Roofing felt keeps flat roofs watertight and has been used for over one hundred years. It’s made from a mixture of bitumen, fibrous membrane and sand; it can be applied using two different techniques. With a lifespan of around 30 years and a mid-tier price point, it’s a solid choice for flat roofs and is suitable for some sloped roofs too.

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Types of roofing felt

There are three types of roofing felt to consider for a roofing project; torch-on felt, self-adhesive felt and shed felt. There are many factors which can influence your decision, including whether you’re installing the roof yourself and the size of the project.

While torch-on felt is one of the quickest and most reliable flat roof installations, self-adhesive roofing felt is easier to apply yourself – you’ll find advantages and disadvantages to each roofing felt.

Torch-on felt

Torch-on felt sees three layers of bitumen felt melt-welded onto the roof with a blow torch, creating a waterproof seal. Traditionally, these layers are made up of a vapour control sheet, reinforced felt and a cap sheet with a mineral finish. If you’re using the torch-on method for a habitable area of a home, building requirements stipulate the use of all three layers. However, for sheds and garages, only two may be required.

While this is one of the most reliable flat roofing solutions, it can be dangerous. The use of a hot flame in the application process means it’s really only suitable to be laid by skilled roofers with experience in this field. To hire an expert to cover a single garage you’d typically be looking to pay around £750-£850. This is cheaper than alternative flat roof coverings such as fibreglass. The lifespan of a torch-on felt roof is in the region of 15-25 years.

Self-adhesive felt

If you’re working on a DIY project, self-adhesive felt is a great option. It allows the roof covering to be laid without the need for naked flames. So it’s a solid choice if you’re planning to install the roofing felt yourself. Once the release film is peeled off the bottom, the roof felt will self-bond to the substrate. This makes it ideal covering for small projects such as domestic flat roofs, garages and porches. It’s also cost-effective and results in a tough and durable roofing solution.

Shed felt

Shed felt is a roofing felt option suitable for sheds and garden buildings. Available in both torch-on and self-adhesive applications, it’s aesthetically pleasing and perfectly complements landscaped gardens. Although these types of constructions aren’t always considered for re-roofing, they depend on a weatherproof and waterproof finish – both of which can be provided by shed felt.

What is roofing felt made from?

These days roofing felt is largely made of modified bitumen mixed with either fibreglass or polyester. Typically, it tends to be the latter as it’s extremely difficult to tear polyester and it copes admirably with the usual movement associated with flat roofs. The combination of this and modified bitumen means felt roofing is tough, reliable and has a good lifespan.

While polyester and modified bitumen form a sturdy roof, it requires a final layer to protect it from the elements. Extreme cold can have a negative impact on felt roofing, but sunlight is the main enemy. It dries out the oils in the system making it brittle and can cause heat expansion movement too. These problems can be alleviated by adding a surface protection layer. There are many different options for that final protective layer including lead flashings and labour-intensive solar reflective paint. However, a mineralised cap sheet will give you an attractive, maintenance-free finish.

What are the advantages of roofing felt?

Felt is a low-cost roofing system that consistently performs. It’s a cornerstone of the roofing industry and offers customers value for money while being effective and easy to replace. As a material, felt is easy to work with and easy to transport, making it a great do-it-yourself option for those embarking on a self-build project.

Maintenance of a flat felt roof is typically quite low and doesn’t usually require a specialist. Repairs can be addressed easily using weatherproof paint and sealant, keeping costs to a minimum. Adding felt to your roof gives it a nice aesthetic appeal. While felt roofs tend to last between 10-30 years, many can be recycled at the end of their lifespan, making this an eco-friendly option.

What are the disadvantages of roofing felt?

Like any roofing option, there are advantages and disadvantages of using roof felt. While it’s easy to repair, amendments can look scruffy and negatively impact the aesthetics of your project. After a series of repairs, it might be worth considering replacing the roof to restore it to its full potential.

The intense heat of summer and the harsh cold of winter can lead to weather damage which can weaken the structure. You’ll also find felt roofing has a shorter lifespan compared with other flat roof materials, with most manufacturers guaranteeing the product for around 10 years.

How long does roof felt last?

There are many factors to consider when determining the lifespan of a flat felt roof. It largely depends on how well it’s been maintained over the years. While it’s true this type of roofing requires minimal maintenance, extreme weather can cause issues that need to be rectified.

However, felt is extremely durable. The guarantee you get will depend on the manufacturer, but they usually offer at least 10 years of cover on a felt roof. This is not necessarily indicative of the product’s lifespan as a well-maintained torch-on felt roof can last beyond 30 years.

When should roof felt be replaced?

With a typical lifespan of between 10 and 30 years, it can be hard to know exactly when your felt roof should be replaced. If you’re confident that the roof was fitted well, has been maintained and promptly repaired when necessary, the chances are your roof will last closer to the 30-year mark. However, nearing the end of its lifespan is not the only reason a felt roof needs replacing. Weather damage and organic growth can signify a replacement is imminent.

Felt roof tears and cracks

Probably one of the easiest problems to spot, tears and cracks can be caused by weather damage or potentially even the age of the roof. While small tears can be repaired cheaply and easily if a split appears across the entire surface of the roof you should consider a replacement. In this situation time is of the essence, as failing to address a large tear promptly means moisture could seep in creating water damage in your home.

Bubbling and blistering in roofing felt

If the adhesive in the roof fails, you’ll notice bubbling and blistering appearing in the felt. When detected in the early stages this is easily repairable, but if left to develop the blisters can grow so large that the entire roof will need to be replaced. The other issue here is the potential for a blister to pop, which would create a hole in the flat roof. Although, this generally only happens due to poor installation or inadequate ventilation in the construction stage.

Ponding on roof felt

If anything is likely to significantly reduce the lifespan of a felt roof it’s ponding. When water is left standing on felt it causes additional stress to the roof and can result in cracking and water penetration. Calling in a specialist is the best way to try to save the roof, but if the problem has been developing over time you might find replacing the roof is the only option available.

Organic growth

A build-up of moss and algae might not seem like anything to worry about, but it does signify the retention of water on your flat roof. It might be because it’s lacked regular maintenance and, if left untreated, you could face additional problems with water damage. This will require the roof to be completely replaced. Of course, the presence of moss might not mean anything more serious is going on. Nonetheless, it’s a good idea to conduct regular inspections to ensure there are no underlying problems developing.

How much felt will I need for my roof?

In order to felt a roof, you’ll need to know exactly how much material is required for the project. For a flat roof, this requires measuring the length and width, but if there’s a slope to your roof this needs to be factored in. To determine the roof area of one side of a slope, simply multiply the length of your roof by the slope height. Most homeowners will have access to this information through building plans acquired with the property, but a local council should be able to help if this is not the case.

Our guide to calculating the roof area for a project provides more information on how to measure your roof.

What are the alternative options to using roof felt?

Felt roofing is a solid choice for a flat roof, but it’s not the only option on the market. GRP (glass reinforced plastic) and EPDM (rubber roofing) are also popular choices in the industry. GRP, in particular, has had a strong impact in the UK, but while its lifespan sits around 20-25 years it can be as much as £30 per metre more expensive than felt.

EPDM rubber roofing has an impressive life expectancy at around 50 years. It repels moisture well and looks aesthetically pleasing, but the installation process is a lot more involved and typically requires a roofing specialist. If you’re embarking on a DIY project, self-adhesive felt is not only cost-effective but much easier to install too.

How to felt a roof

Felting a roof is achievable for most keen DIYers. The process varies depending on whether you’re felting a flat or sloped roof. However, being lightweight, easy to install and budget-friendly, felt is a solid roofing choice. Preparation is key, so remove any existing felt and ensure the surface is clean, replacing any rotten or damaged timber where necessary. You’ll also need to calculate the roof area, remembering to add around 5-7.5cm for overlaps at the edges and gables.

If you’re covering a flat roof you’ll need to lay the felt out to rest before you can manipulate it into place. Whereas with a sloped roof you can begin installing the felt as soon as you’re confident your calculations are correct. You can find a step-by-step guide on how to felt both a flat and sloped roof here.

If you would like further advice regarding roofing felt, speak to our experts! You can use the chatbox in the bottom right or give us a call on 01752 692 760.

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