February 2026

How to Seal a Polycarbonate Roof Properly (No-Leaks Guide)

A watertight polycarbonate roof is essential for keeping your structure dry and long-lasting. Whether you’re roofing a pergola, carport, lean-to, verandah, greenhouse or garden room, the most common issues builders and homeowners face are leaks, condensation and moisture entering the sheet chambers. These problems almost always come from one thing: incorrect sealing.

This guide explains how to properly seal your polycarbonate roofing system so you avoid leaks completely. It covers the correct tapes to use, how to seal glazing bars, how to flash a roof against a wall, and the biggest installation mistakes to avoid.

Why Proper Sealing Matters

Polycarbonate sheets are incredibly durable, lightweight and long-lasting, but they require correct sealing at the edges and joints. Without it, you can experience:

Leaks during rainfall
Moisture entering multiwall chambers
Condensation and fogging inside sheets
Dirt, insects and algae entering channels
UV damage on exposed sheet edges
Structural issues if water runs behind the glazing system

More than 70% of polycarbonate roof leaks come from sealing issues, not the sheets themselves. The good news is that correct sealing is simple once you use the right materials and follow the right method.

1. Seal the Top Edge With Aluminium Tape (High End)

The top edge of every multiwall polycarbonate sheet must be sealed using solid aluminium tape. This prevents:

Rain penetration
Snow and wind-driven water
Dirt, moss and insects
Moisture getting into the chambers

Aluminium tape is fully waterproof, and when combined with an end closure or glazing bar, it forms a permanent seal.

You can find the correct aluminium sealing tapes in our Multiwall Accessories collection:
https://polycarbonatex.co.uk/collections/multiwall-accessories

Why aluminium tape must be used on the top edge

This edge is the entry point for driven rain. If water gets inside the chambers, the sheet becomes cloudy and permanently stained. Once moisture gets into a chamber, it cannot be removed — meaning the sheet must be replaced.

After taping the top edge:

Cover the taped edge with:

An end closure, or
A glazing bar

This protects the tape from peeling or UV exposure.

2. Seal the Bottom Edge With Breather Tape (Gutter End)

The bottom edge is where moisture escapes — so it needs breather tape, not aluminium tape.

Breather tape allows:

Air circulation
Moisture ventilation
Pressure stabilisation
Prevention of mould and condensation

Without breather tape, moisture becomes trapped inside the chambers, creating foggy sheets.

Breather tape also needs protecting, usually with:

A U-profile drip trim,
Or end closures

You’ll find both breather tapes and U-profiles in our accessories:
https://polycarbonatex.co.uk/collections/multiwall-accessories

3. How to Correctly Seal Glazing Bars

Glazing bars are responsible for keeping the roof watertight between sheets. They must be installed properly with the correct gaskets.

The most reliable glazing bar accessories are available here:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-glazing-bar-end-cap

To seal glazing bars correctly:

Ensure gaskets are fully seated along the bar
Do not overtighten fixings (this warps the sheet and causes leaks)
Allow sheet expansion (3mm per metre)
Use end caps to stop water tracking inside the bar
Seal any exposed joints with roof-grade sealant, not silicone

If you use cheap PVC glazing bars or twist them during installation, water will penetrate the joints.

4. How to Seal a Lean-To Roof Against a Wall

One of the most common leak points is the wall connection on a lean-to roof.

Follow this sealing sequence:

Step 1 — Fit the sheets and glazing bars first

This gives you the correct height for flashing.

Step 2 — Install wall flashing

Use aluminium, steel or lead-replacement flashing.

Step 3 — Seal the flashing where it meets the wall

Use LM (low modulus) roofing sealant for maximum flexibility and UV resistance.

Step 4 — Ensure flashing overlaps the top of the sheets

It must sit over, not on, the sheets.
If it presses directly on the polycarbonate, expansion movement will pull it loose and create leaks.

Key rule:

Do not seal directly between the flashing and the polycarbonate surface.
The flashing must be allowed to float above the sheet.

5. Seal Around Fixings and Screw Points

If you are using screw-down glazing bars or roofing fixings, you must seal them correctly.

Use:

Rubber washer fixings
Pre-drilled holes oversized for expansion
Low-modulus roofing sealant only if needed

Avoid:

Driving screws in at an angle
Overtightening
Piercing through chambers
Using standard silicone

Incorrect screw sealing creates micro-fractures that expand over time — a leading cause of roof leaks.

6. Common Areas Where Polycarbonate Roofs Leak

If you already have a leak, it’s usually from one of these areas:

Missing or incorrect top/bottom tapes
Poorly fitted glazing bars
No end caps
Missing drip trim
Flashing fitted incorrectly
Sheets clamped too tightly for expansion
Pitch angle too low

A polycarbonate roof must have at least a 5° pitch. Any lower and water can run backwards and find gaps in the sealing system.

7. The Biggest Mistakes to Avoid

Here are the errors that cause 90% of all leaks:

❌ Using silicone instead of roofing sealant

Silicone breaks down in sunlight and fails within months.

❌ Using aluminium tape on the bottom edge

This traps moisture inside the chambers.

❌ Using breather tape on the top edge

Allows rain to enter the channels.

❌ Overtightening glazing bars

Creates warping and pressure points.

❌ Missing drip trims

This allows water to run behind or underneath the roof.

❌ Buying low-quality glazing bars

Poor gasket seals eventually split and leak.

8. Maintenance Tips to Keep Your Roof Leak-Free

A properly installed polycarbonate roof needs very little maintenance. To keep it watertight long-term:

Clean twice a year with mild soapy water
Check breather tape airflow annually
Remove debris from the gutter end
Avoid abrasive or chemical cleaners
Ensure glazing bar gaskets remain flexible
Inspect flashing every 12 months

If installed and sealed properly, a polycarbonate roof can last 20+ years without leaks.

9. Where to Get the Correct Sealing Components

Using the correct tapes, trims, glazing bars and accessories makes sealing far easier. You can buy all compatible accessories here:

Conclusion

Sealing a polycarbonate roof correctly is straightforward once you know which materials to use and where each component belongs. If you follow the rules — aluminium tape at the top, breather tape at the bottom, well-fitted glazing bars, properly installed flashing — your roof will remain watertight and long-lasting.

A properly sealed roof looks better, lasts longer and eliminates the common issues of condensation, mould and water ingress.