Polycarbonate roofing sheets are one of the best ways to let daylight into a carport, lean-to, canopy, pergola or conservatory roof without the weight or fragility of glass. They are light, strong, easy to cut and available in several thicknesses and tints. But they are also easy to fit badly. Using the wrong thickness, laying the sheet upside down, or sealing the ends incorrectly can cause leaks, condensation and early yellowing. This guide explains the types of polycarbonate sheet, how to choose a thickness, minimum pitch, sheet sizes, fixing and sealing, and how to look after the roof.
What Is Polycarbonate Roofing?
Polycarbonate is a tough, transparent plastic. For roofing it is made into flat solid sheets, multiwall sheets with hollow channels, or profiled sheets that match corrugated or box profile metal roofing. It is much lighter than glass and far more impact resistant, and it usually has a UV-protective layer on one side to slow ageing. It is commonly used on lean-tos, carports, verandas, greenhouses and covered patios. Our guide to patio roof ideas shows where it fits among the other options.
Types of Polycarbonate Roofing Sheet
| Type | Description | Best for | Points to note |
|---|---|---|---|
| Multiwall (twinwall, triplewall and more) | Layers of polycarbonate with hollow channels between them | Carports, lean-tos, conservatories, greenhouses | Better insulation, needs sealed ends and glazing bars |
| Solid sheet | A single flat sheet, clear like glass | Porches, canopies and places needing clarity | Noisier in rain, less insulating, usually costs more |
| Corrugated or profiled | Shaped to match metal or plastic roof sheets | Rooflights in sheds, garages and outbuildings | Profile must match the neighbouring sheets |
If you are mixing clear panels with metal or bitumen sheets, see our guide to corrugated roofing sheets for matching profiles and fixing.
Polycarbonate Thickness Guide
| Thickness | Typical use | Notes |
|---|---|---|
| 4 to 6 mm | Greenhouses and light glazing | Generally too thin for a roof that carries weather loads |
| 10 mm twinwall | Carports, pergolas, small canopies | Often quoted as the minimum suggested thickness for roofing |
| 16 mm triplewall | Larger canopies, walkways, lean-tos | Stronger and better insulated than 10 mm |
| 25 mm multiwall | Conservatory roofs, longer spans | Better insulation and load capacity |
| 35 mm multiwall | Self-supporting and larger spans | Highest insulation, typically for specialist systems |
Thicker sheets are stiffer and insulate better, but they cost more and need suitable glazing bars. Always check the manufacturer’s span and load tables, which depend on snow and wind exposure.
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Tints and Colours
- Clear: the highest light transmission, but it can feel bright and warm in summer.
- Opal (white): diffuses light and reduces glare, with less light passing through.
- Bronze or grey: reduces glare and heat, giving a darker space.
- Solar control or heat-reflecting: some sheets are designed to reduce heat gain while still letting light in. Check the manufacturer’s data.
Sheet Sizes
Sizes vary by manufacturer, so always check the data sheet. As a general guide, multiwall sheets are commonly sold in widths from around 700 mm up to 2,100 mm, with common widths including about 1,050 mm, and in lengths from 2 metres up to 6 metres, or cut to size. Many suppliers will cut sheets to length, and the sheets are usually supplied with protective film on both sides.
Minimum Roof Pitch
Polycarbonate roofs must slope so water drains and does not pool on the surface. A minimum of around 5 degrees is often quoted, with 7.5 degrees or more recommended, and some manufacturers ask for 10 degrees. Check the data sheet for your product, because a shallower roof can leak at the glazing bars and trap dirt. If you are not sure of your roof’s angle, our roof pitch calculator explains how to measure it. For single-slope designs, see our mono pitch roof guide.
Support, Glazing Bars and Fixing
Multiwall sheets are usually supported on aluminium or plastic glazing bars fixed to the rafters, with the sheet edges held in the bars and capped with a top cap. Solid and corrugated sheets are often screw fixed with sealing washers. Points to remember:
- Rafters or purlins must be strong enough for the sheet’s span and the local snow and wind loads.
- Glazing bars are usually spaced to suit the sheet width, and rafter-supported systems typically use maximum fixing centres in the region of 400 mm to 1 metre, depending on thickness. Follow the supplier’s table.
- Do not overtighten fixings, since the sheet expands and contracts with temperature.
- Drill oversized holes for screws so the sheet can move.
How to Fit Polycarbonate Roofing Sheets
This is an overview, and the manufacturer’s instructions always come first.
- Check the structure. Make sure the frame is square, level and has the correct slope and rafter spacing.
- Identify the UV side. The UV-protected side is marked on the protective film and must face upwards and outwards.
- Cut the sheets. Use a fine-toothed saw, jigsaw or sharp knife for thin sheets, and cut along the channels on multiwall.
- Seal the ends. Use solid aluminium tape on the top edge and vented breather tape on the bottom edge, so air can circulate and condensation can drain, then fit end caps.
- Fit the glazing bars. Fix the base bars to the rafters, lay the sheets in position and keep the gaps even.
- Fit the top caps and trims. Clip or screw the caps on and fit side flashing, ridge or wall flashing where required.
- Peel off the film. Remove the protective film only after the sheet is fixed.
Use a low modulus neutral cure sealant, not acidic silicone, which can attack polycarbonate. Our guide to roof coatings and sealants explains the differences between sealant types.
Condensation, Algae and Cleaning
Multiwall sheets can show condensation or algae inside the channels if the ends are not sealed properly, or if the bottom edge is not vented. Sealing and venting the edges as the manufacturer recommends is the best prevention. To clean the roof, use warm soapy water and a soft cloth or brush, and rinse well. Avoid abrasive pads, strong solvents and pressure washers, which can scratch or damage the surface and UV layer.
How Long Does Polycarbonate Roofing Last?
Quality sheets with a UV layer commonly carry guarantees of around 10 years, with a life expectancy of 20 years sometimes quoted, but real lifespan depends on UV exposure, correct fitting and the product grade. Cheap, unprotected sheets can yellow and become brittle much sooner. Always ask what the guarantee covers and what must be done to keep it valid.
Heat, Noise and Light
- Heat: clear sheets can make a space very warm in summer, so consider opal, bronze or solar control tints.
- Noise: solid sheets can be noisy in heavy rain, while multiwall is quieter.
- Light: clear gives the most daylight, while opal is softer and more diffused.
Rules and Safety
A lean-to, carport or conservatory roof can fall under planning permission or building regulations depending on size, height and position. The Planning Portal explains the rules in England, and Scotland, Wales and Northern Ireland have their own systems. Conservatory roofs have extra requirements, as covered in our conservatory roof replacement guide.
Working at height is dangerous. Polycarbonate sheets are not designed to be walked on. Use roof ladders or crawling boards over the supports and never step on the sheet itself.
Asbestos warning. If you are replacing old corrugated cement sheets, they may contain asbestos. Do not drill, break or scrub them, and get them identified by a competent surveyor before any work.
Frequently Asked Questions
What thickness of polycarbonate roofing sheet do I need?
10 mm twinwall is often the minimum suggested for roofing and suits carports and small canopies. 16 mm is better for larger spans, and 25 mm is common on conservatory roofs.
Which way up do polycarbonate sheets go?
With the UV-protected side facing up and outwards. It is marked on the protective film, and fitting it upside down can shorten the life of the sheet.
What is the minimum pitch for a polycarbonate roof?
Often around 5 degrees as a minimum, with 7.5 degrees or more recommended, and some manufacturers asking for 10 degrees. Check the product data sheet.
Can you walk on polycarbonate roofing?
No. It is fragile in the sense that it cannot take a person’s weight safely. Use crawling boards that span the supports.
Why is there condensation inside my polycarbonate sheet?
Usually because the ends are not sealed or vented correctly. Use aluminium tape on the top edge and breather tape on the bottom edge, as the manufacturer recommends.
How long does polycarbonate roofing last?
Often 10 to 20 years for good quality UV-protected sheets, though guarantees and results vary. Poor quality or incorrectly fitted sheets may fail sooner.
Final Thoughts
Polycarbonate roofing sheets are a light, affordable way to bring daylight into a carport, lean-to or covered area. Choose a suitable thickness and tint, make sure the pitch and support are correct, fit the UV side up and seal the edges properly. Stay off the sheets, check the planning rules and treat any old cement sheets as asbestos until proven otherwise.
Get a professional assessment
If you are unsure how much life your current roof has left, a local roofer can give you an honest assessment as part of a free inspection. Browse vetted roofers through our roofer directory, or get a quick replacement estimate using our free cost calculator.