Roofing sheets are the practical choice for garages, workshops, lean-tos, stores and farm buildings: big pieces, quick to fit and light enough for a simple frame. But there is no single best sheet. A sheet that is ideal over a bike store is wrong over a workshop, and one that suits a pitched roof fails on a shallow one. The old advice that bitumen sheets are the best option, or that one type lasts for fifty years, does not survive scrutiny. This guide sets out the main sheet types in plain terms, explains what each does well and badly, and ends with a way to match the sheet to the building.
First, what a sheet roof needs to work
All sheet roofs share the same logic. Water runs down the corrugations or ribs, and the overlaps between sheets stop it getting underneath. That depends on three things. The pitch must be steep enough for the product: each maker states a minimum, and below it, water backs up the laps. The overlaps must be correct, both along the sides, where sheets lie over each other, and at the ends, where one sheet sits above the next. And the fixings must be the right type, in the right place, with sealing washers. A roof that fails on any of these will leak whatever the sheet is made of.
The frame matters as well. Sheets are carried on purlins (horizontal beams across the rafters or walls), and each product has a maximum spacing for them, so a lightweight sheet on a wide span will sag or flap.
Coated steel sheets
Galvanised steel with a plastic or paint coating is the standard choice for sheds, workshops and farm buildings. It is strong, light, fast to fit and available in profiles from the classic corrugated to deeper box profiles that span further. Colours are plentiful. Weaknesses are noise and condensation. Rain drums on thin steel, and a cold surface over a warm, damp space drips. The usual fixes are an anti-condensation coating on the underside, a lining, or sheets with a layer of insulation. The edges of cut sheets can rust unless protected, and scratches in the coating let corrosion start. Our answer on condensation under a metal roof covers the causes and fixes.
Fibre-cement sheets
Modern fibre-cement sheets are made without asbestos, and look much like the old asbestos cement ones. They are heavier than steel, quiet in the rain and fireproof. They can be cut and drilled with care, but they are brittle and can crack if stepped on between purlins, and the surface grows moss over the years. They suit a garage or store where a traditional look matters. If you are replacing old sheets, do not assume the existing ones are fibre-free: older ones may be asbestos cement and must be identified first. Our guide to checking whether a garage roof is asbestos explains how.
Corrugated bitumen sheets
Bitumen-impregnated fibre sheets are a cheap, light and quiet alternative, popular for sheds and lean-tos. They can be cut with a saw, they do not rust, and the corrugation shape resembles a traditional sheet. Their weaknesses are a tendency to sag between purlins in hot weather, colour fade, and a lower load capacity than metal. They need closely spaced purlins and the right fixings, sealed at the crown of the corrugation, not in the valley. Our answer on whether corrugated bitumen sheets are any good weighs them in more detail. The claim in some old guides that bitumen sheets last for fifty years should be treated with scepticism, as no honest installer can promise it.
Translucent sheets: polycarbonate, PVC and GRP
If you want daylight, translucent sheets are the answer. Polycarbonate is strong and light-transmitting, and twin-wall versions offer some insulation. PVC sheets are cheaper but yellow and become brittle in sunlight. GRP sheets are tough, and weather well. Clear roofing suits lean-tos, carports, greenhouses and parts of a workshop, but not a roof that has to keep a space cool or private, since it will heat up in summer and show everything below. Our short answer on clear roof sheets for garages explains when they work and when they do not.
Insulated sandwich panels
Insulated panels have a layer of rigid insulation sandwiched between two metal skins. They are made in lengths, fixed through the surface, and give good thermal performance, reduced noise and far less condensation. They cost more than a single skin and are heavier, so they need a frame designed for them. For a heated workshop, a garage conversion with a pitched or low-pitch roof, or a garden office, they are often the best of the sheet family because the roof, insulation and weatherproofing are one component.
Box profile and standing seam
Deeper profiles, such as box profile, span further between purlins and give a modern look. Standing seam metal, where adjacent sheets are joined with raised seams, offers a higher-quality finish with hidden fixings, usually at higher cost and on a pitched roof. They are not typical for a basic garage but suit a contemporary extension or an architect-designed outbuilding.
| Sheet | Noise in rain | Condensation | Main drawback |
|---|---|---|---|
| Coated steel | High unless lined | Common | Drumming, edges can rust |
| Fibre cement | Low | Moderate | Brittle, heavy, moss |
| Corrugated bitumen | Low | Moderate | Sags and fades, wide purlin spacing fails |
| Polycarbonate or PVC | Moderate | Varies | Heat gain, yellowing in PVC |
| Insulated panel | Low | Minimal | Cost and weight |
Life expectancy: what to believe
Old guides and sales leaflets quote impressive lifespans, and it is tempting to pick the sheet with the biggest number. In practice a sheet's life depends on the environment and the installation as much as the material. Coated steel lasts as long as its coating, and a scratch or cut edge starts corrosion earlier. Fibre cement is stable but gets brittle and mossy. Bitumen sheets fade and can sag. Translucent sheets yellow or craze in sunlight. Rather than trusting a figure, look for a manufacturer's warranty with its conditions, read what it excludes, and treat a long headline number with caution. We would rather tell you how a sheet ages and what to inspect than promise a date.
Fire, wind and weather
Fire performance differs. Steel and fibre cement are non-combustible or highly resistant, and bitumen and plastic sheets are less so, which can matter near a boundary or an attached house. Wind is the other stress: sheets act like sails, and a roof with too few fixings or edges not secured can lift in a gale. Snow load matters on a light frame. Hail can dent thin steel and crack brittle plastic. A roof in an exposed spot, such as an open field edge near Overton or a ridge top outside Alton, deserves heavier fixings and tighter spacing than one sheltered between houses.
Cutting, handling and safety
Sheets are awkward to carry up a ladder and easy to damage. Steel edges are sharp, and cut ends should be treated to stop rust. Fibre cement sheets should be cut with hand tools or a suitable low-dust method and with proper protection, not a dry grinder. Never assume that old fibre cement is asbestos-free: leave the cutting of existing sheets alone until they have been identified. All sheet roofs are fragile until they are fixed, so work from crawling boards and take care at the leading edge. Wind makes handling large sheets dangerous, and a calm day is worth waiting for.
What affects the cost
Sheets are generally among the cheaper roof coverings per square metre, and the cost climbs with thickness, insulation, finish and complexity. A plain galvanised or bitumen sheet is at the low end, a coated box profile in the middle, and an insulated panel or standing seam at the top. The frame, purlins, fixings, flashings, ridge pieces, gutters and any removal and disposal are on top. Asbestos cement sheet removal brings its own costs, as the guide on garage roof replacement cost explains. Your fixed quote follows the survey.
Pitch, purlins and fixings in practice
These three decide whether a sheet roof lasts. A low pitch is the most common cause of leaks: a roof built at a gentle slope with ordinary corrugated sheets will let water creep up the laps in wind-driven rain. Longer laps, sealing strips between sheets and fixings through the crown of the profile all help, but a very shallow roof is better with a membrane. Purlin spacing must follow the maker's table, otherwise sheets flex, loosen their fixings and crack. And fixings must be the right fastener with the right washer, driven to the right tightness: too tight crushes the sheet, too loose leaves a gap. Many sheet roofs that leak after a few years do so at the fixings, not through the sheet.
Replacing old asbestos cement sheets
If the sheets you are replacing are asbestos cement, the new choice is influenced by the old frame. A frame built for heavy sheets can take most modern replacements, while one built for a lightweight material may not take a heavier one. Careful removal of whole sheets is non-licensed work under HSE rules, and the waste must be handled as hazardous. The steps and the rules are laid out in the guide to taking off an asbestos cement roof. Once the sheets are off, the timbers underneath can be assessed, and often need work.
Choosing: a simple rule of thumb
For an unheated store with a modest pitch, coated steel or fibre cement is sensible and economical. For a quiet shed, bitumen sheets can do the job if the purlins are close. For a workshop you use in all seasons, an insulated panel or a lined steel sheet makes more sense. For a lean-to or carport that needs light, polycarbonate fits. For anything flatter than the maker's minimum pitch, change the plan, to a membrane such as EPDM or GRP. Our guide to garage roof options compares sheets with membranes and tiles side by side.
What we do
We survey the building before suggesting anything: the pitch, the frame, the purlin spacing and the use of the space. We photograph what we find, and recommend the sheet that fits, or a different covering if sheets would be wrong for the roof. A fixed written quote follows, and we write our workmanship guarantee, which runs for ten years, into it. Read about the wider service under garage roof replacement in Basingstoke, or call 01256 213956.
