How to Felt a Flat Roof

Felt roofing remains one of the most common and cost-effective solutions for flat roofs in the UK. Whether you are covering a shed, garage, extension, or small outbuilding, roofing felt provides a durable, weatherproof surface that protects your property from rain, frost, and sunlight. Although modern flat roof materials like EPDM rubber and GRP fibreglass have become popular, felt roofing continues to offer excellent value for money when applied correctly. Understanding how to felt a flat roof the right way ensures a long-lasting finish and prevents leaks, cracks, and water pooling.

This guide explains everything you need to know, including the materials required, the correct installation process, and the maintenance steps that will keep your roof in excellent condition for years.

What Is Roofing Felt?
Roofing felt is a flexible sheet material made from bitumen, fibreglass, or polyester reinforced with a mineral surface. It provides a waterproof layer that seals flat roofs against moisture. Felt is typically applied in multiple layers using hot bitumen, adhesive, or torch-on techniques to create a watertight barrier.

Modern felt roofing systems are far superior to older versions. Traditional felt made from paper-based materials tended to crack and peel after a few years. Today’s polyester-reinforced felts are stronger, longer-lasting, and more resistant to UV rays and temperature changes. A high-quality felt roof can last 20 to 25 years if installed correctly.

Why Choose Felt for a Flat Roof?
Felt remains a practical choice for several reasons. It is affordable, widely available, and adaptable to different roof shapes and sizes. It provides strong waterproofing protection and can be repaired easily if damaged. For homeowners working on garages, garden offices, or small extensions, it offers a good balance between performance and cost.

Felt roofs are also relatively straightforward to install for those with basic DIY skills, provided safety precautions are followed. When fitted properly with a slight slope for water drainage, a felt roof provides reliable, long-term protection against the UK’s wet weather.

Preparing to Felt a Flat Roof
Before you begin, preparation is key. Poor preparation leads to leaks, bubbles, and a shorter lifespan. The first step is to inspect the existing roof deck to ensure it is sound and free of rot or damp. The deck should be made from exterior-grade plywood or oriented strand board (OSB3), securely fixed and level.

If you are replacing an old felt roof, all previous felt, nails, and adhesive residues should be removed. The surface must be clean, dry, and smooth. Any damaged boards must be replaced before new felt is applied. Ensure the roof has adequate drainage with a fall of at least 1 in 80, allowing rainwater to flow towards gutters or outlets.

You should also check that the weather conditions are suitable. Roofing felt should not be laid during rain, frost, or high winds. The ideal temperature is between 10 and 25 degrees Celsius, as this helps adhesives and bitumen bond correctly.

Tools and Materials You Will Need
The materials required for felting a flat roof are generally easy to source from most builders’ merchants. You will need roofing felt (underlay and cap sheet), bitumen adhesive or gas torch for torch-on felt, primer, nails, a hammer, utility knife, brush, and roofing roller. If you are working with torch-on felt, a propane gas torch with regulator and safety gloves will be essential. Always wear protective clothing and work carefully to avoid burns or slips.

Choosing the Right Type of Felt
There are three main types of felt roofing systems used in the UK: pour and roll, torch-on, and self-adhesive.

The pour and roll method involves heating bitumen and rolling the felt into it to bond the layers. It is reliable but requires more skill and equipment.
Torch-on felt uses a gas torch to heat the underside of the felt until it melts slightly, bonding to the roof deck or underlay. This is the most popular modern technique because it provides a seamless, watertight finish.
Self-adhesive felt, sometimes called cold-applied felt, has a peel-off backing that sticks directly to the surface without heat. This is ideal for smaller jobs or areas where using a torch would be unsafe, such as wooden sheds or balcony roofs.

Whichever system you choose, it is best to use a three-layer method: a vapour control layer, an underlay, and a mineral cap sheet. This structure provides strength, flexibility, and long-term protection against leaks.

Step-by-Step Guide on How to Felt a Flat Roof

Step 1: Clean and Prime the Surface
Start by ensuring the roof deck is completely dry, smooth, and free from dust. Apply a bitumen primer using a brush or roller to improve adhesion. Allow it to dry fully according to the manufacturer’s instructions, which usually takes two to four hours.

Step 2: Cut and Position the Felt
Unroll the felt and cut it into manageable lengths, leaving extra material for overlaps of around 75mm on sides and 100mm on ends. Lay the first sheet at the lowest part of the roof, allowing any joints to fall in the direction of water flow.

Step 3: Apply the Underlay Felt
For the first layer, position the underlay felt and fix it using bitumen adhesive, nails, or heat depending on the system. For torch-on felt, lightly heat the underside with a torch until it begins to melt, then press it down firmly with a roller to seal. Each new roll should overlap the previous one to create a continuous waterproof surface.

Step 4: Add the Second Layer
Repeat the process with the second layer, staggering the joints so they do not line up with the first layer. This prevents weak points where water could penetrate. Ensure all overlaps are properly sealed and smooth out any bubbles or wrinkles immediately.

Step 5: Apply the Cap Sheet
The top layer, known as the cap sheet, provides UV protection and a decorative finish. It usually has a mineral surface that reflects sunlight. Heat or glue the cap sheet into place, again staggering the joins and sealing overlaps carefully. Pay particular attention to corners, edges, and upstands, using additional strips of felt where necessary for reinforcement.

Step 6: Finish the Edges and Flashings
Secure the felt around roof edges and parapets using metal drip trims or edge flashings. These help direct water into gutters and prevent it from seeping behind the felt. Seal the joints with bitumen and check that all seams are watertight.

Step 7: Inspect and Test the Roof
Once complete, inspect the entire surface carefully. Look for bubbles, uneven areas, or unsealed overlaps. Pour a small amount of water on the roof to check that it drains correctly. If any pooling occurs, you may need to adjust the slope or add additional outlets.

Common Mistakes to Avoid
One of the most common mistakes when felting a flat roof is neglecting the roof’s slope. Without a slight incline, water can collect and eventually seep through seams or cracks. Another frequent error is failing to allow proper overlap between felt sheets, which leads to weak joins that leak.

Applying felt in cold, damp, or windy conditions can cause adhesion failures and trapped moisture. Always wait for dry weather and ensure the surface is completely moisture-free before starting. Poor sealing around edges, vents, or roof lights can also lead to water ingress. Taking extra care at these points will ensure a longer-lasting finish.

How Long Does a Felt Roof Last?
A professionally installed, high-quality felt roof typically lasts between 20 and 25 years. Older systems may last closer to 10 to 15 years depending on exposure and maintenance. Regular inspection and upkeep can extend the lifespan considerably. Recoating the surface every few years with a protective bitumen layer or reflective coating can help preserve its flexibility and prevent UV damage.

Maintenance Tips for Felt Roofs
Maintaining a felt roof is relatively simple but important. Inspect it twice a year, ideally in spring and autumn, to check for cracks, blisters, or moss growth. Clear away leaves and debris that could trap moisture. Ensure gutters and downpipes are clear so water drains freely.

If small cracks or blisters appear, they can often be repaired using roofing mastic or a patch of new felt. Larger or repeated leaks may indicate that the roof has reached the end of its lifespan and needs replacement.

Cost of Felting a Flat Roof
The cost of felting a flat roof varies depending on the size, materials, and labour. On average, professional installation ranges between £40 and £80 per square metre. Torch-on felt systems tend to cost slightly more than cold-applied ones but offer greater durability. DIY installation is cheaper, though mistakes can lead to costly repairs later.

Environmental Considerations
Modern felt roofing materials are designed to be more environmentally friendly than older versions. Many are made using recycled or sustainable materials, and newer systems reduce the need for frequent replacements. When replacing old felt, always dispose of it responsibly through your local recycling centre or waste management service.

Conclusion
Felting a flat roof is a practical and affordable way to create a durable, waterproof surface for garages, extensions, and outbuildings. The process involves careful preparation, the correct choice of materials, and precise application to ensure a long-lasting result. When done properly, a felt roof can last more than two decades, offering excellent protection against the UK’s variable weather.

By maintaining it regularly and addressing any minor issues early, you can extend its lifespan significantly. For those comfortable with DIY, felting a small flat roof is achievable, but for larger or more complex jobs, professional installation is always the safest and most reliable choice.