How to Apply a Liquid Waterproofing System to a Metal Roof

Refurbishment of profiled metal sheets with primer + liquid membrane (illustration only)

This method covers refurbishing profiled metal roofs (steel/aluminium) using a matched metal primer followed by a liquid-applied waterproofing membrane. It addresses corrosion treatment, fastener sealing, lap reinforcement and coating application for a long-lasting, seamless finish.

1) Suitability & typical use

• Industrial/commercial profiled metal sheets and flashings.
• Localised leaks at laps, fixings, penetrations and gutter interfaces.
• When you need a seamless, crack-bridging finish without full roof replacement.

2) Choose your liquid system (examples from our range)

• PU liquids: Desmopol, Mapei Purtop Easy — robust, flexible waterproof membranes.
• Acrylic liquids: Mapei Aquaflex Roof — UV-stable refurb coating with reinforced details.
• Accessories: reinforcement fabric/bandage, anti-slip aggregate (walkways), compatible sealants.

Always match the primer to the metal (galvanised/aluminium/coated steel) and the liquid you’re using.

3) Tools & materials

• Degreaser/cleaner, hose/pressure washer (low–medium), scouring pads/abrasives
• Rust remover/convertor (where specified), wire brushes, scrapers
• Sealant for fasteners/terminations (PU/Hybrid, system-compatible)
• Metal primer (etch/epoxy/PU as specified), chosen liquid membrane, reinforcement fabric
• Rollers/brushes/squeegee, mixing gear, masking, wet-film comb
• PPE: gloves, goggles, harness/fall arrest for roof work

4) Survey & preparation

• Safety first: Edge protection, mansafe lines and access equipment in place. Work to safe-weather windows.
• Inspection: Identify rusted sheets, loose fixings, failed sealant, cut-edge corrosion, porous or chalking factory finishes.
• Cleaning: Remove dirt, moss, and chalking. Degrease oil/soot. Rinse and allow to dry. Abrade glossy coatings for key.
• Corrosion: Hand/MECH abrade to sound metal. Treat with rust converter if permitted, then dust-off.
• Fasteners: Replace perished washers; tighten/replace loose fixings. Seal heads with system sealant.
• Laps & edges: Remove friable paint at cut edges; prep for stripe coat + reinforcement.

5) Priming (metal)

1. Select the correct metal primer (etch/epoxy/PU) specified by your liquid system for the substrate (galv, aluminum, coated steel).
2. Apply uniformly at stated coverage. Allow to dry within the overcoat window.
3. Where adhesion tests are required, perform pull-off or cross-hatch tests before proceeding.

6) Detailing — stripes, laps & fixings (illustration only)

• Stripe coat the following with the liquid membrane: side/end laps, rooflight perimeters, penetrations, fixings, gutters, and cut edges.
• While wet, embed reinforcement fabric over laps and transitions; smooth out air and fish-mouths (50–100 mm overlap typical).
• Seal fastener heads and vulnerable terminations with compatible sealant and/or fabric patches.

7) Main membrane coats

1. First coat: Apply the liquid membrane by roller/brush (or squeegee then back-roll) to the specified wet-film thickness. Work across the profile corrugations for even coverage.
2. Second coat: Apply within the recoat window, cross-rolled to achieve total dry-film thickness and full continuity.
3. Walkways (optional): Broadcast fine/medium aggregate into wet membrane along designated routes; de-flute and encapsulate with an additional pass.

8) Terminations & interfaces

• At abutments, upstands and wall flashings, chase and seal or use termination bars as designed.
• Integrate carefully with gutters; reinforce gutter joints and outlets. Maintain fall and free discharge.

9) Curing & weather

• Respect temperature and humidity limits; check dew point to avoid condensation on metal.
• Protect from rain/dust during cure; observe minimum/maximum overcoat windows.

10) Quality checks & common pitfalls

• Poor adhesion: Usually contamination or wrong primer—improve cleaning/abrading and use the specified metal primer.
• Pinholes at crowns: Insufficient film over corrugation peaks—measure WFT/DFT and recoat.
• Capillary laps still leaking: Missing reinforcement—re-detail with stripe + fabric.
• Condensation blushing: Coated below dew point—wait for suitable conditions.

Maintenance

• Inspect annually: check laps, fixings, walkways and gutter interfaces. Clear debris and standing water.
• Recoat high-wear areas when wear indicators show, following the system’s maintenance guide.

Product availability

All components referenced—metal primers (etch/epoxy/PU), liquid membranes (e.g., Desmopol, Mapei Purtop Easy, Mapei Aquaflex Roof HR), reinforcement fabrics, compatible sealants, and accessories—are available from Clever Shield. Use the website search bar to find exact items and sizes for your project.