The short answer
Choose polycarbonate for a car port that needs daylight underneath and a lightweight roof, and choose coated steel for one that needs to last longest and stay quiet in rain. Polycarbonate usually has the shorter life and clouds with age, while profiled steel is stiffer, spans further between supports and ages slowly. The frame, the pitch and what sits under the roof should settle it.
What each sheet is made of
Polycarbonate is a tough clear or tinted plastic, sold in two forms. Solid sheet is a single skin, glassy in appearance, while multiwall sheet has a row of hollow channels running through it, which makes it stiffer, lighter for its thickness and a little better at holding heat. Both are cut with ordinary tools and fixed with sealed screws and washers.
Steel car port sheets are thin coated steel pressed into a profile. The profile is what gives them stiffness, so a sheet barely thicker than a credit card can bridge a gap between supports that a flat plastic panel could not. Most carry a factory paint or plastisol coating, and they come in a limited run of colours. A box profile sheet, with its flat-topped ribs, is the shape you will see on most modern car ports and garages, and box profile roof sheets have their own page.
Fibre cement is the third traditional option, but it is brittle, heavier than either of the above and not the first choice on a light frame. If the existing sheets are old, grey and corrugated, see the note on asbestos below before you do anything else.
Light, shade and what is parked underneath
The case for polycarbonate is daylight. A car port with a clear roof does not feel like a cave, the space under it is usable for a bench, bikes or a seating area, and the house wall behind keeps its light. A tinted or opal sheet cuts glare and heat while still passing a soft light, which suits a south-facing wall where a clear sheet would turn the space into a greenhouse in summer.
Steel gives you the opposite. It blocks light entirely, so the area underneath is shaded and the paintwork on the car is protected from sun, but the space is gloomier and some people end up adding a light fitting. If the car port attaches to a house window, a steel roof can darken the room behind it.
It is worth thinking about what actually lives under the roof. A car that is out of the weather but warm and bright is fine under either. A workshop, a log store or a dog run is happier under steel, where the shade stays cool. A seating area or a greenhouse-style growing bench suits polycarbonate.
Rain noise, condensation and heat
Noise is where many car port owners change their minds. A thin single skin of steel or solid polycarbonate drums loudly in heavy rain, particularly when the car port is attached to a bedroom wall or sits under a window. Multiwall polycarbonate is quieter than either because the air in its channels damps the sound. Steel can be bought with an anti-condensation felt lining or a thicker foam backing that deadens rain noise as well as stopping drips.
Condensation is the other issue. Warm, moist air meeting a cold underside forms droplets that fall onto whatever is below. Bare steel does this more than most coverings, which is why a lined or insulated sheet is a common upgrade. Polycarbonate forms less condensation on its surface but multiwall sheets can trap moisture inside the channels if the ends are not sealed correctly, leaving green growth or a permanent haze behind the plastic.
Heat is the plastic roof's weak point. Clear polycarbonate lets sunshine heat the space beneath, and a south-facing car port can become uncomfortably warm. If that matters, choose an opal or bronze tint, and make sure the sheets are fixed with room for the thermal movement all plastics have.
How long each one lasts
Lifespan is where steel usually wins. Typical figures, which depend on exposure and workmanship, put polycarbonate conservatory roofing at about 10 years in one source and 10 to 15 years in another (HomeBuilding, 2024; The Eco Experts, 2024). Profiled metal sheet is credited with figures in excess of 40 years by one supplier, and 40 to 70 years in the MyBuilder garage roof guide (updated 2026). The supplier adds that a plastisol coating typically wants its first maintenance after 15 to 25 years, and that coastal or industrial sites shorten the life.
Treat those as ranges, not promises. A polycarbonate sheet that faces the sun and gets no cleaning will yellow and become brittle sooner. A steel sheet with a cut edge left unprotected, or one scratched by a ladder, can start to rust from that point. Fixings matter equally: a screw that is overtightened cracks a plastic sheet, and one that is left loose lets wind lift a steel one.
The frame beneath has a life of its own. A tired timber car port frame with soft ends will outlast neither covering, and a new roof on a rotten frame is money spent in the wrong place. The easiest time to repair the frame is while the old sheets are off.
Frame, pitch and fixings
A car port roof is nearly always a lean-to or a shallow mono-pitch, and the angle matters. Corrugated and box profile sheets need enough slope to shed water along the ribs and a decent overlap at the joints, while multiwall polycarbonate is usually laid to run water down the channels, not across them. A nearly flat car port roof is the one that leaks first, whichever sheet it carries.
Spacing between the supports sets how thick a sheet can be. Steel with a deep profile can span a wider gap, so a car port with widely spaced purlins (the cross members the sheets rest on) suits it better. Polycarbonate wants closer supports, and the thinner the sheet, the tighter they need to be. Fitting one over a frame built for steel can leave sheets sagging and holding water.
Fixings are different too. Steel is screwed through the high rib using self-drilling screws with a washer, so the water has no path along the thread. Polycarbonate is fixed with oversized holes, sealed washers and aluminium glazing bars so the plastic can expand without cracking. At the wall, both sheets need a flashing or a sealed closure strip, otherwise water runs back down the brickwork into the car port. If the house wall carries a damp stain after rain, that edge is the first place to check.
Old sheets and asbestos
A car port built in the second half of the last century may be roofed in grey corrugated sheet, and that sheet may be asbestos cement. Cracks, green growth and a chalky surface are all signs of a sheet near the end of its life, but they do not tell you what it is made of. Do not drill, break or wash a sheet you are unsure about.
If the old sheet is intact and sound, the question to answer first is whether it is asbestos at all, and our page on asbestos surveys for garage roofs explains when testing is worth doing. If a sheet is already damaged, our guide to a broken asbestos roof sheet sets out what to leave untouched and how it is dealt with. A car port that has been quietly sitting there for years is very different from one where the sheets are splitting.
Choosing, and getting it fitted
As a rough guide, pick multiwall polycarbonate where light matters and the car port sits clear of bedrooms, solid or opal polycarbonate where you want daylight without glare, and lined steel box profile where life, strength and quiet matter more than light. A mixed roof is possible too: steel over the car and a clear panel over a walkway or door.
We survey car ports as part of our garage roof work. That means checking the frame, the pitch, the wall junction and the existing sheets, then recommending a covering that suits what is actually there. Once the survey is done the price is set on paper, and at the end the old sheets are carted off and the ground left clear. Our main garage roof replacement page covers the wider service.
