Stone Slate Roofs In Rochdale And Heywood

Key takeaways

• Stone slate is sandstone split into thick flags, not the thin metamorphic slate quarried in Wales.
• It is heavy, and the roof structure beneath it was built specifically to carry that weight.
• It is laid in diminishing courses, largest at the eaves and smallest at the ridge, which is not decorative.
• Replacing it with anything else changes the loading, the appearance and often the planning position.
• Timber pegs and iron nails fail long before the stone does, which is what usually brings these roofs down.

What stone slate actually is

Despite the name, stone slate is not slate in the geological sense. It is sandstone or gritstone that splits along bedding planes into thick flat pieces, quarried locally across the Pennine fringe and used for centuries because it was there. It is far thicker and heavier than Welsh slate, has a rougher texture, and comes in irregular sizes rather than standard ones. Those characteristics dictate everything about how it is used: how it is laid, how the roof beneath is built, and why a roof covered in it cannot simply be swapped for something else without consequences.

Diminishing courses, and why they exist

Look at an old stone roof and you will see the pieces are largest at the eaves and get progressively smaller towards the ridge. That is deliberate and structural rather than decorative. The largest and heaviest pieces sit lowest, closest to the wall that carries them, which puts the weight where the structure can best take it. The sizes then step down in courses up the slope. This means the pieces are not interchangeable: a stone taken from the eaves cannot go near the ridge and vice versa, and a repair has to respect the graded arrangement or the roof will look wrong and drain badly.

The weight, and what it means

A stone slate roof is dramatically heavier than a slate or tile one, and the rafters, purlins and walls beneath were sized for that load. This has two consequences that pull in opposite directions. It means the structure of these buildings is generally substantial, having been built for the job. It also means that once that structure has been weakened by rot at a bearing point or by an ill judged alteration, the load it is carrying is considerable, and problems progress rather than stabilising. Any sign of movement on a stone roof deserves more attention than the same sign on a lighter one.

What actually fails

Not the stone. Traditional stone slates were hung on riven timber battens by oak pegs driven through a hole in the head of each piece, or later by iron nails. Pegs rot and nails corrode, and when they go the stone slips regardless of its own condition. The battens fail for the same reasons. So a stone roof reaching the end of its serviceable life is nearly always a fixing and batten problem, exactly as with Welsh slate, but with a heavier and more valuable covering sitting on top of it. That is the argument for stripping and relaying rather than replacing.

Why replacement is rarely the answer

Sourcing new stone slate is expensive and matching existing material closely is harder still, because it was quarried locally and varies from place to place. Replacing a stone roof with concrete tile or artificial slate reduces the load, which sounds like a benefit and creates its own problems: the appearance of the building changes fundamentally, the structure is then over specified for its covering rather than matched to it, and on any building with heritage protection the change may not be permitted at all. In most cases the sensible course is to lift the existing stone carefully, set it aside in order, renew the battens and fixings, and put it back.

The care that lifting requires

Taking a stone roof off is not the same job as stripping slate. The pieces are heavy and awkward, they break if dropped or stacked badly, and their graded order has to be preserved so they can go back in the right courses. Doing that properly takes longer and costs more in labour than a straightforward strip, and it needs somebody who has done it before. It is also why a quote for work on a stone roof that looks similar to a quote for a slate roof of the same area should prompt a question, because the two jobs are not equivalent in time or in skill.

The heritage dimension

Buildings with stone roofs are often old, and a proportion of them will be listed or sit within a conservation area, where what can be done to the roof is subject to control. Stone roofs are also a defining characteristic of the built landscape in this part of the region, which is precisely why designations tend to protect them. If a property is or might be protected, that has to be established before any specification is agreed, because it can rule out options entirely and it changes what materials and detailing are acceptable. The local authority holds the definitive answer.

Living with one

A stone roof in good order needs very little. Keep the gutters and valleys clear so water leaves the roof rather than sitting against it, deal with slipped pieces promptly since a gap exposes the battens beneath and a heavy stone is a hazard when it moves, and keep an eye on the ridge and any mortar detailing, which have shorter lives than the stone. Above all, resist the suggestion that the roof is finished simply because pieces have started to slip. On this material, slipping usually means the fixings have aged out, and the covering itself may have another century in it.

Finding somebody who has done it before

Stone roofing is a specialism, and the number of people who can lift, sort, grade and relay a stone roof properly is considerably smaller than the number who can strip and recover in tile. The consequences of getting it wrong are expensive: broken stone cannot be replaced cheaply, a roof relaid without respecting the diminishing courses will look wrong permanently and drain badly, and a structure loaded incorrectly is a serious matter. It is entirely reasonable to ask what comparable work somebody has done and to want to see it, and it is reasonable to expect the answer to be specific rather than general. On a building of this kind, choosing on price alone is the most expensive decision available, because the material at risk is effectively irreplaceable. Ask also how the stone will be stacked and protected while it is off the roof, since a roof's worth of graded stone left in a heap in a garden for a fortnight is how a large proportion of it gets broken.

Living with the weight

Because a stone roof is heavy, anything that weakens what carries it matters more than it would elsewhere. That makes some ordinary maintenance disproportionately important: keeping gutters clear so water does not reach the rafter feet, keeping the verge and gable pointing sound so purlin ends stay dry, and dealing with any leak promptly rather than tolerating it. It also means alterations deserve caution. Removing an internal wall, taking out a chimney breast or converting a roof space in a building with a stone roof is not the same exercise as doing it under a light covering, and it warrants proper design rather than assumption.

Frequently Asked Questions

What is the difference between stone slate and Welsh slate?

Stone slate is sandstone or gritstone split along bedding planes, thick, heavy and irregular in size. Welsh slate is a metamorphic rock that splits very thin and regular. They look different, weigh very different amounts, and are laid differently, so they are not interchangeable on the same roof structure.

Why are the stones different sizes up the roof?

It is called diminishing courses and it is structural. The largest and heaviest pieces sit at the eaves where the wall can best carry them, with sizes stepping down towards the ridge. It means pieces are not interchangeable between courses, and a repair has to respect the graded arrangement.

Can a stone roof be replaced with tiles?

Technically it can, and it usually should not be. It changes the appearance of the building fundamentally, leaves a structure built for a heavy covering carrying a light one, and on a listed building or in a conservation area it may not be permitted. Relaying the existing stone is normally the better answer.

Why is my stone roof slipping if the stone is fine?

Because the fixings have failed. Stone slates were hung on timber battens by oak pegs or later by iron nails, and pegs rot and nails corrode while the stone does not. Slipping almost always means the fixings and battens have reached the end of their lives rather than the covering.

Is stone roof work more expensive?

Generally yes, and mostly in labour. The pieces are heavy and awkward, they break if handled badly, and their graded order has to be preserved so they go back correctly. A quote similar to one for a slate roof of the same area is worth questioning, because the two jobs are not equivalent.

Do I need permission to work on a stone roof?

Possibly, since buildings with stone roofs are often old and a proportion are listed or sit within conservation areas. Keeping the original material and putting it back is usually the course least likely to run into difficulty. Establish the position with the local authority before agreeing a specification rather than after.