Flat Roof: Put Torch Down Over Old Metal Roof?

shw001September 13, 2011

I have a flat roof (pitch 1/8 inch per foot) with a 60-year old tin roof. I just got what seems like an unusual bid from a local contractor with a good reputation. He will put the new roof over the old metal without tearing out the old metal. Is this installation OK? What do you think? This is his plan:

  1. Leave the tin roof in place (metal is steel coated with tin alloy, 60 years old)

2. Attach 1/2 inch rigid insulation over the old tin roof

3. Attach 3/8 or 1/2 inch of some type of sheathing board over the insulation

4. Adhere the torch down (also called modified bitumen) to that surface. (GAF smooth surface)

5. Paint the surface with aluminum paint to reflect heat away. Contractor says it needs to be re-painted only every 7-10 years.

Although EPDM and PVC are better, this is tempting because the installer is good and the price is less than half that of the other materials. There will be occasional walking on this roof and use of a ladder to service gutters on a higher roof, paint house, etc.

Is this installation OK?

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Any membrane roofing system is only as good as the perimeter and penetration flashings and termination seals so that is what I would be asking about. PVC in a light color should not need to be painted. I'm not fond of open flames on a residential roof.

    Bookmark   September 15, 2011 at 6:55AM
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Thanks for the response. The installer is very good, experienced, and will use a board over the installation, so the decking will not catch fire. I am confident that he will do terminations and seams well. My cheif concern is how a torch down (modified bitumen) roof will work in this type of installation where there could be some small puddles forming (about 1/8 inch deep)?

Renovator, you are correct that PVC would perform better and last longer. However, it is more than twice the price AND most local roofers don't offer it. For the contractos that do offer it, I have no information on their work quality.

    Bookmark   September 16, 2011 at 9:04AM
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I would consider a 1/8" per foot slope to be a dead flat roof so the membrane must be entirely waterproof so it will not leak even if it does not drain and that means no seams (or factory seams) which means the best material is fully adhered EPDM.

Any other membrane should be installed over tapered insulation so the slope is at least 1/4" per foot. You should include in all cost comparisons the probability of having to do it twice. See if the torch down contractor will replace the roofing if it leaks not just patch it.

    Bookmark   September 16, 2011 at 12:06PM
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Do you think TPO will be better than EPDM? and worth an extra 15-20% over EPDM?

    Bookmark   September 27, 2011 at 10:32AM
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