How to Waterproof a Flat Roof

Waterproofing is the most critical function of any flat roof. There are several proven systems available — the right choice depends on the roof's size, complexity, existing structure, and budget.

Membrane Systems

Membrane systems are the most common approach for new flat roofs. They include EPDM rubber, single-ply TPO/PVC membranes, and SBS bitumen (modified felt). Membranes are installed in sheets, with joints carefully bonded or welded to create a continuous waterproof layer.

Liquid Waterproofing

Liquid waterproofing systems (such as cold-applied polyurethane or PMMA acrylic) are applied as a liquid that cures to form a seamless, joint-free membrane. These are particularly useful for complex roof shapes with many details, penetrations, and junctions where sheet membranes are difficult to detail.

GRP Fibreglass

GRP fibreglass is a liquid-applied system that creates a rigid, seamless surface. It's applied as layers of fibreglass matting and resin, then finished with a coloured topcoat. GRP is extremely durable and is the most popular choice for domestic flat roof extensions.

The Importance of Details

The waterproofing membrane itself is only as good as the details — the edges, upstands, pipe penetrations, and junctions with walls. These areas are where most leaks occur. Good detailing at these points is the mark of quality workmanship.

Choosing the Right System

We'll advise on the most appropriate waterproofing system for your specific project. Contact us for a free survey and recommendation.