A sectional garage is assembled from precast concrete panels slotted into posts, with a roof of corrugated sheets laid over light bearers. It was quick and cheap to build, so rows of them appeared behind houses and on estate garage courts, and plenty still work today. The walls usually look fine for decades. The roof is the part that gives up first, and it causes the most worry, because the sheets on older examples are frequently asbestos cement.
That one fact changes how you plan a replacement. A sound-looking roof can be left alone and watched. A leaking one needs a decision about whether to cover it, remove it or rebuild it, and each choice has different costs, waste duties and effects on the walls beneath. The sections below cover how the roof is put together, how to approach the question of asbestos without creating a hazard, the options for a new roof, the work on site, and what the walls and any shared boundary add to the picture. The wider garage roof replacement hub covers the other garage types.
How a sectional garage roof is put together
The walls of a sectional garage are tall narrow panels, slotted into grooves in concrete posts at the corners and between bays. There is no ring beam of the kind a brick garage has. Instead, the sheets sit on bearers, timbers that span from the front wall to the back, and those are fixed to the panel tops or to brackets cast into them. The result is a light roof on a stiff box.
The roof is usually shallow, falling gently to the back or to one side, and the sheets overlap along the slope and across it. They are held down by hook bolts or screws passing through the crests of the wave, and a ridge or capping runs along the high edge where the roof meets the wall. Gutters, where they exist, hang from the sheet ends or from a short fascia.
Where the water tends to find a way in
Because the roof is so shallow, the laps are the weak spot. In a gale, water can travel sideways under a lap and drip from the underside of the next sheet. Fixings loosen over the years. The sheets themselves become porous and stained, and moss grows in the troughs. Where a panel joint meets the roof, gaps open and let the rain in at the wall head. A leak in one corner of the ceiling often starts at one of those joints instead of at the nearest sheet, which we check from inside and out.
Brown streaks on the wall panels below the sheets show where water has been running, and a torch along the underside shows where it started.
Why these roofs so often turn out to be asbestos cement
Asbestos cement was a standard roofing for garages, sheds and outbuildings for a long time, because it was cheap, weatherproof and light. Prefabricated garages, built to a price, used it widely. Anything built before 2000 is treated by HSE as a likely asbestos site, and that applies to a garage as much as a house. If yours comes from that era, treat the sheets as asbestos cement until a test shows otherwise.
That does not make the roof a hazard to live beside. The risk comes from damage: breaking a sheet, drilling it, scrubbing it with a wire brush, or pressure washing off the moss. Each of those throws fibres into the air, and moss on an old roof is the usual reason people reach for the hose. Please resist. A soft brush is no safer than a hose.
What not to do
- Do not walk on the roof. HSE guidance calls every asbestos cement roof fragile, and a sheet can give way without warning.
- Do not drill into the sheets to fix a light, an aerial or a bracket.
- Do not jet-wash, scrape or brush the surface.
- Do not break a sheet to reach the inside or to take it off in pieces.
Neighbours, tradespeople and well-meaning relatives are the people most likely to offer to tidy a roof up, so it is worth telling them first. If you are unsure, ask for the roof to be assessed before anyone climbs onto it. Our guide to corrugated garage roof sheets shows how the different sheet materials compare, and a survey of your own garage settles which you have.
Three realistic options for the roof
When a sectional garage roof is worn out, the choice is between covering it, taking it off or rebuilding in a different form.
Covering over. HSE asks owners whether new sheeting could go over the old instead of the old coming off. Fixing new sheets above the existing ones avoids breaking the old material up and avoids a disposal bill. It suits only a roof whose sheets are in reasonable order, whose frame can carry the extra layer and whose fixings can be made without cracking the old sheets. The old roof stays on the garage, which matters if you later sell or alter the building, because a buyer's surveyor will notice it and a future owner inherits the question.
Removal and replacement. The sheets are taken off whole, lowered to the ground, wrapped and disposed of as hazardous waste, and a new covering goes on, often on new bearers. The garage ends up free of asbestos, but the handling and the waste route cost more. Our guide to disposing of asbestos roof sheets explains that route.
A different roof form. The frame can often take a low-pitched or flat roof on a new timber deck, which gives better insulation and a longer life but needs falls designed in. IKO, citing the 2018 edition of BS 6229 (since revised), gives 1:40 as the design fall for a flat roof, to leave a finished 1:80.
How we choose
We weigh the sheets, the frame, the walls and what you want the garage to do, then put the smallest job that holds in front of you with photographs of your own roof beside it.
What replacement costs and what moves the figure
On MyBuilder (updated 21/08/2026) the typical UK average for asbestos removal is around £2,000, with prices starting at £50 per m². Its range for removing a single garage roof is £350 to £1,295 and for a double garage £650 to £1,800. Checkatrade gives £950 to £3,750 for removal and disposal. These are national guides and not our prices, and they cover taking the old roof off.
For the new covering, MyBuilder gives £700 to £1,000 for a flat roof on a single garage and £900 to £1,400 on a double, with a pitched roof at £2,500 to £4,500. What lies under the sheets can move the total either way, and a roof that looks the same from the drive can be a very different job once the bearers are visible.
What pushes the cost up or down
- Whether the sheets come off whole or are already cracked, which changes the method and the paperwork.
- The distance to a tip that accepts the waste, and the number of loads.
- The state of the bearers and the fixings to the panel tops.
- Access, including whether the garage backs onto a lane or a neighbour's fence.
- The covering you choose, and any insulation added at the same time.
Nobody can price this from a photograph of the roof. After the survey the written quote arrives as one total, and you can check whether VAT and any scaffold or access tower are inside it. The guide to what a garage roof costs goes through the wider range of garages.
Taking a sectional garage roof off safely
The sequence on site is slower than people expect, and that is deliberate. The ground below is covered, and access is set up so that nobody steps on the sheets. Sheets are unfixed from the edge, lifted whole and lowered. HSE asks for large pieces to be lowered to the ground and for rubble chutes and drops to be avoided.
If a sheet breaks up and creates significant dust, the work becomes notifiable non-licensed work, and the regulator must be told. That is why we ask at survey whether any sheets are already cracked, and why we look hard at the sheets along the high edge.
Once the old sheets are down, they are double-wrapped, labelled and loaded into a lockable skip. Nothing is mixed with ordinary waste. If the garage holds a car or tools, they are covered or moved first, and the site is cleared with everything taken away afterwards. We keep the number of people on the roof low and the time the sheets spend in the open short, so that the wrapped waste is not left lying about.
Using the moment to improve the roof
With the sheets off, the bearers and the panel tops are visible. It is the time to repair a rotted bearer, fit a drip edge, add a ventilated lining or put in insulation. Our guide to insulating a garage roof explains the options and the condensation question, which matters on a concrete box that holds the cold.
A job like this is mostly planning, and the written quote reflects that. You get the quote once the survey has shown the real state of the roof, with photographs of it.
Walls, panels and the neighbours next door
A new roof only helps if the walls underneath will carry it. Concrete panels are sturdy, but a heavy covering asks more of the panel tops and posts than the original designer may have allowed for. A light steel or fibre cement sheet adds little. That is one reason a sectional garage usually receives a light replacement covering and not a heavy tiled one.
We check the panels for cracks, spalling concrete and rust staining where reinforcing steel has started to corrode, and we look at the post bases and joints for movement. Gaps at the wall head are filled before the new roof goes on. Panels that lean or bulge are a structural matter for an engineer or builder, and we tell you plainly if we see signs of it.
Gutters and shared walls
An old sectional garage may have no gutter, and water then runs down the panels and soaks the post bases. Our guide to shared garage roofs deals with the common case of a pair or a row, where one wall or roof edge belongs to a neighbour. Talk to them before work starts, and ask the council or a surveyor whether a Party Wall notice applies. Leasehold garages may also need the landlord's written consent.
Replacing a roof like for like does not normally need planning permission, though a listed building or a planning restriction is a reason to ask the Adur and Worthing planning team first. Building control is a separate question where the roof form changes or the garage becomes living space. Consent is explained to you and, where work is notifiable, we handle the Building Regulations notification. The survey also notes how the old sections will come off the building and where they go afterwards.
