Roofers call each horizontal row of slabs a course. On a roof of slates or modern tiles every course is much the same as the one below. Stone slates from the Horsham area are different, because they were split and sorted by size rather than cut to a standard. They are laid in diminishing courses: wide slabs go on first at the bottom edge of the slope, and each course above them steps down in size, so that the smallest stones finish the roof at the top.
The effect is easy to see from the road and hard to forget. The roof seems to ripple from heavy to fine, and the texture is part of what gives these buildings their character. It is also a working system. Each stone belongs in a course that suits its size, and when a stone is lost, the replacement has to belong in that same course.
This guide explains how the pattern works, how to read it on your own roof and what it means for repairs and consent. It belongs to our Horsham stone roofing service, which covers surveys, repairs and relaying for stone roofs across the area.
What diminishing courses are
A diminishing roof is laid in rows that shrink as they climb. The Stone Roofing Association's guide to Horsham stone roofs describes the stone as random-sized slates, with the biggest laid at the eaves and tapering steadily up to the smallest at the ridge. Random here means the slabs were not cut to one pattern. They were taken as the stone split, sorted into sizes, and then laid in graded order.
If you stand back from a stone roof you can see the result. The bottom rows are made of broad, thick pieces, often with a rough and uneven surface. Higher up the slope the slabs get narrower and thinner, and the joints get closer together. A roof laid this way carries a visual rhythm that a roof of identical tiles cannot match. Every course reflects a decision by whoever laid it, which is why reading the roof closely pays off before anything is touched.
Not the same as a modern tile roof
A factory tile arrives in one size and one thickness. A Horsham slab does not. The stone splits as a flaggy sandstone, so each piece has its own outline and its own thickness, and that variation is one reason sorting by size matters.
The term is sometimes used loosely online, with graded stone slates, random courses and diminishing courses all treated as the same thing. For the work we do, the practical meaning is simple: the further up the roof a stone sits, the smaller it should be.
Where roofs like this stand across our area is covered further down.
How to read the courses on your own roof
You do not need to climb anywhere to learn a lot about a stone roof. Standing in the road or garden, and using binoculars if you own a pair, work up the slope one course at a time. The gradation should be smooth. A stone that is clearly too small for the row it sits in, or too large for the area around it, usually shows where a past repair has been made with whatever was available.
Look next at the joints. In a well-graded roof the vertical joints are staggered, so no gap runs in a straight line down the slope. A line of joints stacked one above another suggests a patch, or a section that has been relaid without care. Where the courses meet a hip, a valley or a chimney, the graded order has to be adjusted to fit the geometry, and these junctions are where local variations often show up.
Signs of an earlier repair
An area that is different in colour, or whose stones are noticeably thinner or flatter than their neighbours, may come from a patch with imported material. A group of small stones in the middle of a course of large ones is a giveaway. So is a row of shadow slates in an area where the original roof would not have needed them.
These details help us understand the history of the roof and which areas are original. They are also useful to you as an owner. If you have a photograph of the roof from some years ago, compare it with the roof as it is now. A change in the pattern can show a slab has slipped or a section has been disturbed.
We record the courses on survey photographs so you can see what we mean, and so a future repair has a reference to work from. That record also helps if the building is listed and consent is needed.
Why a replacement has to match its course
The pattern is not decoration. A stone that sits in the wrong course causes two kinds of trouble. One is physical. A stone that is too small for its row leaves the joints open, so the overlap is too short and water can be driven or run behind it. A stone that is too large presses on the courses around it, rocks on its bed or lifts the slabs above, and may never sit firmly.
The other is visual and legal. A roof that has been repaired out of order shows it from the road. On a listed building or a house in a conservation area, that can matter to the council, because the character of the roof is exactly what the protection is meant to keep.
When a slab fails, the right replacement is close to it in size, and in thickness too, as thickness decides how it lies against its neighbours. If the roof has surplus stone, perhaps from a hidden section, a stone of the right size can often be found there. If it has none, the search widens, and the Horsham stone shortage reported in 2019 makes that harder. Our guide to the repair of Horsham stone roofs covers the choice between resetting, replacing and relaying.
Sorting stone before any work begins
If a section is to be stripped and relaid, the stones are sorted into piles by size as they come off and stored in order. They can then be laid back in graded courses, with the largest at the bottom and the finest at the top, using the stone the roof already had. This is slow work, and it costs more labour than a quick patch. It is also the reason a relaid stone roof can look as if it were never touched.
Single lap, double lap and the courses
Two different laying systems are found on Horsham stone roofs, and both use diminishing courses. The Stone Roofing Association names them as a normal double lap system and a single lap system that it describes as unique to Horsham stone. Both use random-sized slates, graded from large to small.
In double lap, each slab overlaps the one beneath it, as on a conventional slate roof, so the weather is kept out by the overlap. In single lap, the stones are laid edge to edge with a narrower overlap, and each joint between them gets its weathering from a piece of Welsh slate, called a shadow or shade, bedded in mortar. The two systems look similar from the ground, but they behave differently when something goes wrong.
The association's recommendation is to retain or reinstate double lap wherever possible. Moving a roof off single lap touches the load and the stability of the timbers, so building control approval is needed and, for a Church of England church, a faculty. On a listed building it also needs listed building consent from the council. Our page on Horsham stone in the Sussex villages looks at how these roofs appear across our area.
Why it matters to the order of stones
A single lap roof depends on the shadows bedded beneath each joint, so the size of the stone and the width of the joint it makes both feed into the design. A double lap roof relies on the length of the overlap, so the length of each stone matters. When we plan a repair, we need to know which system we are dealing with before we choose any replacement stone, because the courses are read differently in each.
On roofs where repairs have left a mix of the two, the survey photographs help to show where one system ends and the other begins.
Hips, valleys and the ridge
A plain gable slope is the simple case. The harder cases arise where a roof changes direction. At a hip, the outer corner where two slopes meet, the courses of both slopes must come together and the stones have to be cut or chosen to make the angle watertight. In a valley, the inner angle, the stones from each side are brought down to a gutter lined with lead or a similar material. These junctions need the roofer to adjust the graded order locally, sometimes by using stones from a different row.
The ridge is the other end of the scale. By the time the courses reach the top, the stones are small, and the ridge itself is usually capped and bedded in a way that suits the building. On a stone roof the capping and its fixing are a separate question from the courses beneath. Our guide to ridges on stone roofs explains what is involved.
A note on modern fixing standards
BS 5534 has treated mortar as insufficient alone for ridge and hip tiles since its 2014 revision, so those tiles take a mechanical fixing too. The standard was written for ordinary tiled and slated roofs, and tile makers point out that its 2018 amendment allows for heritage roofs where the recommendations may not be appropriate. For a stone roof, the fixing of the ridge is therefore agreed case by case, with the council where the building is protected.
This is another reason to treat the roof as a whole. The ridge, hips, valleys and courses were designed together, and changing one affects the others. We look at all of them at the survey, not only at the spot where water is coming in.
Consent, records and planning the work
Because the courses are central to a stone roof's character, they matter when consent is in question. The South Downs National Park Authority holds that routine upkeep, and small in situ repairs which match what is there, need no listed building consent, while alterations or total replacement of historic fabric that would change the building's character do. Resetting a slab in its own course, or replacing a broken stone with one of matching size and kind, is a repair in kind. Re-grading a roof, or substituting a different material, is a different matter.
If you own a listed building, our guide to listed building consent for stone slate roofs explains the position in detail. Not certain of your home's status? A call to the council's planners will confirm whether it is listed or in a conservation area before work begins.
What we record and what you receive
Our survey photographs show the roof as it stands, course by course, with any faults marked. Our written quote comes afterwards as a single price for the recommended work, with the reasons for it and what each alternative would mean for the stone and the pattern. We aim to suggest repair rather than replacement wherever repair will hold.
Costs depend on access, how much of the roof is involved, the stone that has to be found, and whatever the survey finds beneath the stone. After the survey you receive a written total with photographs of the courses, the battens and the stone as they stand, so the scope is agreed before any slab is lifted.
If you are planning for the long term, keep a record of the roof. Photographs taken now, a note of any stone stored in an outbuilding, and a copy of any previous repair invoices all make the next survey faster and the next repair more accurate.
