Do flat roofs still need stone chippings?

On a traditional built-up felt roof, the top layer is bitumen. Bitumen is waterproof but it ages in sunlight, going brittle and cracking, and it softens when it is hot. A layer of stone chippings, pressed into a coat of bitumen, shades the surface, reflects some of the sun and shields the felt from knocks. The weight of the stone also helps hold the layers down in a gale. Some owners simply like the look of a stone roof, and that is a fair reason to keep it.

That is why so many older flat roofs are grey and stony. The method worked, and still does, but it is only one of the options now. Roofers still use it where a plain felt finish is wanted and the structure can take the weight. The covering you choose decides whether the stones are needed, and our guide to torch-on felt roofing explains the layers that sit beneath them and how each is fixed.

A mineral cap sheet is a felt whose top layer comes already coated with small mineral granules, so the protection is applied in the factory. It does much the same job as chippings with none of the loose stone, and it is lighter, which suits a roof that was not designed to carry a layer of gravel. EPDM is a synthetic rubber sheet that resists sunlight by itself. GRP has a topcoat that carries the colour and the protection, and liquid systems cure into a single unbroken skin.

For these roofs, chippings do nothing useful. They would only add weight and hide the surface. Our comparison of EPDM, GRP and felt sets out how each is finished, so you can see which would suit your roof, your budget and the way the roof is seen from the house.

Chippings hide the surface, which makes a leak harder to trace, because you cannot see a split or a blister without raking the stones aside. They hold leaves and silt, which feed moss, and moss keeps the surface damp. After a few years the top of a stony roof can look green, and cleaning it means lifting the stone. They drift over time, leaving bare patches where the felt is exposed, and they wash into gutters and outlets, where they block the flow of water.

Stone also adds weight to the roof structure. That is not a concern on a sound roof, but it matters if you are adding to or altering the roof. If water is standing on a stony roof, the stones can mask the dip, and our answer on standing water on a flat roof describes the problem and how a survey finds it.

If a stony roof needs patching, the chippings around the area have to be swept back to expose the felt, and put back afterwards. Stone left in a repair can puncture the new material, so the surface must be clean before the patch goes down. A neat patch with stone scattered unevenly around it still reads as a repair from above, so we blend the edges back into the surrounding roof. After a repair, the stones are spread again so the patch is shaded like the rest of the roof, and any bare strips are topped up. If the old stone has gone dull and dusty, washing it first helps it stick, and fresh stone is bedded in where the supply has run short.

When you are replacing a covering, the question becomes whether to keep the stone finish. The survey gives you a verdict and the reasoning in plain words. If the old stone is sound, some of it may be reused, but weathered chippings are often dusty and worth replacing. The answer on whether to repair or replace a flat roof gives the signs that point to a new covering.

For a felt roof, choose between a mineral-surfaced cap sheet and a chipped finish. For an extension, garage or porch that you can see from the road, a neat mineral finish or a coloured membrane often looks better than loose stone, and it never rattles across the roof in a gale or ends up on the drive. For a roof that will be walked on or has equipment on it, think about a protective surface designed for it, because loose stone works its way under feet, into the covering and across the roof. Whichever finish you pick, we agree it with you before the work starts.

Tell us at the survey if you want to keep the stone or lose it, and the written quote, with its one total, reflects that choice, along with the weight and the upkeep that come with it. The main flat roofing page lists the options, and the finish you pick is as much a part of the job as the felt beneath it.