The short answer
Carport and lean-to roofs are usually replaced with polycarbonate sheet, box-profile metal sheet or a felt or EPDM covering on a boarded deck. Which one is right turns on the frame, the pitch and how much light you want underneath. Check the posts and rafters first, because a new covering on tired timber only moves the problem.
Polycarbonate, metal sheet or felt for an open structure
Most carports and lean-tos end up with one of three coverings, and each suits a different frame.
Polycarbonate
Twin-wall or multiwall polycarbonate is light, lets daylight through and cuts easily on site. It suits an open carport where you want the space underneath to stay bright. HomeBuilding gives a typical lifespan of 10 years for polycarbonate conservatory roofs, and The Eco Experts quote about 10 to 15 years for glass or polycarbonate, so that range is a fair guide for a carport as well, with sun and quality of sheet both playing a part.
Box-profile metal sheet
Steel sheet in a box profile is stiffer than plastic and spans further between purlins, the horizontal timbers that carry the sheets. Fitted with an anti-condensation backing, it copes with a carport that is partly enclosed or used as a workshop. It is noisy in rain unless it has a lining, which is worth knowing if the carport sits under a bedroom window.
Felt, EPDM or GRP on a deck
A boarded roof under felt or a membrane gives a quiet, solid cover, but it needs a deck of plywood or boards under it and a frame strong enough to carry the extra dead weight. Falls count here: the recommended finished minimum for a flat roof is 1:80, designed at 1:40 so tolerances do not cancel it out.
Judging the frame before choosing the sheet
A carport roof sits on posts and a beam, with rafters or purlins running across. Because the structure is open, the rafters are exposed to rain from underneath as well as above, and the timber at the wall plate, the strip bolted to the house wall on a lean-to, often rots first. Look for soft timber at the post bases, splits along the beam and rust on brackets and bolts.
If the frame is sound, it can usually take the same weight of covering again. If it is not, then new sheets screwed to a rotten rafter will not hold in a gust. Light polycarbonate is easy to lift in wind, so the sheet manufacturer's fixing spacing needs following exactly, and the fixings are sealed with the washers the makers supply.
Changing from a light sheet to a heavier covering is a different job. On a house roof, a covering that adds 15 percent or more to the weight triggers a structural check under Approved Document A, and a carport frame has far less in reserve, so that goes on the survey sheet. We would rather tell you a post needs replacing than cover it and hope.
Attached lean-tos matter in another way. The joint with the house wall is the weak point, and the flashing there, a strip of lead or metal tucked into a mortar joint, is what keeps water out of the wall. We check that first.
Where a carport roof meets the house wall
A lean-to carport relies on a flashing along the wall. On a stone or brick wall the usual method is a lead flashing let into a chased mortar joint and folded over the covering. The lead code matters: Code 4 is described by Leadworx as the standard for general flashing, though your roof's exposure and length of run influence the choice.
Where the wall is rendered, the render should not run behind the flashing, or water tracks down inside it. If your carport is bolted to the house, we check the fixings go into sound masonry, not into loose pointing.
Gutters help too. A carport with no gutter at the front throws a sheet of water onto the car and the ground beside it. A gentle fall to an outlet, between about 1:600 and 1:350 according to manufacturers, keeps the run clear. Guttering costs about £30 to £70 per metre on MyBuilder's 2026 guide.
Light, noise and heat under the new roof
Clear polycarbonate gives the best light, but it turns the space under it hot in summer. Opal or bronze-tinted sheet softens that. If a car is parked beneath for years, a tint also helps protect the paint and the interior from strong sun.
Noise is the usual complaint with metal sheet. A lined or foam-backed sheet quietens the sound of heavy rain. In a roof over a workshop or a garden room a metal sheet needs ventilation at the eaves and ridge, as moist air will condense on the cold underside and drip.
The profile also decides the pitch. Many sheet products work at a shallow slope, but each has its own minimum, which sits in its data sheet, and a carport roof that is nearly flat needs a product made for that, with laps sealed and the sheet run in as few lengths as possible.
Planning, cost and what the job involves
A like-for-like recovering seldom needs planning permission, but a freestanding structure near a boundary, in a conservation area or attached to a listed house is different. Ask Stroud District Council's planning team if there is any doubt. Our guide to shed and outbuilding roofs covers the neighbouring case of closed buildings.
Cost depends on the size and the covering. MyBuilder's 2026 garage roof guide lists flat roofs at £700 to £1,000 for one garage and £900 to £1,400 for two, and a carport sits in a similar band by area. Old corrugated asbestos cement is a separate matter; if your carport roof is that, read about asbestos garage roofs and selling a house and how the sheet is tested.
On the day, we strip the old covering, check the frame, replace what has failed, fit the new sheets, seal the wall flashing, leave the site clear and remove the waste. A joined row of garages brings neighbours into it, which is covered in shared garage blocks. Once we have surveyed the carport, one written quote covers the work, and our workmanship guarantee runs for ten years on paper. Start from the garage roof replacement page if you are comparing options.