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Flat Roof: Put Torch Down Over Old Metal Roof?

12 years ago

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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