Question answered
Sagging stone roof timbers
The short answer
Sagging stone roof timbers are a structural warning, not a cosmetic flaw. A Horsham stone roof rests on a steep frame of strong timbers built to carry substantial weight, so a dip can point to rot, decayed joints, spreading walls or later alterations. Have a structural engineer or surveyor look before anyone lifts the stone, because the right repair depends on what is moving.
A stone roof that dips between its chimneys or along its ridge is telling you something about the frame beneath. The cause can usually be found, and it decides what the repair must be.
What the frame under the stone must do
Horsham District Council's conservation appraisal describes these roofs as steeply pitched, carried on strong timbers to bear their considerable weight. That one line sums up the design. Rafters, purlins and ridge are heavy sections, because everything above them is stone and not clay or slate.
The load is also constant. Wind comes and goes, but the mass of the slabs presses on the timbers every day, year after year, and any weak point eventually shows as a dip. We quote no weight per slab and no minimum pitch for this stone. The engineer works from your actual roof, which is the only reliable guide.
A sag is therefore not always recent. Plenty of old stone roofs settled gently long ago and have been stable ever since. What counts is whether the movement is still going on, and a survey followed by a few weeks of observation can show that. Marking the ridge height against a fixed point, with dated photographs, is a simple start.
Why timbers give way
Water is the usual beginning. A slipped slab, a failed valley or lead that no longer seals lets damp reach the timber ends where they bear on the wall. Wet oak and softwood rot slowly, and the first sign may be nothing more than a soft rafter foot or a darkened wall plate seen from the loft.
Beetle attack and old alterations add to it. Earlier owners sometimes cut a rafter for a hatch or a flue and never made good. Taking a chimney down inside the roof space can remove the very support a purlin relied on.
Spread is the other cause. If the walls drift outward, the roof loses its bearing and drops. Cracks in the masonry at eaves level, doors that stick and a ridge that sinks in the middle all point that way. This is why the reason for the dip counts for more than the dip itself: re-covering over a failing frame only piles extra load onto the problem.
Looking from the garden and the loft
From the garden, sight along the ridge and the eaves against a straight edge such as a fence line or a neighbour's roof. A shallow, even curve is a different matter from a sharp dip or a bulge where slabs seem to be sliding. Note any shifted slates and any open gaps beside the chimney.
In the loft, with a torch and without stepping anywhere that is not a joist, look for timber that is dark, powdery or soft under a thumbnail, for rafters split at a joint, and for daylight where none should show. Tide marks on the underside show where leaks have fed decay.
Take photographs and date them. We work the same way on survey, tracing from the inside outward, and the written quote comes with photographs taken on your own slope. Stone slate sizes explains the courses you are looking at from outside, with the biggest slabs low on the slope.
When an engineer comes first
Bring in a structural engineer, or a surveyor with structural experience, when a dip is new, growing, or paired with cracks, leaning walls or soft timber. The Planning Portal's re-roofing guidance advises taking professional advice on structure before work begins when a replacement covering will differ markedly in weight, and the same logic holds when the frame itself is in doubt.
Weight matters whenever the covering changes. Approved Document A calls it a significant change when the load on the roof rises by more than 15%, and the Planning Portal gives 15 per cent or more as a point where building control approval is needed. Where a structure cannot take extra load, strengthening or replacement counts as a material alteration. Horsham stone or clay peg tiles follows that thread.
An engineer's report also gives your insurer, and any lender, something firm to read.
What a repair usually involves
Order of work matters. We stop the weather getting in, then plan the repair around the engineer's findings. Typically the stone comes off in sections and is laid out by size on the ground so each slab goes back to its own course, damaged timbers are mended or replaced like for like, and the roof is relaid.
Where only a rafter foot has rotted, the repair can stay local. Where the frame has moved, strapping or tie work may need to go in before any stone returns. We recommend whichever job properly cures the cause and say so plainly if the answer is bigger than you hoped.
Stone that cannot be reused is a separate question. The Stone Roofing Association would have any shortfall filled with suitable new Horsham slabs where they exist; stone roof alternatives looks at the options. Our Horsham stone roofing page covers the rest of the work on these roofs. The price is set once we have seen the frame, and it comes in writing.
Sagging stone roofs: two further points
Is a sagging stone roof always dangerous?
No. Some old roofs settled long ago and have stayed put for decades. A dip that is new, growing, or paired with cracked walls or soft timber needs prompt attention. Keep dated photographs so that anyone inspecting can see whether it is moving.
Can the stone go back once the timbers are repaired?
Usually your own stone returns first, since the Stone Roofing Association puts reuse at the top of its list. Any shortfall is made good with suitable new slates where they can be found, and slates from other buildings only where there is no way round it.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.