How to Install a Polycarbonate Patio Roof: Slope, Purlins, Oversized Holes and UV Side Up
Polycarbonate expands and contracts far more than you expect. Room to move, the right side up and the right slope make it last.
- Intermediate
- 1 to 2 days
- Updated
The short answer
To install a polycarbonate patio roof, build a frame with a slope of at least about 1/12 (more is better), set purlins at the spacing the sheet maker specifies, lay sheets UV side up, drill fastener holes 1/8 inch or more oversized for expansion, fasten with washered screws without over-tightening, seal multiwall ends with tape, and flash the top against the house. Confirm all spacing with the sheet manufacturer.
Polycarbonate is a popular patio roof because it lets in light while keeping rain off. It is light, tough and comes in clear, tinted and opal colours. It is also very different from wood or metal in one respect: it expands and contracts far more with temperature, roughly six times as much as steel, so a sheet that looks fine on a cool Fraser Valley morning needs room to grow on a 30 degree afternoon. Most failed polycarbonate roofs fail for one of three reasons: no room for movement, the wrong side facing up, or too little slope.
There are two main types. Multiwall sheet has hollow channels, is stiffer and insulates better, and needs its end channels taped. Corrugated sheet is single layer, looks like corrugated metal, and is lighter and cheaper. Both are installed in a similar way, and both need the frame to be right first. The figures below are common ones, and the sheet manufacturer's instructions govern for your product.
If the roof attaches to the house, the junction with the wall needs flashing, see how to flash a roof to a wall by cladding type. For other small structures, see how to roof a shed.
Tools
- Cordless drill with bits for plastic
- Fine-tooth circular saw blade or handsaw (for plastic)
- Tape measure and level
- Chalk line
- Caulking gun
- Clamps and straight boards
- Safety glasses and gloves
Materials
- Polycarbonate sheets (multiwall or corrugated), with UV protection on one face
- Sealing tape: solid aluminum tape for the top end and vented tape for the bottom end of multiwall sheets
- Washered screws or fasteners made for polycarbonate, with neoprene or EPDM washers
- H-profile and end caps or closure strips as the system requires
- Top flashing for the house wall and drip edge at the eave
- Pressure-treated lumber or aluminum framing for the structure
Safety first
- Polycarbonate is slippery and cannot carry a person. Never walk on the sheets. Work from a ladder, scaffold or boards across the framing.
- WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more. A patio cover attached to a second-storey wall is high enough to need a proper plan.
- The sheets are light and catch wind like sails. Do not handle them in wind, and keep the protective film on until they are fastened.
- Wear eye protection when cutting and drilling. Plastic chips fly and edges can be sharp.
Step by Step
Step 1: Plan the slope and the layout
Polycarbonate needs slope to shed water and avoid pooling. A common recommendation is at least about 10 percent, which is roughly 1 inch of drop for every 10 inches of run, and some makers allow less with extra sealing. More slope is better, especially where leaves and needles collect. Check the minimum for your product.
Sheets come in set widths, commonly about 24 or 48 inches. Plan so you use whole sheets, and avoid narrow slivers at the ends. Order sheets long enough to overhang the eave by about 2 inches.
Step 2: Build a sound frame with correct purlin spacing
The framing has to carry snow and wind loads as well as the sheets, which do not add structure. Rafters run down the slope and purlins run across. Spacing depends on sheet type and thickness, with thinner sheets needing closer supports. Follow the manufacturer's span table. Check local building requirements, since a cover attached to a house may need a permit and proper structure.
Make the frame flat and true. A twist shows up as stress in the sheets, and a wood frame should be dry and treated. Consider wood sealed on top to stop moisture shrinking and moving the fasteners.
Step 3: Check the sheet orientation: UV side up
Most polycarbonate sheets have a UV-protective layer on one face only. That face must face out, toward the sun. If it is installed upside down, the sheet yellows, becomes brittle and cracks within a few years. The film on the sheet usually says which side is the UV side. Leave the film on until the sheet is fixed, then remove it.
Mark the orientation on each sheet as it comes out of the pack, so there is no mix-up when you cut.
Watch out: Installing the UV face down is the most common failure. The sheet will degrade quickly and the warranty is typically void.
Step 4: Cut the sheets to length
Support the sheet fully while cutting and cut slowly to avoid cracking. Multiwall sheets are cut along the channels so the ribs run down the slope. Plan for the eave end to overhang by about 2 inches. Leave a small gap, about 1/8 inch or per the maker, at rigid stops like walls, since the sheet grows with heat.
Step 5: Seal the ends of multiwall sheets
The top end is sealed with solid tape so rain and dirt cannot run into the channels. The bottom end takes vented tape, which keeps insects and dust out but lets condensation drain. Do not use ordinary duct tape. Each tape type is made for polycarbonate and is sold for this purpose. Finish with end caps or a drip edge as the system requires.
Step 6: Drill oversized holes and fasten without overtightening
The oversized hole lets the sheet slide slightly as it expands and contracts. A screw tight in a tight hole cracks the sheet or pulls through. Use screws with neoprene or EPDM washers designed for polycarbonate, and drive them just until the washer compresses slightly, and no further. Keep screws square to the sheet. Space fasteners as the maker says, often about every 12 to 16 inches along each purlin.
On corrugated sheets, fasten through the crest or the flat as the instructions specify. On multiwall sheets, fasten through the flat, in line with a rib, never at a hollow channel.
Step 7: Join sheets and flash to the house
For multiwall, use H-profile connectors that hold the edges yet let them move. For corrugated sheets, side laps are commonly one or two corrugations. At the house wall, the top of the sheet goes under a flashing that is tucked behind the wall cladding, like the wall flashings in how to install counter flashing. Seal with a sealant compatible with polycarbonate, since some sealants attack the plastic.
Step 8: Remove the film and check the finished roof
Remove the film before the sun bakes it onto the sheet. Clean with mild soap and a soft cloth, and avoid harsh solvents and abrasive cleaners. Check the tapes, end caps and fasteners after the first few weeks and again each spring. Keep debris clear, see how to clean debris off a roof.
Why the Hole Size Matters
Polycarbonate grows roughly 1/24 inch per foot of length across a 50 degree Celsius temperature swing. A 10 foot sheet can change in length by about 2/5 of an inch between a winter night and a hot summer afternoon. Fasteners in tight holes hold it rigid, so stress builds until the sheet cracks at a screw or the screw tears out. Oversized holes, a little end clearance and loose-enough washers are what allow the sheet to move.
Wind and Snow in the Fraser Valley
Patio roofs catch wind off the Fraser River and outflow winds, and wet snow at Hope and up the canyon can load a frame heavily. Sheets do not carry load, the frame does, so build it to the local snow load and use enough fasteners for uplift. Ask your municipality about any permit and design requirements.
Common Mistakes
- UV side down. The sheet yellows, turns brittle and cracks in a few years. Check the marking before you cut.
- Tight holes or overtightened screws. No room for expansion means cracks at the fasteners.
- Too little slope. Water pools, leaves collect and leaks appear at the laps. Give it at least the maker's minimum.
- Open multiwall ends. Dirt, insects and water get into the channels and grow algae. Tape both ends with the correct tape.
- Walking on the sheets. They are slippery and will crack or break.
- Using the wrong sealant. Some silicones and solvents damage polycarbonate. Use a compatible product.
When to Call a Professional
Call a roofer or builder if the cover attaches to a second storey, if the frame needs to carry snow loads, if it needs a permit, or if the wall junction needs flashing through siding or stucco. The structure and the flashing matter more than the sheets.
If an existing polycarbonate roof leaks, cracks or has yellowed, it is worth having someone look at the slope, fastening and orientation before replacing sheets, since the same mistake is easy to repeat.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install a Polycarbonate Patio Roof: Questions
Which side of polycarbonate roofing faces up?
The UV-protected side, which is usually marked on the protective film, faces up toward the sun. Most sheets have this coating on one face only. If installed upside down, the sheet yellows, becomes brittle and cracks within a few years, and the warranty is usually void. Check the marking before cutting and leave the film on until the sheet is fixed.
What slope does a polycarbonate patio roof need?
A common recommendation is at least about 10 percent slope, roughly 1 inch of drop per 10 inches of run, which is close to 1/12. Some products allow less with extra sealing, and more slope is better because it sheds water and debris. Always confirm the minimum slope with the sheet manufacturer.
Why do you drill oversized holes in polycarbonate?
Polycarbonate expands and contracts roughly six times as much as steel with temperature. Holes about 1/8 inch larger than the screw let the sheet slide slightly without cracking or pulling through the fastener. Screws should be tightened just enough to seat the washer, never hard against the sheet.
Do I need to tape the ends of multiwall polycarbonate?
Yes. Use solid aluminum tape on the top end to keep rain and dirt out of the channels, and vented tape on the bottom end so condensation can drain while dust and insects stay out. Open channels collect algae and debris, and look bad. Use tape made for polycarbonate, not duct tape.
How long does a polycarbonate patio roof last?
Quality sheets with UV protection, installed UV side up with oversized holes and sealed ends, commonly last well over a decade. Cheap sheets or poor installation can crack or yellow within a few years. Regular cleaning with mild soap and checking the fasteners and tapes each year helps a roof last.
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This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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