Is Torch-On Felt Still a Good Choice for a Flat Roof?

Torch-on felt is not the old shed felt people remember. Here is how a modern built-up system works, where it performs well and the questions to ask first.

Yes, for many garages, extensions and dormers torch-on felt remains a sound choice when it is a modern multi-layer system laid by someone who knows what they are doing. It is a different product from the single layer of nailed felt found on sheds, and it should not be judged by the roofs that gave felt a poor reputation decades ago.

Whether it is the right choice for your roof depends on the shape, the details, how the roof is used and whether a naked flame can be used safely on it.

What makes up a torch-on system

A built-up felt roof is made of more than one layer. Typically there is a vapour control layer and insulation where required, an underlayer bonded to the deck or insulation, and a mineral finished cap sheet on top. The layers are modified bitumen reinforced with polyester, which gives the membrane strength and some flexibility as the roof moves with the seasons.

The mineral chippings on the cap sheet protect the bitumen from sunlight, which is the main thing that ages it.

Each roll is unrolled and its underside heated with a gas torch until the bitumen melts and bonds to the layer beneath. Laps between rolls are sealed the same way, so the finished roof is one continuous waterproof sheet. Upstands are dressed up walls and finished under flashings or trims, and outlets are sealed into the membrane.

The quality of the bond and the laps decides how long the roof lasts. Poorly heated laps are the most common failure point on felt roofs we are called to repair.

Because it uses an open flame, torch-on needs care around timber fascias, soffit vents, roof lights and anything flammable. Responsible roofers carry extinguishers, keep a fire watch after torching stops and avoid torching in places where the flame could reach hidden timber. On some roofs, cold applied or self adhesive felt layers are used at the risky edges instead.

If a roof has lots of timber detailing close to the membrane, that may push the choice towards rubber or fibreglass, which do not need a flame.

Where torch-on suits and where it does not

It suits straightforward garage roofs, rear extensions and dormers, especially where the roof is not walked on regularly. It copes with a reasonable amount of detail around outlets and upstands. It is less suited to roofs used as terraces, where fibreglass gives a harder wearing finish, or to large simple roofs where a single sheet of EPDM avoids laps altogether.

Our felt, rubber and fibreglass comparison goes through those choices in more depth if you are weighing them up.

Signs an existing felt roof is failing

Blisters that grow each summer, splits along laps, bald patches where the chippings have washed off, water ponding for days after rain and damp patches on the ceiling below are all warning signs. A single split can often be repaired. When the whole cap sheet is cracked and brittle, a new covering is usually the better use of money.

Is torch-on felt the same as shed felt?

No. Torch-on is a multi-layer system of reinforced modified bitumen bonded with heat, while shed felt is usually a single thin layer nailed down.

Is it safe to use a torch on my roof?

It is when the roofer follows proper precautions, keeps the flame away from hidden timber and keeps watch after finishing. Risky edges can use cold applied layers.

Can torch-on felt be laid over an old felt roof?

Only if the existing roof and deck are dry and sound. If the deck is wet or soft, the old roof needs stripping first.

Why are there stone chippings on top?

The mineral finish shields the bitumen from sunlight, which slows the ageing of the membrane.

Do you fit rubber and fibreglass as well?

Yes. We fit felt, EPDM rubber and GRP fibreglass, and recommend whichever suits the roof.

Wondering if felt suits your roof?

We will look at the shape, the edges and how you use the roof, then give you a straight recommendation and a written price.

Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.