How Many Layers of Felt Should Be Used on a Flat Roof?

Installing a high-quality flat roof using the proper materials and techniques is crucial to prevent leaks and damage.

When it comes to roofing felt, using multiple layers is recommended for optimal protection.

Table of Contents

How Many Layers of Felt on a Flat Roof?

Credit: City Building and Roofing Ltd

For optimal results, a three-layer felt roofing system is recommended for flat roofs.

This involves removing old felt layers, typically three in number, and applying two layers of underlay felt with one top layer.

Building regulations often mandate three layers for habitable spaces, while sheds or garages may require only two.

Torch-on-felt roofing, which is applied using heat, is another effective method.

High-performance felts are typically bonded with hot bitumen to minimise failure risks.

Key Points

Three layers of felt are recommended for optimal flat roof waterproofing.
The layers consist of a base layer, middle layer, and top cap sheet.
Proper installation methods and overlaps are critical.

Our Opinion

Based on the research, we believe that utilising three layers of high-quality felt is the best practice for waterproofing and protecting a flat roof.

While two layers may be acceptable in some situations, three layers provide superior durability and insurance against leaks.

Properly installing the layers according to code and manufacturer specifications is also essential.

We recommend consulting roofing professionals to ensure a reliable, long-lasting flat roof.

How Many Layers of Felt for a Flat Roof?

For a flat roof, building regulations typically recommend using 3 layers of roofing felt for habitable spaces like homes, while 2 layers may suffice for non-habitable spaces like sheds or garages.

Here’s a quick overview of how a 3 layer flat roofing system works:

Nail or glue the first layer of felt directly onto the roof deck as an underlay.
The second layer overlaps the first by at least 150mm to provide double coverage.
The top layer acts as a capsheet, providing a weather-resistant seal.

Going with 3 layers rather than just 2 has several benefits:

Having multiple layers of overlapping material makes the roof more impervious to water.
It provides backup protection in case one layer gets damaged or deteriorates.
The layers bond together to create a more seamless barrier.
It improves insulation and weatherproofing.

In the end, the small additional cost of a 3rd layer of felt is well worth it for the added durability and leak protection it provides. So don’t cut corners when installing a new flat roof – take the time to do it right with 3 layers of high-quality roofing felt.

Why Multiple Layers of Felt Are Recommended

Credit: Roof Giant

Several key reasons recommend using multiple layers of felt for flat roofs:

Extra protection – Having more than one layer provides backup waterproofing in case a single layer gets damaged or starts to degrade over time.
Improved sealing – With each additional layer, there are fewer gaps and holes where water can penetrate. The layers bond together for a more seamless barrier.
Increased durability – The combined thickness of multiple layers makes the roof more resistant to wear and tear from weather and foot traffic.
Better insulation – Extra layers slightly improve the roof’s thermal efficiency, helping keep interior temperatures stable.
Redundancy – If a leak occurs in one layer, the layers below still provide protection until repairs can be made.
Industry standards – Building codes often require multiple layers for habitable structures as a safety precaution.

In short, while a single layer of felt may suit low-slope structures like sheds, you should strongly recommend using multiple layers for flat roofs on homes and businesses. The modest additional cost is well worth it for the long-term performance benefits.

Types of Roofing Felt Materials

When installing layers of felt on a flat roof, there are a few different material options to consider:

Traditional roofing felt is made from organic paper fibres or fibreglass matting saturated with asphalt. This type is relatively inexpensive but requires replacing every 10-20 years as the asphalt degrades.

Modified bitumen is an enhanced version where polymers are added to the asphalt coating. This increases durability, with a lifespan of 15-30 years. Polyester or fibreglass reinforcing is also commonly used.

EPDM synthetic rubber sheets provide 20+ years of waterproofing. Seams are sealed with adhesive or tape. EPDM lacks reinforcing though, so care must be taken during installation.

PVC and TPO are thermoplastic alternatives resistant to weathering and punctures. They can last over 25 years with chemical welding of seams. PVC is more rigid while TPO remains flexible in cold weather.

Spray polyurethane foam (SPF) is a seamless liquid application that expands to form a protective barrier. It has excellent adhesion and insulation properties but requires specialized equipment and training.

The optimal roofing felt type depends on budget, expected lifespan, and ease of installation. Multiple layers of any material provide redundancy, though incompatible types should not be combined.

Laying Felt on a Flat Roof Step-By-Step

When installing layers of felt on a flat roof, proper technique is crucial for effective waterproofing. Here is an overview of the step-by-step process:

First, inspect and prepare the roof deck. Fix any damaged areas and clean thoroughly. Apply primer if needed.
Roll out the initial layer of felt, cutting it to fit the roof shape. Make sure overlaps match specifications – typically 50mm side laps and 100-150mm end laps.
Use a propane torch to heat the felt surface at laps, bonding the layers together. Alternatively, use hot bitumen or adhesive.
Apply a continuous bead of sealant along the edge, eaves, protrusions, and terminations to waterproof. Embed the felt into the sealant.
Add the specified number of additional layers in the same manner, offsetting overlaps between each. Stagger end laps at least 300mm.
Inspect for bubbles, gaps, and loose areas. Re-seal as needed to prevent leaks.
Install metal drip edge trim along eaves and rakes. Apply flashing around penetrations.
For an enhanced protective surface, coat the top layer with liquid waterproofing or gravel. This completes the flat roofing system.

Following proper techniques and manufacturer instructions is key to achieving a durable, water-tight roofing felt installation. Allow time for sealants to cure before exposing the roof to weather.

Benefits of Installing Roofing Felt Layers on a Flat Roof

Installing the proper number of roofing felt layers provides important benefits for your flat roof.

Additional layers of felt increase waterproofing redundancy. If you compromise one layer, the layers underneath will still protect against leaks. This is critical if any punctures or cracks occur over time.

Multiple staggered layers also enhance durability. They distribute the roof load, preventing fatigue and deterioration in any single layer.

The right amount of roofing felt helps manage moisture flow. It absorbs water effectively and prevents pooling or ponding issues that can lead to leaks and damage.

With sufficient felt overlap and sealant, you create a continuous water barrier across the entire roof surface. This protects vulnerable areas like joints, seams, and penetrations.

Finally, following manufacturer specifications for the number of layers ensures proper system performance and warranty coverage. This gives you peace of mind that your flat roof will provide lasting water protection.

In short, not skimping on roofing felt enhances the roof’s resilience, longevity and safety. It’s an investment that pays dividends over the lifetime of your flat roof.

FAQ

How many layers of felt do you put on a flat roof?

For optimal waterproofing and durability, use three layers of felt on a flat roof. The three layers consist of a base layer nailed to the deck, a middle layer, and a top layer.

Can you put two layers of felt on a roof?

Yes, it is possible to install two layers of felt on a roof. However, three layers are preferred for better protection, especially on flat roofs. Two layers may be acceptable for some sheds or garages.

What are the three layers of roofing felt?

The three layers of roofing felt are:
1. Base layer – nailed or glued directly to the roof deck.
2. Middle layer – installed over the base layer.
3. Top layer or cap sheet – final protective and weatherproof layer.

Which way do you lay felt on a flat roof?

Lay the felt layers on a flat roof in the same direction as the flow of water, starting at the bottom edge and working upwards in overlapping rows. Lay the top layer perpendicular to the bottom layers. Proper seams and overlaps are key.

Conclusion

In summary, it is recommended to use three layers of felt for a flat roof installation. The three layers provide optimal waterproofing, durability, and protection compared to using fewer layers. Following proper application methods for installing the base, middle, and cap sheet felt layers is also important. Using high-quality felt and ensuring proper overlaps will result in a well-protected flat roof. Understanding how many layers of felt are on a flat roof is vital.