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metal roof installed directly on shingle roof?
is it ok to put a metal roof directly screwed to an existing shingled roof. the current roof is not in bad shape. it is about 15 years old. my contractor says hes done 100's of these with no problem. he says the shingles are just like the sheath that you would put down if you were starting from scratch!
To avoid abrasion between the metal panel and the mineral surface of the old shingles, I suggest the installation of an underlayment between the two. I would suggest either 30-pound felt or (especially in warmer climates), a poly-based underlayment or felt with a layer of red rosin paper on top of it.
Using a screw down metal roof over existing shingles is not a good idea. Not even with an underlayment.

1. Shingles under a metal roof are prone to "cupping" caused by the cooking of the shingles by the metal roof. The cupping of the shingles will cause oil canning in the new panel. This condition is more prevalant in standing seam panel than screw down.

2. Attaching to an uneven and soft suface like shingles allows for a screw gap between the panel and the sheathing. This occurs after a few years when the shingle shrinks from cooking. Causing loud chatering noises in high winds and potential leaks around the washer.

The end point, your paying top bucks for a good roof, do it right the first time, pay for the tear-off.

PS: The 2 top items are compounded if you have a double couse of shingle on the house. Adding a 26 gage roof adds approx 1#/SF (24 gage = 1.2#/SF). A double shingle course and the new roof may exceed your rafter or truss loading, check your local code.
Some metal roofing systems are designed and tested specifically to allow them to install directly over existing ashphalt shingles and maintain or improve the fire rating etc. A 30 lb felt paper should be used as a separator to stop any abrasion. The larger the flat area in some of the generic vertical rib sheet products can diplay some telegraphing of the existing roof material and are best set up on strapping or have the existing material removed. Consult the appropriate manufacturer once you have chosen a product on its suitability.
Dan DiCicco perhaps makes some good points concerning some metal roofing profiles. But, Al Reid states correctly that some profiles are designed specifically to install over asphalt shingles. The roofing manufacturer and the contractor will be the best persons to decide if the style you choose can be placed directly over the shingles, the type of recommended underlayment, etc.
So, assuming one wants to go the easiest route and install directly on top of existing asphalt shingles (or should I say on top of 30 felt) which type of galvalume metal roof is best or specifically made for this type of use?
Most of the shingle, shake, and tile profiles manufactured today are suitable for installation over existing roofs. Because of their combination of horizontal seams or impressions, as well as vertical seams, they are designed to be forgiving of uneven surfaces.

Particularly with existing roofs that are reasonably smooth, many of the heavier gauge (24 gauge or heavier steel or .032" or heavier aluminum) vertical seam (standing seam and through-fastened) products can also be installed over old shingles.

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