When faced with roof replacement its natural for many homeowners to look for ways to save money.
While there are some legitimate ways to cut costs that will not jeopardize the integrity of your roof, others ways do more harm than good in the long run.
One such dangerous way to save money is to install a new roof over the old layer of shingles, instead of tearing it off. Another, is to forgo repairing old plywood/sheathing before installing a new roof.
Many roofers desperately looking to get your business by luring you in with “big savings” may suggest to go for one or both of these options.
I will describe a recent job that will clearly illustrate why its NEVER a good idea to try to save money in this way. Instead: 1. Tear off the Old Shingles 2. Repair/replace plywood
Last winter we worked on replacing an old roof that sustained massive Ice Dams damages, resulting in a destroyed kitchen, hardwood flooring and walls.
Before any work began, the homeowner wanted to leave old shingles in place to cut down on job costs.
Although I explained to the client the negative consequences of making this decision, the possibility of saving $2000+ on tear-off and plywood costs was too attractive.
So, the homeowner continued to insist on putting a new roof over the existing 1 layer of shingles.
However, while doing our initial inspection on that roof, we found that removing the old shingles and fixing the roof substrate was not only the right thing to do, but also necessary. Here is why:
Outdated Construction Standards
The house was built in the 1960’s and back then the building code required a minimum of 3/8″ plywood – something no good contractor would think of doing today. Now, any new roof (in Massachusetts) must have a minimum of 7/16″ sheathing with H clips.
However, it is best to use 5/8″ CDX plywood or OSB for overall roof integrity. Additionally, the majority of roofing manufacturers require the same substrate specs for warranty purposes.
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As we stepped on the roof, I felt that I was going to break through it with my feet, if I stepped between rafters.
That was an immediate call to change the job specs – we simply could not install a new roof onto the plywood that was in place. After changing the terms with he homeowner, we began removing the old shingles.
What we found below was scary to the say the least!
Lazy / irresponsible Builder / Roofer
After we opened the roof up, we found 3/8″ thick plywood that was old, dilapidated, broken in many places and had several spots of water damage.
Additionally, the rafter spacing was not a standard 16″ on center and not even the odd 24″ OC – rafters were not evenly spaced at all. Some had 17″ of spacing – some were 19″, 20″, 21″ etc. Whoever built that house, really did not care much for the quality, or the additional work required in laying roof substrate / sheathing.
What If a New Roof was Installed Over this Old Shingles / Plywood?
Basically there are a lot of what ifs, so lets look at different scenarios:
1) What nail length a roofer would use?
Most contractors use 1.25″ long roofing nails on new construction projects. This meets the code, but isn’t good enough in my book. A 1.5″ or 1.75″ long nails hold much better.
But if a roofer uses 1.25 or 1.5 inch long nails to install a second layer, its “Houston we have a problem”. Assume that each layer is almost 1/2″ thick, so 2 layers = 1″ give or take. 1.5″ nails will barely penetrate the plywood. 1.25″ won’t penetrate plywood at all!
At the same time, all roofing systems require at least 0.5″ penetration, meaning the nail must protrude at least half an inch past plywood. So basically such a roof would not be nailed properly.
2) Dry / rotted plywood does not hold nails as well as new plywood
As was the case on this roof, dry rotted 3/8″ plywood would not hold the nails even if they were long enough – you could pull nails out by hand – that is how weak the plywood was.
3) Will the roofer install underlayment?
On most “over the top” jobs (second layer installs) roofers do not use felt or any other underlayment.
While it makes some sense, if you are already doing a second layer, you should put down a synthetic breathable underlayment, for leak protection, and to not trap any attic moisture.
4) Added weight:
Asphalt shingles are HEAVY. On average 2.5 lbs / sq. ft. 2 layers equal 5 lbs / sq. ft. By comparison, clay tile and slate are about 8 lbs / sq. ft. But those heavy roofs come with increased snow load framing already built into the roof structure.
MANY asphalt roofs have 2×6 framing, or 2×8 spaced 24″ on center. With an average roof being about 1600 sq. ft., this framing is not designed to hold 8000 lbs of roofing shingles and a snow load on top (this winter we had 97″ of snow, of which 70″ fell within 30 days). Do the math.
5) Increased thermal mass:
Asphalt shingles are the least energy efficient residential roofing system in US / Canada.
Not only they fail to reflect solar heat, they also absorb it and store it (thermal mass). That is why the top floor of nearly every home in US is always so hot in the summer, even long after the sun goes down. Now, two layers of shingles is double the thermal mass – try to cool that with an AC.
6) Most important – decreased longevity and NO warranty:
To get the somewhat useless warranty on your new shingles, you need it installed by the spec, which includes adequate (not damaged, proper thickness) substrate – plywood or boards. Moisture protection barrier (felt / underlayment), ice and water barrier where required by building code and/or shingles installation specs, proper ventilation, and some other nuances.
Installing a new roof as a 2nd layer will void that warranty. Also, because the shingles will not be properly ventilated (even with adequate attic ventilation) they will dry out much faster, and will last maybe 1/2 of their intended lifespan.
All of these what ifs VERY often result in “new” roofs lasting less than 3 years.
So there you go – trying to save $1200-2000 on a new roof by not removing old shingles and not repairing / replacing rotted plywood, will usually cost $4000-6000 more just in few short years. And this does not include any interior damages.
Think about it before you spend your money.
Oh and also – if the roofer offers or insists on doing the second layer – my advice to you – kick them out immediately.
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