Well, not anything…
We live in a rural area. And I have noticed more and more metal roofs on homes when I drive around the area. And I understand why… they have 50 year warranties and are very low-maintenance.
Our (shingled) roof will need replaced in about 9 years. And I will seriously look into installing a metal roof.
[del]What’s your bankcard number and confirmation code?[/del]
Damn.
I had my roof replaced this year. I had two companies come out. They both gave me a specific dollar amount, and were within a thousand dollars of each other.
I had one or two days that I needed to make sure my car was outside of my garage early enough.
I opted to have a secondary water barrier installed, and a few solar attic fans. They replaced some rotted wood in the roof, repainted the trim, and matched the critter caps to the barrel tile they were installing.
I’d suggest reporting this bid to your insurance company, since they recommended the roofer. It’s definitely not standard practice, and they may want to rethink their recommendations.
I guess I will be the tiny minority here where I would actually prefer the time and materials bid. But I do that all the time in my contracting (to the Gov’t) work. The one time I tried to get a builder to offer me such a bid he refused. All my building bids have been firm fixed price.
The difference is risk. Risk is an expense like any other expense. The difference is simply who pays it. On a FFP bid, the contractor is assuming the risk and so the price will be higher. On a time and materials bid the owner is assuming the risk and so the price will be lower-unless something goes wrong. In general a T&M contract will be cheaper for both sides compared to a FFP. Note that the risk is in every contract regardless. The only question is who is paying for it.
In most cases for a one time contract like a roof for a homeowner, I guess the FFP model is best. Certainly all the contractors think so. Primarily because it is simpler. You don’t have to worry about disputing the time the contractor charges for the job-and figuring out in the first place what is reasonable and having a mechanism in the contract to resolve the dispute at all. On the other hand, the material cost is easier for you. You will have the receipt and you can call suppliers and confirm that the cost charged isn’t padded.
So, while a FFP is easier for both sides, the T&M is generally cheaper if the buyer is experienced enough to manage the contract. That second bidder could be thought of as paying you a compliment. Or taking advantage of you, depending on your view of contractors… 
Confirmed, though no cite.
You’re adding weight, without knowing what the original architectural specs for the roof trusses’ load bearing capability is.
You’re sealing in potential weak points (whatever caused the replacement in the first place) and making them impossible to find later.
And you’re attaching the shingle to … shingles. Shingles are not a structural material.
It’s the lazy, cheap, and quick way to do the job. When is that *ever *a good choice?
When you’re judgment-proof and don’t have a state license to worry about? Another vote for “don’t even think about using this contractor”. A time and materials bid is not necessarily sketchy, but refusing to tender a fixed-price bid is.
On the layering shingles thing, I suspect it varies by climate. You never want to do that down here (Florida) because we get so much rainfall. Any damage to the shingles means there *will *be water damage to the framing.