How to waterproof a flat roof (UK guide)
Flat roofs leak at their joints — the seams, laps and upstands where traditional felt or sheet systems meet. That's why liquid-applied waterproofing has become so popular: it goes on as a fluid and cures into one seamless, jointless membrane that wraps every detail. This guide covers how to do it well.
Why liquid: a flat roof's weak points are its seams and the awkward junctions at upstands, outlets and pipes. A liquid coating has no seams and self-details around those junctions, so there are far fewer places to fail.
Step 1: Prepare the surface
Good preparation is 80% of a lasting flat roof. The surface must be clean, sound and dry: sweep off all debris, remove moss, algae and loose material, cut out and make good any splits or blisters in the existing covering, and let the roof dry thoroughly. Any trapped moisture or contamination under the coating will undermine adhesion.
Step 2: Check the falls and drainage
A “flat” roof should never be truly flat — it needs a slight fall so water runs to the outlets rather than ponding. Standing water shortens the life of any system. Clear and check the outlets and gutters before you start, and address any serious ponding areas, because waterproofing won't fix a drainage problem.
Step 3: Detail the tricky bits first
Upstands, parapet junctions, rooflight kerbs, pipe penetrations and outlets are where roofs leak. With a liquid system these are reinforced (often with a scrim/reinforcing fabric bedded into the coating) and carried up and over the detail so the waterproofing is continuous from the field of the roof right up the upstand. Do these details before or alongside the main area.
Step 4: Apply the coating
A high-build flexible coating like The GOAT Flexible Waterproof Coating is roller-applied across the roof to build a thick, elastic, seamless membrane. Its flexibility lets it bridge small cracks and accommodate the thermal movement a roof goes through, and because it's liquid it bonds the field and the details into one continuous skin. Apply to the recommended coverage and film thickness, keeping a wet edge and avoiding thin or missed areas.
Step 5: Let it cure properly
Give the coating the full cure time before exposing it to heavy rain or traffic, and check the weather window before you start — applying in the wrong conditions is a common cause of early failure.
Common mistakes
- Coating a dirty or damp roof. Adhesion fails and the membrane lifts.
- Ignoring ponding. Fix the falls and drainage; don't waterproof over standing water.
- Skipping reinforcement at upstands and outlets. These are the first places to leak.
- Under-applying. Too thin a film won't give the designed life.
Related reading
- Which waterproofing product do you need?
- How to waterproof a wet room before tiling
- How to stop penetrating damp through an external wall
Frequently asked questions
Can I apply a liquid coating over my existing felt roof?
Often yes, provided the existing covering is sound, clean, dry and well prepared, with splits and blisters made good first. A badly deteriorated roof may need stripping back.
Why does my flat roof keep ponding?
Insufficient fall or blocked outlets. Waterproofing reduces the harm, but correcting the falls and drainage is the real fix.
Is a liquid coating better than felt?
Its big advantage is being seamless — no laps or joints to fail, and it self-details around upstands and penetrations, which are a traditional roof's weak points.
Not sure which coating suits your roof or how much you need? Our technical team can help you spec and estimate it.