Sealing polycarbonate roofing sheets where they meet a wall is essential to prevent water ingress, draughts, and structural damage. Whether you’re installing a lean-to, carport, or greenhouse, following the correct method will keep your roofing watertight and long-lasting.
This guide explains how to professionally seal clear roofing sheets to a vertical wall using tried-and-tested materials.
🔧 Tools & Materials You’ll Need
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Polycarbonate roofing sheets (cut to size)
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Aluminium or uPVC wall flashing
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Silicone sealant (external grade, preferably UV-resistant)
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Fixing buttons or screws with rubber gaskets
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Foam eaves filler (optional for corrugated polycarbonate)
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Drill and screwdriver
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Spirit level
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Measuring tape
Step 1: Install the Sheets with Correct Overlap
Before sealing the edge to the wall, make sure your polycarbonate sheets are:
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Properly fixed to the rafters or structure with minimum 50mm overlap at the high (wall) end.
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Sloped at 5° or greater to allow effective water runoff.
✅ A slight pitch ensures water doesn’t pool and leak at the wall junction.
Step 2: Fit Wall Flashing or Upstand Trim
To create a clean and weatherproof joint where the polycarbonate meets the wall:
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Measure and cut your wall flashing (aluminium or uPVC).
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Position it so that it overlaps the top edge of the roofing sheet by at least 30–50mm.
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Drill pilot holes into the masonry wall using a hammer drill.
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Fix the flashing into place using appropriate wall plugs and screws.
✅ For conservatory or lean-to builds, an “F-section” wall plate trim can also be used.
Step 3: Apply Silicone Sealant
Once the flashing is securely in place:
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Run a continuous bead of external-grade silicone sealant along the top edge of the polycarbonate sheet where it meets the flashing.
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Smooth it out with a gloved finger or sealant tool to ensure a tight seal.
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Add extra sealant behind and over the screws to prevent moisture tracking through.
✅ Always use a UV-stable, flexible sealant designed for outdoor roofing applications.
Step 4: Check for Gaps and Reinforce If Needed
Inspect for:
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Small gaps between the sheet and the wall
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Uneven contact between flashing and wall render/brick
If needed:
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Add foam filler beneath corrugated sheets
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Apply more sealant to exposed joints or gaps
Maintenance Tip
Check the seal every 6–12 months, especially after heavy storms or frost. Reapply sealant as needed to maintain a watertight connection.
Summary: Key Points
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Ensure proper slope and overlap of the roofing sheets
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Use flashing for a professional wall junction
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Seal all joints with weatherproof silicone
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Perform regular checks for long-term durability
Sealing polycarbonate roofing sheets to a wall isn’t difficult — but doing it right means no leaks, better insulation, and a longer lifespan for your outdoor structure.
👉 Looking for high-quality sheets? Visit Bonus Roofing for a full range of sizes and thicknesses.