing them.
Matching thickness and colour
Within each band we keep slates of a similar thickness together where we can, so that a course lies flat on its battens and does not rock. Weathered faces are kept facing out. When new stone has to be added, it is graded into the same bands as the old, and the English Heritage policy reproduced in the council's guidance wants any new slate to match the old stone for geology, colour, texture, size and thickness.
The stack decides the roof. Once the slates are sorted and counted by length, we can say how many courses the slope will take and where any new stone needs to go, before a single batten is fixed.
Getting the sort right is slow work, and it is the part of a stone roof job that is easiest to rush. A rushed sort shows up later as courses that wander, laps that are too thin, or a band of odd slates that looks wrong from the lane for as long as the roof stands.
Gauging the battens as the slates get shorter
On a modern tiled roof the battens sit at one spacing from eaves to ridge, because every tile is the same size. On a stone roof that is impossible. The spacing between battens, called the gauge, has to shrink as the slates shorten, and it changes course by course.
How a gauge is worked out
Stone slates are laid double-lapped: every part of the finished roof is covered by at least two thicknesses of stone. Each slate's tail reaches down past the slate immediately below it and overlaps the head of the slate two courses down. That overlap is the headlap, and it is what keeps driven rain out. So for each band of slates, the gauge comes from the slate's length from peg hole to tail, less the headlap, divided by two. A long eaves slate gives a wide gauge; a short cock near the ridge gives a narrow one.
Setting out the slope
Because the stack, not a catalogue, sets the lengths, the battens are usually set out a band at a time as the roof rises, rather than marked from eaves to ridge in one go. The roofer checks how much slope is left against the slates still to be laid, and adjusts the bands so that the courses reduce evenly and finish neatly under the ridge. A course that is suddenly much shorter than the one below looks like a step from the ground, and a lap squeezed to make the numbers work is a leak waiting to happen.
Keeping the lines true
Every course needs a straight tail line across the whole slope, even though the slates in it vary in width. We run a line along each course and check that the tails sit on it. Wavy tails are one of the clearest signs of an old roof that has moved or been patched with the wrong slates.
Laying a course: pegs, joints and the finish at the ridge
Traditionally a stone slate is hung over its batten by a peg of cleft oak, occasionally a pair, driven through a hole near its head. The guidance describes exactly this fixing. Nothing nails the slate down: gravity and the weight of the courses above keep it where it is. That is part of why the sort matters so much. A slate that is too short for its course does not get enough cover, and a slate whose hole is in the wrong place hangs out of line with its neighbours.
Breaking the joints
Because widths vary, the roofer picks each slate from the band so that its side joints fall over the middle of a slate in the course below, never over another joint. That side cover is what stops water running straight down through the courses. On a stone roof it is done by eye and by selection, one slate at a time, which is why laying is slower than on a uniform covering.
Valleys and the ridge
Where two slopes meet in an internal corner, the traditional detail is a swept valley: the courses curve round the corner in stone, without a lead gutter. The guidance says these are built with specially cut slates, named bottomers, lie-byes or side skews, and skews, and it rates close-mitred and open lead valleys as a cheaper, less satisfactory option, best avoided. The diminishing courses have to carry on through the valley, so the cut slates are graded too.
At the top, the short cocks finish under the ridge, the horizontal line where the slopes meet. According to the guidance, the ridge on these roofs was always cut from dressed stone, shaped as a plain angle. When we bed a stone ridge now we add mechanical fixings as well: the 2014 revision of BS 5534 stopped treating mortar on its own as enough to keep ridge and hip pieces in place.
Where diminishing courses go wrong
A common thread runs through the faults we find on graded stone roofs: the courses themselves. A repair done with whatever slate was to hand often uses one that is too long or too short for its course. Too long, and it sticks out below the tail line; too short, and the headlap underneath it is thin enough for wind-driven rain to get past.
Uniform or sawn replacements
Sawn slates of one size, or a uniform manufactured product, cannot be graded and do not lap the way split stone does. The English Heritage policy reproduced in the council's guidance cautions that slates cut with a saw, rather than split, may fail both technically and in appearance. It also says a sandstone slate must never stand in for a limestone one, or the reverse, and that second-hand stone belongs only on the building, or the group of buildings, it was taken from. Our guide to reclaimed or new stone slates explains the sourcing side.
Weight
Stone is heavy. The guidance puts a stone slate roof at about 1,000 kg for every 10 m², and the timbers underneath were built for that load. Adding stone to a roof that once carried something lighter, such as a matching extension, can increase the loading by 15 per cent or more, which the Planning Portal says needs Building Regulations approval. Check with building control, and expect a structural check first.
Lost courses
The quietest fault is a slope that has been re-laid without regard to the original gauge, so the course lines no longer match the slope beside it. That is why, before stripping anything, we measure the existing courses and photograph them, so the slope can go back exactly as it was.
Re-laying a slope in its own diminishing courses
When a slope has failed across its width, the answer is usually to strip it, sort it and lay it again in its own courses. Our guide to re-laying a Cotswold stone roof follows a full job from scaffold to completion. In brief, the survey records the existing gauge and the condition of the slates, pegs and battens, with photographs of your own roof. The slates come off and are sorted by length, the battens are renewed, and the slope is set out and laid band by band from the eaves to the ridge, with new stone graded in where the old has perished.
Cost and paperwork
Checkatrade's slate roof cost guide, last updated September 2026, puts stone slate generally at about £160 to £210 per m², depending on where the slate comes from, if it is new or reclaimed stone, and where you live; it is a national figure, not a Cotswold one. MyBuilder's 2026 guide puts scaffolding for a re-roof at about £1,125. Your own price comes after the survey, in a written quote that covers scaffold, stripping, sorting, new stone, labour and waste. A whole-roof re-lay goes past the Planning Portal thresholds of 25 per cent of the building envelope and 50 per cent of the roof, so a Building Regulations application normally applies; we handle that notification and explain any consent needed on a listed house.
The finished courses are photographed before the scaffold comes down, and the work carries our 10-year workmanship guarantee, set out on paper.