What ridge goes on a Horsham stone roof?

The ridge is the horizontal crest of the roof, where the two slopes meet. Slabs on each side stop short of the very top, and the ridge caps the gap so rain cannot enter from above. A hip is the sloping crest running down an outside corner, and it is finished in the same manner. Because the ridge is the highest and most exposed point of the building, it takes the worst of every gale.

On a heavy stone roof the ridge is a small structure in its own right. The pieces have to be bedded true, set to a line and pointed tidily, or the roof looks wrong and water finds a way in. From the ground the crest is hard to inspect, so faults here often go unseen until a mark shows on a bedroom ceiling. A pair of binoculars on a still day will show a lot, but only a close look from the scaffold proves the bed is sound. Cracks at the joints between pieces, gaps beneath them and pieces that rock when touched are all signs that the bed has let go.

Several kinds of ridge sit on stone roofs. Half-round or angled clay ridge pieces are common, and they can be supplied to match a given colour. Stone ridges, cut from a single block into an inverted V, were used on some of the best buildings and survive on a number of them. On other roofs a lead roll is used: a strip of lead wrapped over a timber core, which is flexible and copes well where a roof moves.

Which one goes back is a matter of what was there. The survey studies the existing crest, any old photographs you may have and the neighbouring houses. On a listed building the council will expect like-for-like, so we keep the same type unless it has plainly failed. Our comparison of Horsham stone and slate shows how the capping changes with the covering.

Ridge pieces are bedded in mortar so they sit level and shed water. On an old stone roof that mortar should be lime-based. Lime is softer and more flexible than the cement mixes that replaced it, so it moves with the roof rather than cracking away from it, and it lets damp escape from the old work. A hard cement bed on a soft old roof tends to crack and trap water behind it, which is why so many tired ridges are found with failed pointing on both sides. Our guide to lime mortar on old roofs goes into the reasons in more detail.

The mortar is not the only thing keeping the crest up, though. It is there to seal and to give a bed, not to carry the load of a gale. That job belongs to the fixing.

Marley, the tile maker, advises against leaning on mortar by itself for the grip of ridge and hip pieces, since each bedded piece also needs a mechanical fixing. That follows the 2014 revision of BS 5534, the code for slating and tiling. For a stone roof the same thinking applies, though the code itself concedes that it may not suit certain older roofs built in traditional materials. Our answer on BS 5534 and old stone roofs covers the balance.

In practice we fix the ridge to sound timber below with stainless steel, hidden beneath the bedding so nothing shows from the road. It does not change the look of the roof, and it means a bed that fails in time no longer takes the ridge with it. That small detail is what keeps a crest in place long after the pointing has weathered. It is also the part of a ridge repair that you will never see, which is exactly why it is worth asking about before the work is agreed.

A cracked bed, pointing that has gone or a piece that has shifted can often be put right without disturbing the slabs. We rake out the failed mortar, rebed in lime, fix the pieces and point to match, working along the crest a section at a time, so no stretch of the roof stays open overnight. If the pieces themselves are in poor shape, or the timber beneath has rotted, the ridge is taken off and rebuilt.

That is also the time to examine the top courses, since the smallest slabs sit close to the crest and are the easiest to lose. See how diminishing courses work for the reason. When we have surveyed it, you are sent a single total price on paper, plus pictures of the crest. To see the other stone work we take on, visit the Horsham stone roofing page.