I've been asked to install a metal roof (galvanized aluminum) over existing shingles.   There are a few spots where the shingles have failed and water has leaked through to the ceiling tiles below.  I believe the reason the shingles failed is that there is little ventilation to the roof. 

My question:  Is it okay to go over shingles with the metal roof? 

If it is okay I think I should at least tear the bad shingles up, check the water damage and possibly replace the sheathing.  

What do you think?

Chad, IL

-proabably long since dealt with this, but I would always advise a tear-off. Then you can see the existing roof deck, repair damage, inspect everything--especiallt chimney flashings, repari as necessary.
-you can then start with a good flexible membrane at the gutter edge as well as new roofing felt.
-most metal systems rely on a smooth level deck--the locking tabs, or seamoverlaps work better (and look better) if they are going on level to begin wiith

A tearoff would be advised if, and only if, there is considerable damage to the roof sheathing. Otherwise, overlaying the metal is a simple and inexpensive solution to a bad shingled roof. We normally use galvalume steel panels, either painted or bare. These 29 gauge panels have the structural strength to span small weak spots in the sheathing, and as long as there are no more than 2 layers of shingles in place, we do not remove them. This allows for a roof that will last 3 to 4 times as long as shingles, in our climate, and at a cost close to the cost of replacing with shingles if you include tearoff and dump fees in your calculations. By the way, I have never used galvanized aluminum, and I think I would be more comfortable with our galvalume steel (stronger and less sensitive to environment).