Multiwall polycarbonate can create a bright, durable greenhouse, patio or sundeck cover when the framing and glazing details let water drain and the sheets expand. Most failures come from reversed sheets, blocked flutes, insufficient slope, over-tight fasteners or incompatible accessories.
This is a planning guide, not a substitute for the panel manufacturer’s span tables or local building requirements. Confirm loads, slope, supports and fastening for the exact product before construction.
Quick answer
Frame to the manufacturer’s support spacing and minimum slope, install the UV face outward with flutes down-slope, cut cleanly, remove swarf, seal the high and low ends with the specified tapes, allow thermal expansion and use compatible profiles and fasteners without crushing the sheet.
What you need to know
- Flutes run vertically on walls and down-slope on roofs.
- The top and bottom flute ends usually receive different tape treatments.
- Protective film stays on during handling but must not become trapped under profiles.
- Never stand directly on the sheet.
Tools and materials
- Specified multiwall panels and installation guide
- Engineered framing and flashings
- Compatible H, U or standing-seam profiles
- Solid and ventilating anti-dust tapes
- Approved fasteners and washers
- Polycarbonate-compatible sealant
- Fine-tooth cutting tool and swarf removal
- Access, fall protection and PPE
Step-by-step method
-
Confirm structure and slope
Build square, adequately braced framing to the current span and load tables. Provide the minimum drainage slope and required edge support.
-
Plan sheet orientation and lengths
Run flutes with drainage and use full lengths where possible. Mark the UV-protected face and cutting layout while the protective film remains in place.
-
Cut and clean
Support the sheet, cut with a suitable fine-tooth blade and remove chips from the flutes without scratching the surface. Keep the film on both faces during handling.
-
Treat flute ends
Apply the specified solid tape at the high end and ventilating/anti-dust tape at the drainage end, then protect the tape with the correct closure profile.
-
Allow thermal expansion
Set edge clearances, connector engagement and hole sizes from the manufacturer’s temperature and length guidance. Do not clamp the sheet rigidly.
-
Install profiles and fasteners
Keep connections square, place fasteners only where the system permits and tighten until weather-sealed without crushing the walls or preventing movement.
-
Flash, drain and inspect
Complete head, side and sill details so water sheds away. Remove exposed protective film promptly, confirm open drainage paths and inspect after the first heavy rain.
Common mistakes
- Insufficient roof slope
- UV face installed inward
- Flutes running horizontally on a roof
- Both ends sealed airtight
- Screws overtightened
- Sealant incompatible with polycarbonate
- Protective film left to bake onto the sheet
Safety
Polycarbonate roofing does not support a person between framing members. Use supported crawling boards, scaffolding and fall protection appropriate to the installation.
Check permits, snow and wind loads, fire requirements and structural design with the local authority and a qualified professional where required.
Frequently asked questions
Which end gets vent tape?
Normally the lower drainage end, while the high end receives solid tape. Follow the exact system drawing because details vary.
Can I screw through the sheet?
Only where the manufacturer permits, using the specified oversized holes, washers and spacing. Some systems fasten connectors rather than the panel.
How do I remove chips from the flutes?
Use the panel manufacturer’s approved air or vacuum method after cutting, keeping oil and water contamination out of the channels.
Why is there condensation inside the flutes?
Moisture can enter if tapes, closures or drainage are wrong. Some systems use ventilated lower tape specifically to manage vapour while excluding dirt and insects.
Get the right materials for the job
Tell our team what you are making and how the finished part will be used.
Shop multiwall polycarbonate and glazing accessoriesAsk our technical teamView the original printable project sheet.
Sources and further reading
This guide provides general educational information. Products, substrates and working conditions vary. Follow the current technical data sheet, package directions and safety data sheet for every material you use. Structural, code-regulated or safety-critical work may require a qualified professional.
