If a roof has a single layer of shingles put on without tar paper on a plywood base is there any problem with putting on a second layer?
I’m not a roofer. I did the roof on my own home, 20 years ago and it was a VERY BIG DEAL to get the old roof as smooth as glass before we laid down the Tar Paper and Shingles.
I don’t know why, but it worked.
We spent two days tearing that stuff off and decades enjoying the fruits of our labors.
This method is commonly referred to as a “nail over” or “layover” and adding tar paper is not common. Obviously, you save the cost of removing the original layer of shingles but the 2nd layer tends to not last as long as a clean install.
This is a common practice with homeowners needed a new roof while realizing they will probably sell before it’s time to replace the roof or simply do not have the money to tear off the original layer.
Many reputable roofer will not do it while others point out that the manufacturer may not honor warranty claims. Check with the shingle manufacturer on their position on a layover.
To me it’s a big deal because the first layer was put on incorrectly. Any current spec includes roofing paper or snow and ice shield under the shingles. Water can easily blow under shingles. They are not made for leak protection as a single element of a roof. It’s the paper under the shingles that prevents leaks. This roof needs to be removed and done correctly. Check any shingle manufacturer’s installation instructions or just read the side of any bundle of shingles.
I’ve heard the opposite tar paper with tack holes and nail holes is not a waterproof layer.
The waterproofing comes from the water flowing off of one shingle onto another.
Tar paper is usually stapled presenting a much smaller chance of water infiltration and serves as a last line of defense against a leak. What you say about shingles shedding water is true but under the right circumstances, wind can temporarily lift a shingle allowing wind driven rain to blow under it and this is when the tar paper serves its purpose. Water can also infiltrate roof flashing and tar paper can also prevent or slow damage.
OK, but the nail holes holding the shingles also penetrate the tar paper, so it’s not just tiny staple holes.
True and you make a good point. I would still not hesitate to install tar paper because it does serve a last line of defense purpose.
What’s your opinion of that rubber roof membrane? Its claimed to be leakproof for 25-30 years.
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Ripping off the old shingles allows you to inspect the sheathing plywood for rot. This can become a big deal. Shed money up front and do it so you don’t shed tears later.
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If you live in an area that gets decent-sized hail, tear off the old roof. If you shingle over the old layer your roof has more flexible spots that the hail will find and tear holes in. There are a number of places where building codes prohibit 2nd layers.
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If a roof has a single layer of shingles put on without tar paper on a plywood base you may already have a rot problem if you get a lot of rain. Consider what might happen if you do have rot, and you add the weight of another layer of shingles. Grab a bundle of shingles sometime and then imagine what a whole roof’s worth must weigh. As far as I’m concerned (and I am nobody of consequence) a roof w/o tar paper is like white pants w/o underwears.
The second layer of shingles over the first is a common, successful practice. Contractors insisting on removing the first layer are only inflating the job. By code most places, you can’t do a third layer.
The membranes are a common practice for flat roofs. I see no reason they wouldn’t work on a sloped one. Metal roofing is gaining for residential roofs. Regardless of the roofing type, flashing makes a huge difference. The newer practice of running an elastomeric material up whatever and screwing strips to hold it seems to be a good solution.
tar paper is a water barrier and a friction release. the shingles will have movement due to temperature that is different from the roof deck.
a second layer of shingles doesn’t work as well or last as long. it might void the warranty on the shingles. poor condition of shingles below and poor installation methods get magnified in the top layer.
if you are where there is snow in winter and you are not superinsulated so you have no heat loss through the roof then an ice dam membrane on the edge is well worth installing.
Since the pros have already answered I’ll thow in my 2 cents.
We bought our house 13 years ago. It was built in 1942. The original layer of shingles is still up there, over tar paper over 1X12 spaced 3" apart. (no plywood)
A third layer of shingles was installed a year before we purchased the house. So far we’ve had no problem, but I’m sure when it comes time to redo the roof, it will cost a fortune.
The biggest problem, I was told, is the weight of the whole mess could cause a failure.
I installed a second layer of shingles directly over the old ones in 1996. They are still in good shape and withstood a hail storm that lead to replacement of many roofs in my neighborhood.
If the first layer is installed properly then putting a second layer is not a big problem. But the first layer was put on improperly so I would remove it and install a new roof properly. Also I would check for any other mistakes and correct.
Thanks all. This is basically what the city inspector said.