Customs and practices vary from place to place. We don't have any building codes here. In all the time i've been doing this i can only remember one roof that had more than one layer of shingles.
Around heeyah, it's standard practice to completely strip the roof and re-nail the boarding. The main reason for doing this is because they didn't have galvanized nails back in the day. The old fashioned common bright nails have pretty much rusted out and there's not much holding the boarding to the roof.
You have to watch out for the older roofs that have more than one layer of shingles. They're like a time capsule that may contain toxic nasties. The reason for this is because up until the 1970's some manufacturers reinforced their asphalt shingles with asbestos fibers. There's really no way of knowing what you're dealing with unless you send a sample out to the lab first. It's the same with the tar paper underneath. It's not as common, but i have seen one place that had tar paper with asbestos added. When you do the tear off that stuff gets up in the air and you get covered in it. It's not a big huge hairy deal. It's just one of those job hazards that you need to be mindful of.
~ Times have changed. It's an entirely different ballgame now. ~
The big change happened when oil ( and scrap ) prices topped out awhile back. It was getting pretty bad for the shingle industry because oil prices were so volatile. They couldn't price a truckload of asphalt shingles delivered to the building supply yard until the day it was delivered. This was causing all kinds of problems and they were losing money big time.
The shingle manufacturers had been pushing for fiberglass based shingles since they were first introduced in the 1980's. Fiberglass based shingles earned a bad reputation when they first came out because they didn't hold up. The roofers were getting a lot of call backs so they didn't want to have anything to do with them.
The industry used the global oil shortage & volatile prices as an excuse to stop making
asphalt based shingles. Instead, they shifted over to
fiberglass based shingles and forced the change on the building industry.
As you would expect, it was a blast from the past. The fiberglass shingles started blowing off the roof and the bigger roofing guys in this area took a terrible financial beating on callbacks. When the roofers called on the manufacturer to honor it's warranty the roofers were left holding the bag. The manufacturer claimed that the shingles weren't installed properly.
As far as i know ..... The shingles available to work with these days are fiberglass based. They look about the same as the old asphalt based but there's a big difference in how you install them.
First: No more multiple layers of roofing. They need to be laying on a flat uniform surface so they can seal down properly.
Second: No more boarded roofs. Just plywood roofs. If you've got a boarded roof put a layer of plywood over it.
Third: Give some thought to your underlayment. The plastic tarp stuff is new to the market and hasn't proven itself yet. The #15 felt out of China is brittle & tears easily. Use a #30 felt.
Better yet ! Use Grace Ice & Water Shield. That way the roof won't leak when the fiberglass shingles blow off !
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