How Thick Should Roofing Felt Be?

Choosing the correct roofing felt or roofing membrane can be complex. There are many fantastic choices on the market, but all serve different purposes, as we will find out in this post. We’ll be looking at the various thicknesses of felt and their suited purpose.

Shed Felt Thickness (160gsm – 230gsm)

Typical shed felt is made from bitumen and thus is naturally thick to protect the shed from breaking while also being easy to install for the typical DIYer. GSM for shed felt can vary but typically ranges from 160 to 230gsm — if you are looking for a more premium product. Anything over that would be overkill, and less would make your job more challenging as the felt could tear quite quickly, making your job suddenly more expensive and time-consuming.

Generally speaking, shed felt products don’t get much mention of their GSM in their specifications due to them being widely applicable and all the time made from roughly the same quality of material and mineral finish.

Torch-on Roofing Underlay Felt (2mm)

Torch-on underlay is a superb product used across many flat roofing surfaces on top of office buildings, garages and even homes. It tends to be around 2mm and is a solid, hard-to-break material that is flexible. Unlike shed felt, it cannot be nailed down or applied with some felt adhesive. To apply torch-on underlay, you need to be a trained roofer, as you’ll be using heat welding tools such as Oxyturbo kits. If you’re looking to save money, you can get some training from YouTube tutorials. However, we do not recommend this method as this job can be dangerous if you’ve never undertaken this project or have no one with you with relative roofing experience.

Breathable Membrane (95gsm – 175gsm)

If you’re building a new roof or simply re-roofing an existing one for better breathability and durability, then choosing a breathable membrane is a solid choice. However, this product range has varied product lines, meaning you can get lite options that are pretty thin to extreme/pro options that can be nearly double in thickness. These breather membranes can be acquired from companies such as Marley, Cromar, and others in the roofing world.

Non-Breathable Membrane (0.5m – 1mm)

Non-breather membranes and felts are a great alternative if you’re not looking to let in any moisture into your building and are possibly looking to build a cold roof. They tend to be 1mm or 0.5mm in thickness and are sold by many manufacturers, such as Sark-IT, Protect A1, and Marley.

Abbie Wilson
Abbie Wilson has years of experience in the DIY & construction field having covered roofing, plumbing, and general DIY for various award-winning publications.

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