Modified Bitumen (Felt) vs EPDM

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Grant Findley of Findley Roofing & Building

. ThiGrant Findley of Findley Roofing and Building compares using modified bitumen felt and EPDM rubber membrane on your flat roofing projects.

FOR OVER half a century, and right up until the early 2000’s, bitumen felt had been at the forefront of flat roofing in the UK. This material was used in the construction of many houses with garages, extended bay windows and porches during the 1960’s. However, it didn’t take long for homeowners to lodge complaints about the failure of this roof covering.

In the decades since, improvements have been made to felt roofing that have reduced its risk of failure to a certain degree. And there are still a huge amount of
old-school roofers set in their ways who choose to only install felt. For the rest of us, we are fortunate that over the past 15 years, newer developments in flat roof coverings have genuinely innovated the industry – delivering great success to flat roofers.

One product in particular, proving hugely popular for both its durability and longevity, is EPDM rubber. EPDM (Ethylene Propylene Diene Monomors) found its way into Europe following a growing, glowing reputation in America. Year on year, its popularity led to large quantities being exported around the globe, from manufacturing facilities in Pennsylvania.

What are bitumen felt and EPDM roofs made of?
EPDM: this synthetic, man-made rubber membrane is formed by mixing a number of raw materials together, which are pressed and rolled into strips through a process called extrusion. Layers designed to separate the fabric are then installed, before the rubber is placed in an oven and heated, alongside sulphur, to improve its strength and elasticity. This process, scientifically known as ‘vulcanation’, is the last stage before the rubber is shipped and installed on properties across the globe.

Modified Bitumen: Modified bitumen, or felt, is made of three layers: boards, tar and mineral. The bitumen-based tar is laid, using a naked flame, and over the top of the tar is a crushed slate (normally Westmorland slate, on older flat roofs).

Which is the best for your flat roof?
My personal view when comparing modified bitumen felt and EPDM rubber roofing is not only completely impartial, but based on the vast experience I have in installing both of these types of roofs – as well as the years of research I’ve put into finding the best flat roof coverings for my clients. It’s always best to know the facts before you jump into replacing your flat roof. For me, I remained completely unbiased until I had trialled all of the available options. There are certain occasions where a specific roof covering will be more suited to a certain roof type, and vice versa. But I’ve found the pro’s of using EPDM on most flat roofs will drown out the cons by a country mile.

What sets EPDM apart?

  • Life expectancy – in the majority of cases, the life expectancy of a roof will boil down to the skill of the contractor chosen to install it. EPDM has a life expectancy of 50 years, comparing incredibly well to its bitumen felt competitors. The average guarantee a company will provide when installing EPDM on a residential client’s property is 20 years. Mineral felt has a general life expectancy of 5-10 years, but, on more occasions than I can count, I’ve witnessed felt fall short of that. However, this is more often down to the installers’ lack of skill, than the product itself. The average guarantee a company will offer to install a felt roof is 5 years.
  • Price – as the old saying goes, you get what you pay for – and I firmly believe this. EPDM’s price per square meter is more than double that of felt, but it actually more than triples the life expectancy. So, if you’re looking long-term for a client living in their forever home, paying more for an EPDM roof that may never need to be replaced again should be a no brainer. Essentially, if you buy cheap, you get cheap.
  • Installation – one of the major concerns with felt roofing is the torch-on application and use of a naked flame. Most commercial workplaces have banned the use of a gas torch on site. This is because it constitutes the largest danger to workers, with many examples of major fires breaking out every year. EPDM’s heat- and flame-free installation is safer, easier to install. It is usually fully stuck down using a recommended water-based adhesive.  Mineral felt can be purchased 1m wide by up to 10m in length. This means on wider projects there will be the requirement for lots of joins and seams. EPDM, depending on the manufacturer, can be fitted bespoke to the size of your roof space. Some manufacturers make their product up to 20 metres in width by 20 metres in length. This covers larger roofs in one piece without the need for any joins, seams or nails – and nearly all flat roof failings come through the joins. EPDM is regarded as one of the easiest and safest systems to install.
  • Durability – There is no comparison between the two, bitumen felt vs. EPDM systems, when it comes to how durable both roof coverings are. The only similarity is that they are both there because of their waterproofing properties.
    Rubber is extremely resistant to all types of weathering, which is why it’s now being installed in all different climates across the globe. Plus, rubber is not affected and does not deteriorate when exposed to UV rays. It is extremely flexible, and will stretch up to 200% of its natural size. Unless you plan to really go to town on the rubber with a chainsaw, then the membrane will not pierce, tear or split! For extra strength and longevity, classic bond now sell a fleece back rubber membrane. Foot traffic is not a cause for concern with EPDM, which requires little to no maintenance whatsoever.
    Felt, when exposed to certain criteria, will deteriorate in quality over time and demands regular maintenance. Roof spaces with constant foot traffic will see the crushed mineral protective layer wear. Should the bitumen base below be exposed to the sun’s UV rays, it will crack and split. Felt has no flexibility and will tear if pierced by sharp objects. The installation process is more difficult and more prone to human error. Felt deteriorates quicker in extreme low and extreme high temperatures.
  • Aesthetics – Felt roofing is generally grey. It can be installed in different colours and tends to look slightly old fashioned, as well as dull. When compared to bitumen felt, EPDM rubber certainly has a more modern aesthetic to it. It is much smoother and flush when installed correctly, without the unsightly seams and joins you see in a felt roof. Aluminium powder-coated drips and edges attain a smart fascia-like finish. Rubber is a dark matt black colour. If it’s appearance that is the deciding factor, then EPDM will win hands down.

Essentially, EPDM speaks for itself. Last year alone, one north east roofing firm, Findley Roofing and Building, installed over 2,000 EPDM rubber roofs on residential properties in the region – proving that it’s fast becoming the future of flat roofing. Findlay Roofing and Building operates in Wearside, Northumberland, Durham, the Tees Valley, and Newcastle.



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