Drive down almost any lane lined with garages and you will spot the ribbed grey or green steel on at least one of them. Box-profile sheet is a long panel of coated steel pressed into a repeating pattern of high ribs and flat troughs, the cross-section of the ribs being square-shouldered rather than rounded like old corrugated sheet. It is light, quick to lay and sheds water well, which is why it is the usual answer when a cement or bitumen garage roof reaches the end of its life.
The product has its own details, though, and most of the problems we are called to are really detail problems: sheets fixed in the wrong place, laps left unsealed, a cut edge left bare. This guide covers the panel, the backing, the fixing and the ageing. If the reason you are here is a wet underside, the fault itself is covered in why a metal garage roof drips. For the wider choice of coverings, see garage roof replacement.
What a box-profile panel actually is
The panel starts as a coil of steel, galvanised so that zinc protects the base metal, then coated on its upper face with a paint or plastic layer that gives the colour and does most of the weatherproofing. The coil is rolled through a machine that forms the profile and then cut to length, so panels can be ordered to run from eaves to ridge in one piece. A single long sheet has no horizontal joint for water to find.
Two things set it apart from corrugated sheet. The ribs are flat-topped and stiff, so a panel spans a wider gap between supports. And the flat crown of each rib gives a firm surface to screw through, a point that matters when we reach fixings.
Terms worth knowing
- Crown: the raised, flat part of the profile.
- Trough: the low channel between crowns, where water runs.
- Side lap: the strip where each sheet overlaps its neighbour.
- End lap: the overlap between sheets stacked up the slope, needed only on long runs.
- Purlin: a horizontal timber or steel beam across the rafters that the sheets rest on.
Single skin and insulated panels
Single-skin sheet is one layer of steel and is what most garages use. Insulated panels sandwich a core between two skins, and are a different, heavier product that belongs with a garage that is being lived in; the roof side of that job is in our guide to garage conversion roofs.
Anti-condensation backing: what it does and where it stops
Cold steel above a warm, damp garage is a condensation machine. Warm air carrying moisture meets the underside of the metal, cools, and the vapour turns to drops that fall as if the roof were leaking. A single-skin steel sheet has no way to absorb any of this.
Anti-condensation sheet answers the problem with a layer fixed to the underside, usually a fleece or felt-like material. The layer takes up the moisture as it forms and lets it go again when the air dries, rather than letting droplets gather and fall. Think of it as a buffer, not a barrier.
The limits of the backing
It copes with an occasional damp spell, a car brought in wet, a washing machine door left open. It cannot cope with a steady supply of moisture. A garage where a tumble dryer vents inside, or where the floor is permanently damp, will overwhelm any backing, and the answer there is to remove the moisture source or add ventilation. The wider rules on moisture in buildings are set out in BS 5250, which Marley cites when it explains how a roof void should be ventilated.
Backing at the edges
The backing is only as good as its edges. Where a sheet is cut on site, the fleece is exposed at the cut and can wick water along the sheet. A good installer seals or trims the cut. At the eaves, the end of the backing needs to stay above the gutter line so it does not sit in standing water.
When we would not specify it
An unheated store holding only a bicycle and a lawnmower seldom needs it, and the plainer sheet is cheaper. For a workshop, a damp floor or any room that will be warmed, we would choose the backed version and look at ventilation too.
Fixing: crowns, troughs and the lap
Every manufacturer publishes its own fixing instructions, and those are the reference, not any rule of thumb from us. But there are common principles that explain why some sheet roofs last and others lift in their first gale.
Where the screw goes
On profiled steel, screws are driven either through the crown or through the trough, and the two are not interchangeable. Fixing through the trough puts the screw where water runs, so it is sealed with a washer and relies on that seal for the life of the roof. Fixing through the crown keeps the hole above the water line. Which a given product wants depends on the profile and the maker, which is why the data sheet comes first.
Screws and washers
The fixing is a self-drilling screw with a rubber sealing washer that compresses against the steel. Over-tightening crushes the washer; under-tightening leaves a gap. On a failed sheet roof, screws that miss the purlin or go in at an angle show up quickly under inspection. Fastener finish counts too: a poor screw rusts at the head and stains the coating below.
Laps and sealing
Side laps are the water joints of a sheet roof. Where the pitch is shallow, a strip of sealant tape in the lap closes the gap that wind-blown rain would otherwise pass through. The lap should also be stitched with fasteners at the spacing the manufacturer states, because a loose lap flaps and fatigues.
Pitch
Sheet roofs shed water at lower slopes than tiles, but they are not flat roofs. A minimum pitch applies to each profile, and we read it from the maker's own literature. If the garage has a very shallow slope, the manufacturer's laps and sealing instructions become non-negotiable, and a pitch below the stated minimum is a case for a different covering.
Supports, weight and what the frame has to do
One of the quiet advantages of steel sheet is how little it weighs next to a tiled covering. Concrete tiles are heavy enough that swapping them for sheet takes a lot of load off the roof; a frame built for sheet needs checking before tiles are ever laid on it. Going the other way, a light frame suits sheet, so long as the purlins sit at the spacing the sheet requires.
Purlins and rafters
The sheet spans from purlin to purlin, and the allowable span is listed in the maker's data for each gauge of steel. Where the old roof was asbestos cement on timber purlins, the existing timbers are checked for rot, slackness and level before the new sheets go on, and any that are spaced too widely are supplemented. A sheet fixed across a gap bigger than it can span will dip, collect water and eventually leak at the fixing.
Uplift in wind
Light roofs have less dead weight to hold them down, so fixings carry more of the job. That puts the focus on the wall plate and the way the rafters tie into the walls. A sheet roof that is only as secure as a few rusty nails is a risk on a lane with no shelter, and we check the connection down to the masonry, since a tidy new sheet on a loose frame is wasted effort.
Weight and the rules
Because steel sheet is lighter than most coverings, the 15 per cent weight threshold that the Planning Portal gives for Building Regulations seldom bites when you go to sheet. It can bite when the garage is changing to something heavier, and then a structural check comes first. On an attached garage, whole-roof replacement can go past the Planning Portal's limits (work to less than 25 per cent of the building envelope and less than 50 per cent of the roof), in which case building control is involved and we handle the notification. A small detached garage is often exempt altogether: the Planning Portal says one under 30 m² would not normally need approval if it is under 15 m², or at least a metre from any boundary, or built substantially of non-combustible materials. Building control can confirm which applies to yours.
How long a sheet roof lasts, and what shortens it
How long a sheet roof lasts depends on exposure, upkeep and the quality of the coating, and it is the coating, not the steel, that sets the clock. The steel itself is protected by zinc and by paint, and once the paint chalks, scratches or peels, the zinc takes the strain and rust starts when it runs out. A manufacturer's guarantee on the coating is a statement about the paint, no forecast of the roof's working life, and that holds for every covering.
What wears a sheet roof
- Cut edges. Steel cut on site is bare at the edge. Left alone, it rusts first and the rust creeps under the paint.
- Scratches. A dragged ladder or a dropped tool takes the coating off and exposes zinc.
- Standing water. Leaves and moss held in the troughs, or a blocked gutter, keep a section wet and shorten the coating's life there.
- Overhanging trees. Drips and debris from branches wear at the surface and clog the gutter.
- Chemicals. Sap, bird droppings and cleaning products can mark and attack a coating.
Maintenance that earns its keep
Clear leaves from the troughs, keep the gutter running and look for rust blooms around the screws and cut edges once a year. Touching up a scratch with the right paint is cheap, and the fixings that are weeping at the washer are cheaper to replace early than late. Do not pressure-wash a coated sheet; use a soft brush and water.
Noise
Single-skin steel is loud in heavy rain. It does not damage the roof, and for a garage few people mind, but it matters if the space is a workshop or a studio. Backed sheet softens it a little, and insulated panels more.
Replacing a failed garage roof with box-profile
Box-profile is a common choice to replace corrugated cement or a worn bitumen roof, and it suits a pitched garage whose frame is sound. If the sheets coming off are asbestos cement, the removal comes first; under HSE guidance, careful removal of intact asbestos cement sheeting is non-licensed work, while broken-up or heavily damaged sheeting is notifiable. Our guide to overlaying an asbestos roof explains the case for going over rather than removing, and where it falls down.
What the survey checks
We measure the run from eaves to ridge, to see whether sheets can be single lengths, and the pitch against the manufacturer's minimum. We look at the purlins, rafters and wall plates, at the gutters and fascias, and at what is under the eaves. The quote that follows, in writing, covers scaffold or access, stripping, sheets, fixings, trims, labour and rubbish removal.
A sense of cost
MyBuilder's garage roof guide, updated in 2026, gives £2,500 to £4,500 for a pitched garage roof. That covers all coverings and is a national range, not a Supreme price, and a box-profile job can land at any point within it depending on the size of the garage, how reachable the roof is and how sound the timbers are. Disposal can add noticeably to the bill when asbestos is involved, and MyBuilder's 2026 asbestos figures put single-garage removal at £350 to £1,295.
Related problems
Once the new sheets are on, any leaks that turn up are almost always at laps, fixings or flashings where the roof meets a wall; our guide to leaking garage roofs runs through the causes in order. If the building is a shed, not a garage, shed and outbuilding roofs compares the smaller-scale options. Workmanship is guaranteed for 10 years, set down in writing; the garden is left tidy.