Is Roofing Felt Waterproof? [A Useful Guide]

If you’re dealing with a leaky shed, a flat roof repair, or you’re replacing the underlay on your house, you’ve probably come across roofing felt.

It’s one of the oldest and most widely used materials in UK roofing, playing a crucial role in protecting the building—but many homeowners still ask:

Is roofing felt waterproof?

The short answer is yes—but with limits.

At Roofing Newcastle, we install and repair all types of felt roofs and underlays, so we know exactly what it can (and can’t) do. In this article, we’ll break down how roofing felt works, when it’s waterproof, and what you need to watch out for.

Additionally, it’s important to note that roofing felt should comply with British Standard BS 5250 to ensure proper ventilation and effectiveness in roofing systems.

What Is Roofing Felt?

Roofing felt is a layer of protective material used on roofs, either as:

• A primary waterproofing layer on flat roofs (known as felt roofing), or
• A secondary underlay beneath roof tiles or slates on pitched roofs

There are different types of roofing felt, each with distinct qualities, costs, and suitability for specific applications. These classifications can impact the choice of materials and methods in roofing projects.

It’s typically made from bitumen, fibreglass, or polyester, and often coated in mineral granules or sand.

It’s flexible, strong, and designed to provide protection against water, wind, and UV damage.

Is Roofing Felt Waterproof?

Yes—when installed correctly.

Roofing felt is designed to repel water, not absorb it, even when it becomes wet. It forms a barrier that keeps rain, snow, and moisture from penetrating the roof structure.

However, how waterproof it is depends on:

• The type of felt used
• The quality of installation
• Whether it’s the main covering or an underlay
• The condition of the felt over time

Let’s look at both main uses.

1. Roofing Felt on Flat Roofs

Flat roofs often use a multi-layer felt system, usually applied with heat (torch-on) or cold adhesive. This is a primary waterproofing method.

When properly installed, a felt flat roof is fully waterproof.

However, traditional bitumen sarking felt can become brittle and rot when exposed to sunlight and other environmental conditions, which significantly affects its lifespan and maintenance needs.

Modern felt roofing systems are much more durable than older versions and can last 15–25 years if maintained.

2. Roofing Felt as an Underlay (Pitched Roofs)

On a pitched roof, roofing felt sits beneath the tiles or slates. It acts as a secondary barrier—protecting your property from leaks and moisture damage in case of tile movement, heavy winds, or water ingress.

While this felt layer is water-resistant, it’s not designed to withstand constant exposure to the elements. It relies on the roof covering above it.

If your tiles are missing or broken, felt can hold off leaks for a while—but it’s not a long-term substitute.

Types of Roofing Felt and Their Waterproofing Levels

Not all felts are created equal. Here are the main types:

We offer a comprehensive range of roofing felt products, suitable for various applications and catering to different needs and preferences in roofing materials.

Torch-On Felt

Applied using heat to bond layers together. Strong, durable, and very waterproof—ideal for flat roofs.

Self-Adhesive Felt

Sticks without the need for heat. Easier to install but may not last as long. Best for smaller projects or sheds.

Cold Adhesive Felt

Bonded using special glue. Good for DIY, though waterproofing depends heavily on surface prep and weather during installation.

Breathable Felt (Underlay)

Used under tiles or slates. Water-resistant but vapour-permeable to reduce condensation. Waterproof against light exposure—but not designed as the main roof surface.

How Long Is Roofing Felt Waterproof?

• Torch-on felt roofs: 15–25 years with proper care
• Standard shed felt: 5–10 years
• Underlay felt (pitched roofs): lasts as long as the tiles protect it

If felt becomes cracked, blistered, or begins lifting at the edges, it may no longer be watertight and needs attention.

Common Signs Your Roofing Felt Is Failing

• Water stains or leaks indoors
• Blisters or bubbles on flat felt roofs
• Torn or loose edges
• Mold or damp in the loft (underlay failure)
• Bits of mineral felt in your gutters

If you spot any of these signs, it’s worth having your roof checked before the damage gets worse.

Is Roofing Felt Enough to Waterproof a Roof?

On Flat Roofs

Yes, if installed as a multi-layer system with proper overlaps and sealed joints, flat roofing felt effectively meets diverse roofing needs by providing excellent waterproofing capabilities, durability, and versatility.

However, flat roofs are more vulnerable to standing water and UV damage, so regular checks and occasional re-coating may be needed.

On Pitched Roofs

No—roofing felt is only part of the waterproofing system. The tiles or slates are your main line of defence.

If the outer covering is damaged or missing, felt might offer temporary protection—but it won’t hold off the weather forever.

Do I Need to Replace Old Felt?

If your roofing felt is brittle, cracked, or over 20 years old, it may be time to replace it.

Upgrading to modern felt—especially breathable membranes—can improve ventilation, reduce condensation, and give you better peace of mind.

At Roofing Newcastle, we assess the condition of your current felt and recommend the best solution for your roof type. We assist customers in selecting the best roofing products for their needs, ensuring they find suitable solutions for their projects.

Final Thoughts

So, is roofing felt waterproof?

Yes—when used correctly and in good condition. It’s a reliable, weather-resistant material that plays an important role in both flat and pitched roofing systems.

But it’s not magic. It works best when installed properly, protected by other materials (like tiles or coatings), and maintained over time.

If you’re not sure about the condition of your roofing felt, get in touch with Roofing Newcastle. We offer free inspections across the North East and can help you decide whether your felt is still doing its job—or needs an upgrade.

Better safe than soaked.