Roofing felt is one of the most common and cost-effective ways to waterproof a flat roof. Made from bitumen or polyester-based materials, it forms a durable, weather-resistant barrier that protects the roof deck from rain, wind, and UV exposure.
It’s a popular choice for sheds, garages, outbuildings, and small extensions thanks to its affordability and straightforward installation. Whether you're replacing worn felt or fitting a new roof, a well-laid felt system can provide reliable protection for years to come, especially when installed with care and attention to detail.
In this guide, we’ll walk you through the tools and materials you’ll need, how to prepare your roof, and the step-by-step process for installing roof felt correctly and safely.
Why Choose Roofing Felt for a Flat Roof
Roofing felt has long been a go-to option for flat roofs, and for good reason. It’s a practical solution that offers solid protection without the high cost or complexity of some modern flat roofing systems.
Key Benefits
- Weatherproofing: Felt creates a sealed, watertight layer that protects your roof deck from rain, snow, and standing water.
- Cost-effective: It’s one of the most budget-friendly flat roofing materials on the market.
- DIY-friendly: Cold-applied felt systems are suitable for competent DIYers with basic tools and safety equipment.
- Lightweight: Ideal for sheds, garages, and small extensions where heavy materials might be unsuitable.
- Available in multiple formats: From self-adhesive rolls to traditional torch-on systems, there’s a felt product for every job.
While felt is a reliable option, it isn’t the longest-lasting. Most felt roofs last around 10–15 years with proper care. For high-traffic areas or where longer lifespans are needed, other systems like EPDM or GRP may be worth exploring, though they usually require specialist installation.
For most residential outbuildings and simple flat roofs, felt remains a tried-and-tested option, especially when installed correctly over a solid, dry deck.
Tools & Materials You’ll Need
Before you begin installing roof felt, it’s important to have everything ready. This will save time and help you work safely and efficiently.
Roofing Materials
- Roofing felt: Choose cold-applied, self-adhesive, or torch-on felt depending on the job and your skill level. Most flat roofs use a two- or three-layer system for best results.
- Felt adhesive (cold-applied): For bonding the layers of felt together (not needed with self-adhesive rolls).
- Galvanised clout nails or roofing tacks: To fix the base layer of felt securely in place.
Tools
- Tape measure: To measure felt runs and overlaps accurately.
- Stanley knife or utility knife: For trimming felt cleanly.
- Hammer: For securing nails or tacks.
- Paint brush or roller: For spreading cold adhesive evenly.
- Straight edge: To guide cutting and ensure square edges.
- Broom or brush: To sweep the roof deck clear before starting.
Safety Gear
- Work gloves: Protect hands when handling bitumen-based materials.
- Safety goggles: For working near adhesives or when trimming.
- Sturdy boots: Non-slip footwear helps with footing on a flat roof.
- Ladder or scaffold tower: Ensure safe access and secure footing at all times.
Step-by-Step Installation Guide
Whether you’re felting a new flat roof or repairing an old layer, following the correct installation process is key to a watertight and long-lasting finish. This guide is for cold-applied felt – the most common and DIY-friendly method.
Step 1: Measure and Cut the Felt
- Measure the total roof area, allowing for overlaps and overhangs at the edges (typically 50mm at sides, 75mm at eaves).
- Cut your felt rolls to size on the ground before lifting them onto the roof.
- If you're laying multiple layers, prepare each one in advance to ensure clean alignment.
Step 2: Lay the First (Underlay) Layer
- Start at the lowest edge of the roof and work upwards.
- Position the felt and fix it in place with galvanised clout nails or tacks along the top edge and sides (every 100–150mm).
- Avoid stretching the felt too tightly – allow slight slack for movement and expansion.
- Use a straight edge to trim the ends neatly and fold down over the edge where needed.
Step 3: Apply Adhesive Between Layers
- Once the first layer is secured, apply cold felt adhesive to the surface using a roller or brush.
- Apply in manageable sections to prevent premature drying.
- Carefully roll out the second layer on top, pressing down to ensure full contact and removing any trapped air.
Step 4: Install the Cap Sheet (Top Layer)
- Repeat the adhesive process for the top layer (usually a mineral-finished cap sheet).
- This layer should be staggered slightly to offset the joints from the underlay layer.
- Ensure a minimum 75mm overlap between sheets and 150mm at end laps for full waterproofing.
Step 5: Finish the Edges
- At the eaves, fold the felt over the edge and secure it to the fascia using tacks or adhesive.
Use a timber batten to tidy the edge if required.
For corners and upstands, neatly cut and fold the felt for a clean, sealed finish. Apply additional adhesive where needed.
Step 6: Check for Gaps and Secure Final Fixings
- Inspect all overlaps, joints, and edges to ensure they are fully bonded and secure.
- Add extra nails or adhesive where necessary, particularly in corners or exposed sections.
- Leave to cure fully as per adhesive manufacturer instructions before exposing to heavy rain or walking on the surface.
Tip: Always work in dry weather. Moisture beneath roofing can often lead to blistering, poor adhesion, and future leaks.
Aftercare and Maintenance
Looking after your felt roof doesn’t require much effort, but a few regular checks will go a long way in keeping it watertight and in good condition.
- Inspect twice a year: Ideally in spring and autumn, check the surface for signs of wear, especially after bad weather.
- Look for damage: Keep an eye out for blisters, cracks, tears, or loose edges that may allow water in.
- Clear debris: Remove leaves, moss, and other build-up that can block drainage or cause ponding.
- Check edges and overlaps: Ensure they remain fully sealed. Reapply adhesive if needed.
- Maintain surrounding areas: Keep nearby gutters, outlets, and flashing clear and in good condition to support effective water runoff.
With basic upkeep, a well-installed felt roof can provide years of reliable protection with minimal fuss.
Felt Roofing Made Simple with the Right Support
Installing roof felt on a flat roof is a job that’s well within reach, especially with the right preparation, tools, and attention to detail. From garages and sheds to outbuildings and extensions, felt provides a reliable waterproof layer that protects your roof for years to come.
At MP Moran, we stock everything you need to get started, from roofing felt and adhesives to nails, tools, and safety gear. Pop into your local MP Moran branch for friendly advice and product support, or sign up for a Trade Account to access exclusive prices, quicker ordering, and tailored service.
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