Stand under the roof of an old Stroud mill cottage and the first thing you notice is the timber: thick rafters, big purlins and joints cut by hand. Those sections were not chosen for show. The covering above them is stone, and the cottage was framed to hold it. The Cotswold District Council guide to stone slate roofing (2000) puts the load at about 1,000 kg for every 10 m² of roof. That is a hundred kilograms on each square metre, before any snow or the weight of a person on a ladder.
Roofs age, and so do the frames inside them. This guide covers how stone's weight travels through the structure, what a sagging slope tells us, how timbers are repaired or strengthened, and the loading rule that applies when the covering changes. For the wider service, see Cotswold stone roofing.
A hundred kilograms on every square metre
The figure from the council's guidance is worth turning into something you can picture. A single slope of 50 m² would carry about 5,000 kg of stone, five tonnes, resting on a few dozen timbers. Add the second slope and the frame is carrying a very large weight indeed.
That load is permanent, which makes it different from wind or snow. A timber under constant stress behaves differently from one that is only occasionally loaded. Over decades it can creep, bending a little further each year, and if it gets damp it weakens as well. A stone roof therefore tends to show its structural age sooner and more visibly than a roof of lighter material.
Why the old builders over-sized the timber
The sections in an old stone roof are generous because the builders knew what they were carrying. Rafters are often closely spaced, purlins are deep, and the wall plates (the timbers laid along the top of each wall to take the rafter feet) are substantial. Cutting any of this back, even in a good cause such as a loft conversion, removes margin the roof was designed to have.
The stone itself varies
Stone slates are not a uniform product. Because the courses are graded, with the biggest slates low on the slope, the load is heavier near the eaves than near the ridge. Our guide to diminishing courses explains the grading.
How the roof frame carries its load
Knowing the parts makes the faults easier to follow. Working down from the stone, there are four main elements.
- Battens are the laths the slates hang from, fixed across the rafters.
- Rafters are the inclined timbers reaching from the apex to the lower edge of the roof, and they carry the battens.
- Purlins are horizontal beams set across the rafters partway up the slope. They stop long rafters from bending under their load.
- Trusses and wall plates pass the weight on to the walls beneath.
Each of these hands its load to the next. A failure in one shows up as movement in another. A weakened purlin lets the rafters above it sag between supports. A rotted wall plate lets the rafter feet slide outward, and that thrust can push a wall out of line. Something that looks like a hollow in the slope is often a problem lower in the frame.
Walls and thrust
Pitched roofs push outward at the eaves unless something holds the feet in. In a stone house, the thick walls and the ceiling joists or tie beams do that job. If a tie has been cut away or a joist has rotted, the walls take more of the push, and cracks in the wall below the eaves can tell us about the roof above.
An opening in a roof slope, such as a dormer, a rooflight or a stair void, also interrupts the rafters. It needs trimming members around it that are strong enough to carry the load that the cut timbers can no longer take.
Reading a sagging slope
From across the lane, a healthy stone roof shows straight ridge and eaves lines and flat slopes. Old stone roofs are rarely perfectly true, and a gentle undulation is normal in a roof that has settled over generations. What concerns us is a change: a dip that is new, one that is getting deeper, or one with slipped slates and cracks beside it.
What we look at from outside
The ridge line should be level. A dip in the ridge suggests a problem in the frame beneath it, such as a failed truss or a weak purlin. Bellying slopes, where the slates bulge outward, can mean rafters have bowed. A step in the slope often marks a purlin that has dropped at a joint. Photographs taken at the start let us compare the roof with how it looks later.
What we look at from inside
The loft shows what is really happening. We check the rafters and purlins for bending, splitting and joints that have pulled apart. We look for staining that points to old leaks, and for dust or frass (the fine powder that wood-boring beetles leave) beneath timbers. Cracks in the walls beneath the eaves, and gaps where the wall plate meets the wall, tell us whether the roof is pushing.
One finding rarely settles it. We read the roof as a system: the look of the slope, the state of the timber and the movement in the walls together. If the picture suggests a structural issue, we put it in writing and recommend a structural engineer before any slate work begins.
What makes timbers fail under stone
Heavy loads fail timber slowly and for a short list of reasons. Most of what we find under stone slates falls into one of these.
Damp and rot
Water is the main enemy. A slipped slate, failed lead flashing or blocked gutter can feed moisture into the timber for years. Rafter feet and wall plates sit at the eaves, where water collects, and these are where rot most often takes hold. Cement pointing, which holds damp in, can make it worse; our guide to lime mortar on stone roofs explains why.
Insect attack
Wood-boring beetles work through damp or older timber. The damage is hard to judge from outside, because the surface can look sound while the heart is hollowed. Where we suspect it, a specialist survey is the right next step, not a guess from us.
Past alterations
Old roofs have often been altered by people who did not know what the frame was doing. A purlin cut to make room for a loft stair, a rafter notched for a pipe, a tie removed to open up a room: each of these can sit unnoticed for decades and then start to show as the timber creeps. We look for cut and notched members in every stone roof loft.
Overloading
Stone laid on a frame built for something lighter is a direct route to sagging. This happens when stone is laid over a frame built for something lighter, or when a second layer is added during repairs. It is why the loading rule below matters.
Strengthening: from a single repair to a new frame
What is done to a weak roof depends on how much of the frame has failed and how much of the original timber is worth keeping. The work usually falls into three levels.
Local repair
Where only an end of a rafter or a section of wall plate is rotten, the timber can be cut back to sound wood and spliced with a new piece, or a new timber can be bolted alongside it. This is sometimes called sistering. It keeps the original frame and the historic fabric. It suits damage in one place, once the cause (usually a leak) has been stopped.
Added support
Where a slope has sagged because the frame is under-strength or has been weakened, extra timbers or steelwork can be introduced. New purlins or a strengthened truss can take load without replacing everything. The design belongs to a structural engineer, who calculates sizes and fixings, and building control may need to be told.
Renewal
When most of a roof frame has decayed, replacement may be the only dependable answer. Because the slates must come off for this, it makes sense to combine it with a re-lay. Sound slates are sorted and kept, and the roof goes back in the same courses; our guide to re-laying a Cotswold stone roof follows that job.
On a listed house, timber repairs affect the historic fabric, and the council may want like-for-like materials and traditional joints. Check with Stroud District Council before any structural change.
The loading rule when the covering changes
Because stone is so heavy, changes of covering raise a regulatory question in both directions. The rule is in Approved Document A of the Building Regulations. That document defines a significant change as loading increased by more than 15%, while the Planning Portal says a covering that adds 15 per cent or more. With the wording split, we treat roughly 15% as the point to consult your building control body.
When it applies, the work needs Building Regulations approval, and the frame must be shown able to cope. Where it cannot sustain the increase (overstressed members, say, or deflection), strengthening follows. Get a structural engineer or surveyor involved before the work begins.
Which way does it cut?
Stone over a lighter frame is the obvious risk, for example a new extension roof that matches an old stone one, or a re-roof of a slate house in stone. Our answer on matching stone slates on a Stroud extension deals with the matching question; the structural check comes first.
Replacing stone with something lighter reduces the load and is not a loading problem, but it may fail a planning or conservation test, because substitutes are treated as unsuitable on historic buildings in the policy the Cotswold council reprinted. Check with the planning team for a listed home.
Reusing your own stone on its own frame keeps the loading unchanged. A heavier covering, or added insulation and boarding that raises the dead weight, is where the rule can apply.
Temporary weight and safe working on a stone roof
The weight that matters most to our crew is the one we add ourselves. Stripping a stone roof means stacking slates, and a stack of stone is far heavier than a stack of tiles. Pallets of stone are not left on a slope, or on a floor that was never designed to carry them.
Spreading the load
Slates coming off are lowered in small amounts and sorted at ground level where possible. If slate has to be stored on the scaffold, the platform is rated for it and loaded evenly. Inside, we avoid piling weight on ceiling joists, and we protect plaster ceilings from vibration while the roof is open.
Walking on old timber
An old roof frame is not always as strong as it looks. We use boards and crawling ladders to spread our own weight, and we do not trust a rafter that has been weakened by rot. On a cut or notched section, we secure the area and stay clear of it until it has been propped or repaired.
Costs, in plain terms
For scale, Checkatrade's cost guide for slate roofs (September 2026) puts stone slate at about £160 to £210 per m², varying with where the stone is sourced and where you live. Treat it as a UK-wide guide with no Cotswold premium built in, and note that timber repair sits outside it. Where the survey finds timber work, the written quote covers it too, so the structural repair is allowed for alongside the stone. For the full cost picture, read the guide to Cotswold stone roof costs. Our workmanship guarantee runs for 10 years, in writing, and we take the waste away.