Corrugated sheet roofs
How corrugated roof sheets are fitted on a garage or outbuilding
Corrugated sheets are quick to lay on a garage or outbuilding, and they are easy to get wrong. This post follows a roof from the old sheets coming off to the last fixing going in.
A corrugated sheet roof is one of the simplest coverings in building. The sheets are large, the profile sheds water on its own and the roof can look finished within a day or two. That simplicity is also the trap, because the details that keep a sheet roof dry are small ones: how far each sheet laps the next, where the fixings go and what the sheets are resting on.
This post is written for the owner of a garage, shed or workshop who needs to know what a sound job looks like, whether they are asking a roofer for a quote or weighing up doing the job themselves. It covers the three common sheet materials, what to check before anything is laid, the order of work, the fixings and overlaps, and the safety points that decide who should be on the roof at all.
It does not replace the maker's instructions for the sheet you buy. Those set the minimum pitch, the laps and the fixing positions for that product, and they win over anything general.
01Side by side
Fibre-cement, metal and bitumen sheets
Three families of corrugated sheet cover most garage and outbuilding roofs. They look alike from the ground and behave differently once they are up.
Each has its place. Fibre-cement sheets are grey, rigid and cut with care. A new one is a modern product, but an old one belongs to a different era, because HSE says a building built or refurbished before 2000 must be assumed to contain asbestos unless shown otherwise. Metal sheets are light, which suits a lightweight frame, and they come in a range of coatings and colours. Bitumen sheets flex, which helps on awkward shapes, and they are easy to carry onto a roof, though they need supports at the spacing the maker gives.
Whichever the material, the corrugation does the same job: it gives a thin sheet enough stiffness to span between supports and sends water down the troughs to the eaves.
Which you pick matters less than whether the sheet suits the frame, the pitch and the use. Our guide to the best material for a garage roof compares sheets with felt, EPDM and GRP, which is worth reading if the roof has a gentle pitch and you are not tied to sheets. If you want the wider picture of a garage re-roof, start from our garage roof replacement page.
| Sheet | What it is | Worth checking |
|---|---|---|
| Fibre-cement | Cement reinforced with fibres, grey and rigid | Whether old sheets contain asbestos; brittleness underfoot |
| Metal | Profiled steel or aluminium, often coated | Coating, cut edges and noise in rain |
| Bitumen | Fibre sheet saturated with bitumen, flexible | Support spacing and the maker's fixing pattern |
02In order
What to check before a sheet goes on
The preparation decides the result, and it happens before the first sheet is lifted. A new roof laid over a poor frame, or an old roof of unknown material, is a risk whatever the covering.
Corrugated sheets: your questions
How much should corrugated sheets overlap on a garage roof?
The overlap depends on the sheet and the pitch, so it comes from the maker's data sheet rather than a rule of thumb. A shallower pitch needs a longer end lap than a steep one. If a quote does not say what lap it will use, ask before you agree to it.
- What is there now. If the existing sheets are old, identify them before they are touched. Our post on asbestos garage roofs explains why.
- The frame. Check the rafters or purlins that carry the sheets: that they are sound, level and at the spacing the maker's data sheet calls for.
- The pitch. Every sheet has a minimum pitch in its data sheet, and a roof flatter than that will let water back up under the laps.
- The edges. Decide how the eaves, the ridge or top edge and any wall abutment will be finished, since each needs its own piece.
- Access. Plan how people and materials will reach the roof without standing on it.
None of this is exciting, and all of it decides the outcome. A frame that has dropped or twisted gives sheets that do not line up, and a gentle pitch under a sheet that wants more is a fault that no amount of sealant cures.
Where the frame is in poor shape, new timbers belong in the quote. We photograph the frame and show you what the survey found, since it is hidden from the garage floor by the old sheets.
03Ridges
The order of work, eaves to ridge
On a sound frame, a corrugated roof goes on in a sequence that is much the same for every sheet type. The maker's instructions give the detail, and these are the stages they follow.
- Fix any eaves support, such as a filler strip or eaves piece, so the lowest course has something to sit on and rain is kept out of the garage wall.
- Start at the eaves and work up the roof, laying each course so that it overlaps the one below, which is the end lap.
- Lay the sheets side by side, each overlapping its neighbour, which is the side lap, in the direction the instructions give for the weather the roof faces.
- Fix each sheet to the frame, not to the other sheets, using the fixings and the positions in the data sheet.
- Finish the top edge with a ridge piece, or a flashing at a wall.
- Seal and trim the edges, then check the whole roof from below for daylight.
On a small garage, the temptation is to lay everything in one go, but it pays to check the lines as you go. A sheet that is out of square at the first course carries that error all the way up the roof, and the gaps that result do not close on their own.
Cutting is the next point. Cut edges should be treated according to the sheet material, and dust from cutting some materials is a hazard in its own right, which is another reason to know what you are working with before the saw comes out.
04Fixings
Overlaps, fixings and sealing
The laps and fixings are where a corrugated roof is won or lost. They are also where a quick job and a good one look most alike from below, until the first heavy rain.
Two overlaps matter. The end lap is where one sheet runs up and over the one below it, and it has to be long enough for the pitch: a shallow pitch needs a longer lap than a steep one, which is why the data sheet ties the two together. The side lap is where neighbouring sheets meet, and it should face away from the weather that the roof meets most often.
Can I walk on corrugated roof sheets to fix them?
No. A sheet roof can be fragile, and HSE calls AC roofs "always fragile". Use crawling boards or edge protection spanning the supports, plan access from the ground and keep untrained people off the roof. The sheets alone will not carry a person's weight safely.
| Point | What to check |
|---|---|
| End lap | Length for the pitch, and a sealed joint where the maker asks for one |
| Side lap | Direction, and whether it is fixed through both sheets |
| Fixing position | Which part of the profile takes the fixing, crown or trough |
| Washers | Seals under the heads, and no fixing driven so hard it crushes the sheet |
| Holes | Pilot or clearance holes where the instructions require them |
A fixing that is overtightened can crack a brittle sheet. A fixing that is too loose lets a sheet lift and rattle in wind. On an exposed site the fixing count and positions deserve a question, and the maker's table, not guesswork, holds the answer.
Do not rely on sealant to rescue a lap that is too short. Sealant ages, and a lap that depends on it is a roof waiting to leak.
05Key points
Fragile sheets and who should be on the roof
A sheet roof can be fragile, which means a person who steps on it can go through it. HSE's A14 guidance calls AC roofs "always fragile", and the caution applies to any old sheet roof you have not been told is safe to stand on. Moss, age and a wet surface add to the danger.
That shapes the job in three ways.
- No one should stand directly on the sheets, new or old. Crawling boards that span the supports, or an edge protection system, spread the load and stop a fall.
- Materials should be lifted by a method that does not need a person on the surface, such as a hoist or a ladder with a platform.
- Anyone who is not trained for work at height should be at ground level.
For a homeowner doing a small job, the honest advice is that the roof is the riskiest part. A garage is low, but a fall from low height onto a concrete floor through a broken sheet is still serious.
Where a contractor does the work, CDM 2015 hands a homeowner's client duties to the contractor and requires a construction phase plan for the project. A good roofer will tell you how the roof will be accessed and protected without being asked. If the answer is vague, ask again before the work is booked.
If the sheets have already started to leak, a survey will show whether the sheets, the laps or the frame is at fault.
06The detail
Doing it yourself, or paying for it
A new corrugated roof on a small garage is within reach of a competent DIYer in terms of the tasks involved. What makes it a harder decision is the access, the old material and the weather.
Published cost guides give a sense of the scale of a hired job. MyJobQuote puts a garage roof replacement at between £750 and £3,500 on average in 2026. The range is wide because garages differ in size, material, frame condition and access, and it does not say what a sheet roof specifically costs. The price of the sheets themselves is a small part of it.
Do old fibre-cement sheets contain asbestos?
They may. HSE tells you to presume asbestos in a building built or refurbished before 2000 until you know otherwise. Identify the material before any sheet is drilled, cut or broken. A survey tells you what you have, and the result decides who can remove the sheets and how.
What sets the price is the size of the roof, the sheet type, whether the old roof has to be taken off and disposed of, the condition of the frame and how the roof is reached. You get one written total price after a survey. A quote should say whether the old sheets are being removed, what happens to them, and whether scaffold and VAT are in the price.
Permission and repair
A written quote is also the time to check the permission side. Whether a garage re-roof needs anything beyond Building Regulations depends on the building, and our answer on planning permission to replace a garage roof sets out the test.
For a garage that has a worn roof and no plans to change, the question is usually repair against replacement. Patching a sheet roof can buy time, but it does not suit a roof that is cracked across many sheets or sitting on a poor frame, and we tell you which side of that line yours falls on after the survey.
Tell us what's wrong with the roof.
You'll get one written total price.