Is it ever a good idea to put new roof shingles on top of old roof shingles?

[NNN0LHI]

I have heard over the years some people who recommend doing this and other who recommend not doing it.

After trying it both ways I do not recommend ever doing it.

If your old shingles are dried and curling your new shingles that are installed on top of those will be curling up in the same way as the old ones in just a few years as they take the shape of the old shingles and it will look like you need a new roof. The new shingles will not help the old shingles left on the roof. It will be the opposite. The old shingles will screw up your new ones quickly.

Then you are still going to have to pay someone to remove two layers of shingles now because a reputable roofer will not put a third roof on because of too much weight. Especially in areas where there is a lot of snowfall.

After trying this both ways I would highly recommend never putting new shingles on top of old ones. Its a waste of money.

Anyone else discovered this through trial and error?

Don

[Berserker]

1. All of what you said

is true.
The most important reason for removing the shingles is to check the sheathing for leaks and deterioration. Just because you have not detected a leak on the interior does not mean your shingles are not leaking and rotting the sheathing.
Also the flashing can be checked and replaced due to rust or poor workmanship.
Most areas will not allow a 2nd layer of shingles you will need to check with your local building official.

[txlibdem]

2. Never a good practice

How could you ever know if your roof sheathing is rotted out from previous water damage?

Definitely agree that you take it down to the bare sheathing and remove and replace all the flashing - and caulk like mad. Also ice dam prevention if you're in that type of clime, chances are that you need to put on a nailable waterproof membrane that goes from the edge to 24" up the slope of the roof. Sometimes when you get all of the stuff off the roof you might find flaws that previous workmen just covered up - huge gaps, flimsy construction, etc.

Not to deflect the topic or anything but have you set your heart on asphalt shingles? Since you're replacing the roof you might be able to go standing seam, cool roof, or even green roof if it's beefy enough structurally. All of these options will reduce your energy usage and last a heck of a lot longer. Just food for thought.

If I had to replace the roof sheathing I'd also consider using SIPs (structural insulated panels) if the budget isn't shoestring.

[Berserker]

4. The IRC

(International Residential Code) states that the Ice dam Membrane sheet shall be used in lieu of normal underlayment and extend from the eave's edge to a point 24 inches inside the exterior wall line of the building.

24 inches as you stated would not meet code requirements in Northern climates.
We install the Ice and Watershield 6 feet up from the bottoms of all roofs in Minnesota and Wisconsin regardless of the local code. We also install it in all the valley areas.

[txlibdem]

5. Nice

I like to see someone with the ability to think ahead and head off problems from the start! Way to go!

I live in Texas and don't have the inside info on the northern states' building codes so please forgive my ignorance.

[Paper Roses]

3. I believe it is a law around here that no more than two layers

of shingles can go on your roof. At least that is what my last roofer said. For a lot of reasons this makes sense. Wood and flashing problems unseen, rot, fire hazard, curling shingles. weight--probably even more.

I am on the 3rd roof for this old house. Removed the old ones a few years after we bought this place as they had started to leak. They were that old asbestos shingle, triangle shaped. Ugly. Second time was because a tree fell on the roof. Lets hope this one lasts until I don't. Expensive!! About 30 years ago we had the roof done, $850.00 and a good job.

The last one was almost $8000.00 and, although it is a good job with long lasting shingles, it almost broke the bank. It was a shock...and I got 3 estimates.