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I am looking to diagnose reasons why my first attempt at torch-down patching of my almost-flat roof (1/12) failed. I used this inexpensive torch fueled by a small, camping propane tank, for compactness and ease of transport to the roof:

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To make a small patch around a fart fan roof vent using this material:

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I couldn't get the other side (mating surface) of the material to melt and become adhesive, so it wouldn't stick. The environmental conditions were around 35°F and windy (probably 15 mph wind, which visibly interfered with the flame and was "sailing" the material as I was holding it with groove joint pliers). I did make sure the roof surface was dry.

Could it be because:

  1. Inadequate (cheap) torch
  2. Inadequate fuel tank (should have used 20 lb like for grilling, which would make it harder to get up on the roof)
  3. Environmental conditions (temp + wind)
  4. Lack of workman skill
  5. Something else?
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    I think you answered your own question. If you can't get the heat to be absorbed by the material it won't melt. Tank size is irrelevant assuming adequate fuel. – isherwood Feb 4 at 16:34
  • I think the charge in the tank was low. Could that cause low pressure and consequently heat? – amphibient Feb 4 at 16:36
  • I don't think that material is meant to just be torched and put in place. It usually need hot tar. – JACK Feb 4 at 16:39
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    I mentioned that this would be a problem when you asked about the adapter last week... – Ecnerwal Feb 4 at 16:59
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    A 1 lb tank will not supply enough gas to heat a large area. I think my burner is 300k btu and if on full I don’t think it would last 30 minutes on a 20lb tank if it did not freeze up first that small tank just can’t provide the gas needed to properly heat the surfaces. My small tank I think 10lb freezes in just a few minutes but is enough for most patches. Tank size matters with high flow. There are several types of rolled roofing that is put down with a torch prior to membrane roofing this is how we did it but adding tar is usually required on rocked style. – Ed Beal Feb 4 at 17:02
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The 1 lb tank will not supply the flow needed. you used the equivalent of a small hand torch with that bottle adapter to the 1 lb tank. There is a smaller than 5 gallon 20 lb tank I think it is 10 lb and will supply the proper flow of propane if you don’t have or want the standard size tank but need a few minutes of high flow. This is the most likely cause but the low temp and wind added or even compounded the problem. Hot tar can be an art but it’s easy to learn.

Getting the surface to be bonded hot enough to “wet” or turn the material surface to liquid then start sticking it down and continuing working it until sealed. With small patches I usually will add tar around the hole or in the area. I have seen guys step on the hot tar to smush it together but I was taught to use a roller so the tar is not smushed out in one spot next to a spot that is not smushed the roller helps for an even pressure, for a small patch the back of a garden rake works. And at last resort light pressure from the ball of your foot not full pressure and make sure to overlap but remember you only have 10-30 seconds depending on how warm you got everything. If you don’t have 10 seconds you are not heating the original surface enough.

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  • I just googled, they have 11 and 5 lb that are smaller than 20. Do you think 5 would work for small jobs? – amphibient Feb 4 at 16:53
  • 5 is awfully small with a high flow it may freeze up in just a minute or 2 . I think I have the 11lb was guessing at the size. I actually got it for patches years ago much easier to climb a ladder with the smaller tank. It will freeze up but I am usually just about done when the flow chokes down. – Ed Beal Feb 4 at 17:20

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