Slate Roofing in South London

On a Victorian terrace each slate hangs on two nails, driven into riven laths or sawn battens over bare rafters or close boarding. There is no felt underneath. Instead the slates were often torched — lime mortar and animal hair pressed up between the battens to stop draught and driven snow. If you put your head into the loft and find grit and crumbs on the joists, that torching is on its way out and the roof is relying on the slate alone.

Most of these houses also have a butterfly roof: two slopes falling inwards to a lead-lined valley gutter that runs front to back behind the parapet. That gutter is the commonest cause of serious water damage in SE5, SE15, SE22 and SW9. When it blocks with moss and leaves, or the lead splits, the water has nowhere to go but into the middle of the house.

Welsh slate will do a hundred years and frequently a good deal more. The nails will not. Iron and early galvanised nails corrode in the damp along the batten line and let go, usually somewhere between sixty and a hundred years in, and the slate slides out. That is nail sickness, and it is what most old South London slate roofs are actually suffering from.

You can read it off the roof from the pavement. One slate down is an accident. Slates down in a different place after every gale, lead tingles clipped over half a dozen tails, and rusty nail heads collecting in the gutter — that is a roof telling you the fixings have gone across the whole slope.

Slate itself does fail, but differently and much more slowly. Cheaper imported slate with a high carbonate or pyrite content takes up water, oxidises inside and starts to come apart along its cleavage planes. You see rusty freckles, soft crumbling edges and layers lifting like a wet book. Good Welsh slate absorbs almost no water at all, which is why it does not do this.

Not every slate problem is a re-roof, and a roofer who opens with one has not been up there and looked.

Once the fixings have gone across a slope, the honest job is to strip it and start again. Scaffold goes up, front and back if both slopes are being done. We strip by hand and sort the slates as they come off — on a Welsh slate roof, half to two-thirds usually survive.

Then the timbers get looked at, and this is the part nobody can price from the ground. Rot hides in the rafter feet, the wall plate and the ends buried in the wall either side of a valley. We photograph whatever we find and ring you with a price before we cut anything out.

The new roof is a breathable membrane, treated 50 × 25 mm battens gauged to the lap, and slates fixed with copper or aluminium nails — never steel, which is what put you here. The lap, meaning how far each slate covers the one two courses below, is set by the pitch and how exposed the roof is; on London terraces it is usually somewhere between 65 and 115 mm, and BS 5534 governs the fixing specification. Ridges, hips and verges are mechanically fixed, because since the 2014 revision of BS 5534 mortar on its own is no longer accepted as the only thing holding them down. New Code 4 and Code 5 lead goes to the valleys, stacks and abutments, and we ventilate at the eaves and ridge so the membrane does not trap moisture in the roof space.

Re-covering more than about a quarter of a roof is a material alteration, so Building Control has to be notified before a slope is stripped.

The right slate is usually the one already on the house — particularly where the terrace has an unbroken roofline, and in the conservation areas that cover much of Dulwich, Herne Hill, Clapham and Brixton.

The test is the state of the fixings and how far the failure has spread, not the age on the deeds. If under about a third of a slope has gone and the nails elsewhere still hold, repair it and leave it alone. If slates are coming off in several places every winter, or the battens crumble when you lift one, you are buying the same repair three more times before you buy the roof anyway.

Doing one slope at a time is a legitimate middle answer and we quote it often. The front in matching slate because it is seen from the street, the rear when the money is there. What we will not do is take money for patching a slope we have told you is finished.