How to buy and use a gas torch for roofing

A gas torch is a vital tool in laying a proper felt roof. Properly applied, the torch allows the roof to be completely sealed, making for a cost effective and durable roof. It’s a skilled process, and involves playing with a lot of heat. We strongly recommend hiring a professional rather than doing this yourself – if you make a mistake the risk of hurting yourself or severely damaging your property are extremely high. So, if you're wondering how to torch on felt, carry on reading.

This article covers how a professional would use a torch.

Choosing a torch for roofing

The most common choice of gas for roofing is propane – it’s widely available and relatively cheap.
Torch-on felt needs a wide column of flame, so the torch should be fitted with a large burner - most torches sold for roofing come with a 50mm bell. Roofing torches are usually fitted with a long enough neck to allow the user to stand up straight whilst applying flame to the roof. Anyone bending over while working will quickly find themselves developing a crick in their back – even if the roof they’re working on is a very small one.

Small propane bottles that connect directly to the torch are not suitable for laying torch-down roofing. Aside from being heavy and unwieldy, they don’t hold much fuel and will need to be changed often. Larger propane bottles connected by a hose are a better choice. The hose should be long enough that the user can complete the layout of a single strip without pausing to move the bottle.

Safety precautions

Because it involves fire, using a torch can be hazardous, and there are some precautions that should be taken before starting any work.
The area should be cleared of dust and debris – a broom and/or a blower should be used to clear the working area of anything flammable. A suitable fire extinguisher must be on hand in case of any accidents. Some people recommend having a wet towel handy so they can lay the torch down immediately after extinguishing it without damaging anything.

In terms of clothing, anyone handling a roofing torch should be wearing gloves, boots and goggles at the very least. Long trousers are recommended – even better if they’re flame retardant. A propane blow torch can get as hot as 1,995°C at the very hottest part of the flame. Not something you want to mess around with!

Before starting work, the torch and its peripherals should be checked to make sure it’s in good condition. Hoses should be examined for any signs of wear or damage, and the torch should be tested for leaks. One of the best methods to check for leaks is to hook everything up and, with a little gas flowing through the system, trickle soapy water over the joints at the hose ends and where the regulator meets the propane bottle. If any bubbles appear, then there’s a leak that needs to be corrected.

More detailed instructions will depend on the model of the torch – anyone using one should read through the manual.
A special note here: a flame should NEVER be used to check for leaks! That’s a quick way to earn a trip to A&E, if you’re lucky.

The roof layup

A felt roof is usually laid in three layers: two layers of bitumen felt, with a layer of mineral felt on top. The under-layers should lie perpendicular to the top layer. All the overlaps need to be staggered so that you never have two seams lying on top of each other. The felt should be laid out so that water never runs directly into any of the overlaps – felt higher on the roof should always rest on top of the felt lower on the roof.

Using the torch to lay felt

The mineral felt should be rolled out about ten feet to test how it lays on the roof, and to plan the overlaps. The aim is to have 3” of overlap on the edges, and 6” of overlap on the ends of the roll. Once positioned, it should be carefully rolled back up, making sure it’s kept straight and tight, without forming a cone.

The torch should be passed over the exposed underside of the roll and along the edge of the overlap. The user should move the torch evenly and smoothly over the area, working in an L-shape. Approximately 80% of the heat should be applied to the roll, and 20% to the substrate. In colder weather they might need to adjust this to 60%/40%, possible also moving the torch at a slower pace.

The pass is started at the lap side of the roll, moving out to the far edge. The torch is then pushed out a little so the flame is also catching the substrate as they sweep back to the lap edge. As a final touch, he torch is swept along the lap edge before pulling it back to the starting point.

You can tell when the bitumen is at the right temperature when it forms a slight sheen. There is a protective coating on the underside of the felt – this should completely burn off when the application temperature is met.

Heavy smoke means the temperature is too hot, and the user should back off.

As the felt is unrolled and the joins form, ideally there will be about 3/8” of bitumen flowing out from the overlap. Less will starve the seam, and if there is more than an inch of flow-out then the felt is being overheated.

Pressure is applied just behind the seam to help achieve good flow-out and ensure proper adhesion by the overlap. There are weighted rollers used for this, but it can also be done by walking next to the edge – as long as everyone is careful not to step in the bitumen.
After laying down the felt, the seams should be checked carefully. Any areas where the seam has been starved of bitumen can be fixed by heating the end of a trowel with the torch, and sliding it under the raised area of the overlap. When the area is hot enough, the trowel can be slid out again and pressure applied to the joint. If it has worked there will be flow-out, and good adhesion at the seam after it has cooled.