Drive down almost any 1950s to 1970s estate in Basingstoke, Tadley or Aldershot and you will see them at the end of the drive or in a block behind the houses: single-storey garages made of grey concrete panels, with a low-pitched or near-flat roof of corrugated sheets. They were sold as a kit and bolted together on a concrete base in a day or two. Their walls often outlast their roofs by a long way. This guide explains how these garages are put together, why the roof usually goes first, what you should check before work begins and how a new roof is fitted to a structure that was not designed with replacement in mind.
How a sectional garage is built
A sectional garage is a kit of precast concrete parts. Upright posts are set in a concrete base or bolted to it, and tall rectangular wall panels slot between the posts, often held in grooves or by bolts through the posts. The panels are usually thin, with a steel reinforcing mesh or a timber frame cast into them. At the top, a beam or cross member runs from side to side, and the roof is laid across it.
There were several roof types. The commonest by far is a low-pitch roof of corrugated sheets, supported on timber or concrete purlins (horizontal beams that carry the sheets) and sloping gently from front to back. Many of those sheets were made of asbestos cement, as the material was cheap and light. Others used corrugated metal or fibre cement without asbestos. Some designs have a flat or monopitch roof of concrete or timber panels covered with felt. A few have a pitched roof with a ridge and tiles or sheets. Because the kits came from a handful of makers, but were widely copied, there is plenty of variation from one garage to the next.
Why the roof goes first
The walls of a concrete garage need little attention. The roof does not enjoy the same luck. Corrugated sheets weather and crack, and moss builds in the troughs. The overlaps between sheets are tight and shallow, so wind-driven rain gets under them, and the fixings are often bolts with small washers that corrode and let water in at each hole. The low pitch makes matters worse, since water drains slowly and every joint is tested for longer.
Condensation is the other problem. The roof sheets are cold and the garage air is damp, because cars come in wet and nothing is insulated. Moisture condenses on the underside of the sheets and drips onto the beams, which are often softwood and slowly decay. By the time the roof leaks visibly, the timber underneath can be soft. Where the roof is a felt-covered flat panel, the common fault is the felt itself, which cracks, blisters and lets water through the joints between panels. Our answer on why a garage roof leaks goes through the usual causes in detail.
Check for asbestos before anything else
If the garage was built before the end of the 1990s and the roof is corrugated and grey, treat it as asbestos cement until a sample proves otherwise. That does not mean panic. Intact sheets are considered lower risk, and careful removal of whole sheets is non-licensed work under HSE rules. It does mean nobody should drill, scrub, pressure wash or break the sheets, and anyone working on the roof needs to know what it is. Our guide to how to tell if your garage roof contains asbestos explains the clues, and the fuller guide to asbestos garage roof removal sets out the rules, process and costs.
Other parts of a sectional garage can contain the material too. Gutters, downpipes and flat-sheet fascias were sometimes asbestos cement, and a garage that once housed a heater may have a flue pipe in the same material. A thorough survey looks at all of it.
What we look at in a survey
A sectional garage is not like a house roof, so the survey is specific. We look at:
- The type and condition of the roof covering, and whether it is asbestos cement.
- The beams or purlins that carry it, checking for rot at the ends where they sit on the panels.
- The top of the walls, where the panels meet the roof, for cracking, rust staining and loose bolts.
- The posts and their fixings, which can rust at the base and shift with time.
- The gutter, its brackets and the downpipe, since many prefab garages have shallow gutters that overflow.
- Any pitch or fall, because a roof that has sagged holds water.
- The doors and the wall behind the roof, where leaks often show first.
New roof options
With a low pitch and a light structure, the replacement has to match the building. The main choices are these.
EPDM rubber membrane
A single rubber sheet bonded to a new deck of boards. It suits low-pitch and near-flat garages, has no joints in the middle of the roof and can be dressed neatly at the edges. A published cost guide gives a typical UK range of £1,200 to £2,500 fitted for a single garage roof. Our answer on whether EPDM is good for a garage roof covers the pros and cons.
Fibre-free profiled sheets
Modern corrugated or box-profile sheets made without asbestos look similar to the original. They suit a garage that has to stay in keeping, and they are light enough for most sectional frames. Because they are laid to a minimum pitch with proper overlaps and sealed fixings, they perform better than the original sheets did, but they still need ventilation to avoid condensation. Insulated or lined versions help.
Felt
Mineral-faced felt over a boarded deck is a budget choice for a low-pitch garage. It is easy to repair, though it has a shorter life than the alternatives.
GRP or a pitched roof
GRP gives a hard, jointless finish on a flat deck. A pitched, tiled roof gives a more traditional look, but needs a frame strong enough to carry the load and may need approval, since the roof structure is changing. A garage that is being converted or extended is a different matter, which our guide to the garage conversion roof covers.
How the job runs
The first step is the survey and sampling, followed by a fixed written quote. On the day, the garage is emptied or covered, and if the sheets are asbestos cement they come off whole and are packaged as hazardous waste. The beams are inspected, and any that are rotten are replaced or doubled up. If the roof is to be boarded, the deck goes on with a fall to the back, and the new covering follows with its edge trims and a gutter.
Connections to the walls are the delicate part. The panels are thin and the bolts that hold the beams may be rusted or seized. We avoid forcing them. Where a fixing is lost, new brackets are fitted using chemical anchors or through-bolts, and the wall top is repaired with a suitable mortar or sealant so water has no way in. A damp-proof strip or flashing is added where the new roof meets a house wall, as many sectional garages are attached.
Walls, bases and doors
A new roof sits on the garage as it stands, so we also note what else is wrong while we are there. Panels can crack where the steel reinforcement inside has rusted and swollen, a fault called concrete cancer in its mild form. Small cracks can be sealed. Larger ones, or panels that bulge, need to be flagged, because a wall that moves will pull on the new roof edge. The base can settle or crumble, particularly at the corners, and the garage door frame can drop out of square, so the door catches or leaves a gap. None of this stops a roof going on, but it affects how the edges are detailed, and we would rather tell you now than leave you to find a leak at the front edge next winter.
What affects the price
For a sectional garage the cost splits into removal and disposal of the old roof, repairs to the timbers and fixings, the new covering and the finishing. Asbestos cement sheets add specialist handling and disposal to the first line. A rotten beam adds timber. A tall or awkward garage adds access costs. The choice of covering matters too, with a simple membrane at one end and a pitched tiled roof at the other. Our garage roof replacement cost guide sets out the published ranges and the factors in more detail.
Insulation and ventilation
A garage roof does not have to be insulated, but if the garage is used as a workshop or store for damp-sensitive items, it is worth considering. The same moisture rules apply as on any roof: an insulated cold roof needs a clear ventilation path, while insulation above the deck avoids the problem. Our short answer on whether you should insulate a garage roof discusses it.
Guttering and rainwater
Water leaving the roof has to go somewhere, and on many sectional garages it does not. The original gutters were small, often of the same asbestos cement or of thin pressed metal, and they sit at a shallow fall. Blockages are common, since leaves and moss wash down from the roof and collect. When the gutter overflows, water runs down the front panel and the door, soaks the base and can splash back up under the roof edge. A new roof is the moment to fit a proper gutter with brackets fixed to a sound fascia or edge board, and a downpipe running to a drain or soakaway. It costs little next to the rest of the job and prevents a surprising amount of damp.
Planning and regulations
Replacing the covering of a garage is normally a like-for-like repair that does not need planning permission. Changing the shape of the roof, raising its height or converting the building raises different questions, and the council's planning team or your building control body will tell you what applies. If a garage is within a conservation area or the roof is visible from a listed building, ask about it first.
Booking a survey
We cover Basingstoke and the surrounding towns. First comes a free visit where we photograph your garage roof and walls, then a fixed written quote that itemises removal, waste and the new covering. Our workmanship is guaranteed in writing for ten years. Further detail on our garage work is on the garage roof replacement in Basingstoke page, and the office line is 01256 213956.
