There are several routine crimes committed against roofs out of expediency that should never be done. They do get done, because a homeowner wants to believe a fantasy, and a contractor is willing to do something mediocre just to get the job. So don’t do these.
Existing roofing should never be covered over in a reroofing situation.
You see this done with asphalt shingles to ostensibly save a few bucks now and then, but it is against code, it covers up potential problems with the decking and the roof just does not last as well.
Critical flashing areas can just be nailed over rather than properly flashed with proper flashing metals. A chimney flashing will be much worse off after a recover than with the original roof. The valley flashing on a chimney is a critical point of risk and adding a recover here is at least doubling your chances of really serious problems right from day one of a recover.
Asphalt shingles work really well when it is well supported against a solid flat deck. Recovering over the existing levels the new shingles poorly supported over the roiled surface of the existing shingles.
The recover can trap moisture, cause the new materials not to lay properly and, if push comes to shove, you basically have no warranty.
We also have to be very sensitive to slope.
Any slope flatter the 3:12 which means a 3 foot elevation gain for every 12 feet of surface run… is the flattest slope that works with the standard spec for sloped roofing. You see roofers on occasion flirting with low slopes. Maybe it is s remodel, and the roofer is a roofer in name only, but you see people putting on roofing and adding an extra layer of underlayment, maybe, to help keep water out.
Once upon a time there were roofing specifications for low slope roofing that called for a double layer of underlayment. But you don’t want this. If you get into a low slope situation, you want a waterproof specification just like for a flat roof. With a low slope spec, you are hoping the water runs off before the water soaks through. Particularly, if you have a lot a long slope to drain, makes the situation even worse.
There are new self sticking membranes used in sensitive areas of the roofing that reseal themselves around nailheads, but this is not as good as using a roofing spec designed to keep out water.
Your good old standard roof spec can run into low slope situations as part of the roof slope configuration. Different slopes can be tricky, particularly in a situation where a roofer is moving at high speed; say, new construction. Make sure you also see the discussion on getting the roofing details right.
You might want to check out the page on the problem home. If you have a chronic problem and the contractor is making excuses, then you might have a problem home with unique little structural traps build right in. Check out the at risk home here.
What did I miss?
Do you have a question or a comment; feel free to speak up. What you say might help someone make a better decision.