How to Felt a Flat Roof in the UK

Why Felt Roofing is Still Popular

Despite the rise of modern materials like EPDM rubber and fibreglass, traditional bitumen felt remains a popular choice for UK flat roofs. It is one of the most affordable coverings available, relatively straightforward to install, and effective when applied correctly. Felt is particularly common for garages, sheds, and small extensions. However, to achieve a roof that lasts, it needs to be laid with care, proper preparation, and adequate attention to detail.

Preparation Before Felting

A successful felt roof starts with good preparation. The existing deck, usually made from OSB or plywood, must be in sound condition, clean, and completely dry. Any rotten or damaged boards should be replaced before work begins, as felt needs a stable surface to bond to. The roof should have a slight fall of at least one in forty to allow water to drain away, and any gutters or outlets should be clear. It is also important to plan the work for dry, mild weather, as felt does not bond properly in wet or very cold conditions.

Laying the Underlay Felt

Felt roofs are usually built up in layers. The first is the underlay, which provides a base for the top layer and helps improve waterproofing. The underlay is rolled out across the roof, cut to size, and laid in position with overlaps of around 75mm on the edges. It can be nailed into place on timber roofs, or bonded with cold adhesive or hot bitumen depending on the system being used. Joints must be neat and well sealed to prevent water ingress.

Applying the Top Layer

The top layer of felt is usually a mineral surfaced cap sheet, designed to resist UV damage and provide the final waterproof barrier. It is rolled out over the underlay in the same way, with staggered joints to reduce weak points. This layer is usually bonded with hot bitumen, torch on application, or cold adhesive depending on the type of felt chosen. The laps are sealed thoroughly to create a continuous surface, and extra reinforcement is added at corners, edges, and upstands where leaks are most likely.

Detailing the Edges and Upstands

The weakest points of any flat roof are the edges and areas where the roof meets walls, chimneys, or roof lights. Here, the felt must be carefully dressed up vertical surfaces and securely fixed with timber battens or metal flashing. On exposed edges, the felt is folded neatly and protected with edge trims or drips to encourage water to run into gutters. Good detailing is crucial to the longevity of the roof and is where poor workmanship often leads to early failure.

Maintenance and Lifespan

A well installed felt flat roof can last between 10 and 15 years, with some high-performance systems lasting longer. Regular maintenance helps to extend its life. This means clearing gutters, checking for ponding water, and inspecting joints and edges for cracks or lifting. Minor repairs can often be made with patch felt or cold applied sealants, but if the roof develops widespread issues, full replacement is usually more cost effective.

Costs of Felting a Flat Roof

The cost of felting a flat roof in the UK is typically between £40 and £60 per square metre for basic torch on felt systems. This makes it one of the most affordable coverings compared to alternatives like EPDM or fibreglass. However, it does have a shorter lifespan, so while upfront costs are lower, homeowners may face replacement sooner than with longer lasting materials. For garages, sheds, and smaller buildings, felt remains a practical and economical option.

Conclusion

Felting a flat roof is a well-established method of waterproofing that, when installed correctly, provides reliable protection for a decade or more. Success depends on proper preparation, correct application of the underlay and cap sheet, and careful attention to detail around edges and upstands. While it may not last as long as modern coverings, felt remains one of the most cost effective and straightforward options for flat roofs in the UK.