I do believe you are mistaken about Grace I&W applied over old roofing underlayment. Read technical bulletin #5.

You may want to look up Grace Ultra.

All aside, I have seen Grace baked between layers of roofing during tear-offs. Appears someone thought it may be a good idea to peel back the bottom foot of covering down to the decking & apply I&W for the first foot, with the balance adhered to the roof covering. I have seen it more than once, and it bakes the adhesive, rendering it useless.

I should add, the membrane was between layers of roof covering. Also, if by chance your idea was to use the I&W as a roof covering, there is a window, which the membrane can be exposed to the elements. Can't remember off hand, but it's something like 30, 60 or 90 days.

This post was edited by Roof35 on Wed, Mar 20, 13 at 13:13

Torch-down roofing should have a modified-bitumen/asphalt top membrane so a self-adhering modified -asphalt membrane like Grace Ice & Water Shield should adhere to it well but the surface must be clean.

Did you have any leaks through the old roof?

Once the leaks occur stripping becomes far more important to check the roof deck for hidden damage.

Or you can hope the roofer does not find any 'soft spots' applying the new roof.