The short answer
For a shed, lean-to or light outbuilding, yes, they are a reasonable budget choice if laid on a frame with close supports and a sufficient pitch. They are light, easy to cut and quiet in rain. They are poorer on a garage with a wide span, a shallow slope or heavy snow, because they sag, soften in hot sun and fade. They also are not a like-for-like replacement for an asbestos cement roof without checking the frame.
What they are
Corrugated bitumen sheets are wave-profiled sheets made from fibres impregnated with bitumen and pressed into shape. They look like corrugated iron, weigh a fraction as much and can be cut with a handsaw. They are fixed through the crests with special nails or screws fitted with sealing caps, and overlap at the sides and ends to shed water. They are sold in a few colours, most of them dark.
Because they are lightweight, they put little load on an old frame, which is one reason they turn up on sheds, sheds with sagging timbers and some older garages. They are also quiet. Rain on a bitumen sheet is far less noisy than on metal.
Where they work
On a small outbuilding with a sound timber frame, they do well. The supports, called purlins, must be close enough to carry the sheet without sagging, and the closer, the better. The slope needs to be enough to shed water and wind-driven rain over the laps, so a near-flat roof is not a good candidate. Where a roof is flat or almost flat, a continuous covering such as EPDM or felt is safer. Our comparison of which roofing sheet is best lines up the options.
They also make a cheap cover for a log store, a lean-to over a bin store or a carport over a light frame, where appearance matters less than keeping things dry.
Where they struggle
Heat is the first problem. Bitumen softens in strong sun, especially on dark sheets, and unsupported areas sag between purlins. Over the years the surface fades and becomes brittle, and the nail caps can lift and let in water. Cold and snow bring the opposite fault, since a sheet that is too lightly supported can crack under a load of snow.
Span is the second. Garages are wider than sheds, and a roof across a double garage needs more structure than a shed roof. A wide span needs deeper rafters and closer purlins, and if the existing frame was built for a heavier covering, a switch to bitumen sheets may leave the roof more flexible than you want. The sheets also have a limited life when compared with metal profile sheet or a good membrane. Our page on garage roof materials sets out the longer-lasting alternatives.
Replacing asbestos cement with bitumen sheets
Many older garages have corrugated asbestos cement sheets, and bitumen sheets look like an obvious like-for-like swap. The profile is similar and the weight is much lower. But the asbestos has to come off first, or be dealt with, and that has rules of its own. Careful removal of asbestos cement sheets without breaking them is non-licensed work, but it needs a risk assessment, controls and trained workers. It becomes notifiable if the sheets will be substantially broken up. Overlaying is sometimes an alternative. Do not drill, break or scrub the sheets yourself. See our guide on asbestos garage roof removal.
Cutting, fixing and weather
The sheets are easy to cut, but the cut edges should be sealed where the maker advises, and fixings placed carefully near the ends so the sheet does not split. They are best laid in mild weather, because they stiffen in the cold and soften and mark in heat. Stand on boards laid over several corrugations, not on a single crest. Sheets stored on edge in the sun can warp before they are fitted. A roof laid in the wrong conditions or with the wrong fixings is the usual reason that people think bitumen sheet is no good.
Getting the best from them
If you do use bitumen sheets, fit them to the maker's instructions. Use the correct fixings at the crests, not the troughs, and follow the lap rules. Keep the purlin spacing within the maker's limits and make sure the pitch is as recommended. Ridge pieces and flashings against walls must match the profile. A properly laid sheet roof can give a good number of years on a modest building, though you should expect to replace it sooner than a metal or membrane roof.
The free survey for a garage or outbuilding includes photographs of the frame and the existing covering, so you can see what supports a new roof. We say honestly whether bitumen sheet is a sensible option or whether something else suits your building better, and give a fixed written quote either way. The finished work is covered by a ten-year written workmanship guarantee. For the whole range, see garage roof replacement.
Every roof is different. A free survey with photos settles it for yours.
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