We are getting quotes for a new roof & would like your opinions: should we remove the existing shingles or overlay them? Code allows for two layers, so most of the roofers just want to install the new shingles on top of the old ones. (The old ones are the original roof, about 13 years old, basically in good shape, no curling, etc.). We're concerned about the added weight on a fairly low-pitched roof (5/12). Any opinions on this, one way or the other? By the way, both roof are fiberglass shingles.

Usually the first re-roof is applied over the existing shingles, and if there were no problems with them, there is no reason to remove them. It will cost no less to remove one roof now than two roofs twenty years from now, not counting inflation.

If there have been problems, then it's possible that the sheathing has been compromised. either by plywood de-laminating, or OSB swelling, either caused by water getting where it shouldn't have been.

Not sure about your climate, but if snow is an issue, one reason you might want to remove the original roof now would be so that the roofers can install waterproffing shingle underlayment before installing the new shingles, to give you some protection against ice dams and the associated leakage. The current building code requires this to extend at least two feet inside the exterior wall, but you will not be required to conform to the current code if you are simply re-roofing.

Thanks, Richard,
I had another roofer come out today. He said because we only have gable end vents (no ridge vent), there is a good chance some of the plywood has been damaged due to the extremes of heating & cooling, resulting in delamination. There is at least one place where it looks like the plywood is sagging. He recommends installing metal vents every 8-10 feet & tearing off the old roof first. Does that sound right to you?

If there is a reasonable suspicion of sheathing problems, yes, removal of the old roof does sound right.

If the old roof is removed, a ridge vent could be installed. Be certain it is external-baffle type. The ridge vent will only be fully effective if soffit vents are also installed. Does your roof have overhangs? If so, is the soffit vented, or can it be easily vented?

Thanks again. Yes, we have a 2' overhang with numerous soffit vents. What exactly do you mean by an external baffle ridge vent? The ones I've seen are rolls of plastic. The last roofer said he didn't like them since plastic doesn't hold up well in our extreme temperatures. He prefers the old fashioned metal cans.

External baffle ridge vents have a fin that extends up from the roof about an inch, and is outside the ridge vent. Its purpose is to prevent rain water from blowing into the ridge vent.

I'd certainly remove the old roof. As was mentioned, there is no good way to determine the condition of the wood under the shingles without actually looking at it. Make sure they remove the tar paper as well as the shingles. Roofs are only designed to hold so much weight and I've seen some houses built so that you'd be surprised they hold up the plywood, much less shingles. Now would be a good time to have a thermostat controlled vent attic fan installed if you've ever thought of doing that. I've found that this can cut cooling costs dramatically, but make sure you have enough ventilation for adequate airflow.

Another issue that has not been addresssed is the fact that 1 layer roof will last longer than a roof that is overlayed. This a due to the heat build up of extra layer causing the new layer to degrade quicker. If you plan on staying in the house for a long time go with architectual shingles also.You will get a lot more than 13 years out of your new roof

Do not use "mushroom" vents every 8-10 feet. A ridge vent with soffit venting is just as good (if nod better) and far more attractive. We live in mic-Missouri and there is nothing like a new development of $500,000 homes sporting "mushrooms" all over their roofs. The better builders in this area use ridge vents exclusively. You should see what happens to the mushroom vents in a hailstorm. Metal or plastic, they all were ruined in no time. Stidk with ridge vent with soffit vent.

Just because local codes allow something, does not make it the best practice. I never recommend re-roofing over the existing shingles. The shingle manufacturers allow layering, but do not recommend it as well. Layering the shingles will dramatically reduce the life of the new shingles, due to increased heat retention. Also there could be existing leak issues that have not shown up yet, that will not be found if the existing shingles are not removed. Just because there is no evidence of a leak inside the house yet, does not mean there are no leaks. Over the years, I have found alot of flashing issues on roofs, that needed to be addressed, that I would not have found had I not torn the existing shingles off of the roof. Your roof is the most important part of your house, as far as keeping the home in good condition. A poor roof job will almost certainly lead to other problems down the road. It is always worth going the extra mile and making sure everything is in good shape with the roof. Proper ventilation is very important, for the life of your roof, as well. The best way to achieve proper ventilation is through a combination of soffit vents and ridge vent. Ask your contractor, they should be able to tell you how much venting you need for the size of your attic.

Ridge vents should never be combined with gable-end vents, since this combination will allow, under certain wind conditions, the infiltration of wind-bourne rain or snow into the attic, through the ridge vent. If a ridge vent is used, it should be in combination with a soffit vent system, allowing the same net-free area of air infiltration at the soffits as is being exhausted out of the ridge vent.

The fins, or external baffles, on the better grade of ridge vents are there primarily to create negative air pressure (partial vacuum) in the area directly behind them, where the hot attic air is exhausted, in order to enhance the air-exchange of the system. As wind blows over the roof and hits the baffle, it is kicked upward, creating negative air pressure in the trough immediately behind the baffle, drawing out the air immediately behind the exhaust vents. This, in turn, creates a pressure reduction in the air at the peak of the attic, which is filled by the rising column of incoming air from the soffit. As the sun beats down on the attic roof, it heats the air within the attic, which, being less dense than cooler air outside the attic, will rise due to thermal convection, drawing additional cooler air into the attic at the soffit. As long as there is a thermal gradient bretween the soffit and the ridge, the system will work.

The addition of a thermostatically-controlled attic exhaust fan behind the gable-end vent is a good idea, if the vent where the fan is located is covered with a sheet of 1/2" plywood, from stud-to-stud, and closed up at the bottom and top of the stud bay containing the gable vent, with the diameter of the fan shroud cut out of the center of the plywood, and the fan mounted over this hole. The result is that the fan will exhaust its full rated capacity of air, in cubic feet per minute (CFM), due to the venturi effect of the plywood with the fan-diameter hole, with no blow-by in the corners, or loss of efficiency.

The color of the roof has as much as anything to do with how long the roof will last, since darker colored roofs absorb more solar energy than do light or white roofs, and the absorbtion or reflection of heat from the sun will largely determine the resonable expectation of life from a roof, assuming proper installation and proper venting. New asphaltic shingles have a certain amount of plasticity, or flexibility, which excessive heat will cook out of the shingle, shortening the life of it.

I recommend you strip the roof first. Many suppliers will void thier warrantee if the roofing is a second layer.
The code allowing two layers refers only to structural issuues, not material longevity.
Also, the second layer will not look as good because the first layer will telegraph its pattern to the second.
finally, two layers tend to trap moisture and lead to related problems.