Do you know how many layers of roofing there are on your house? It may sound like a random question, but in most states there are rules, regulations and such about how many layers of roofing can be on a home. Not to mention common sense. Can you guess how much roofing shingles weigh? If you guessed about two pounds per square foot you would be about spot on. So, if a small house, say one with a one thousand square foot roof, has three layers of roofing that would weigh in at about three tons! The math for that is 3 layers of shingles x 2 pounds per square foot each layer = 6 pounds per square foot x 1,000 square feet = 6,000 pounds, that’s 3 tons! That is about what four inches of fresh snow would weigh or even an elephant!
We recently encountered a house with six layers of roofing on it! That is like having a couple of elephants or close to a foot of snow on the structure at all times. What would happen if it snowed an additional foot? Just how much weight will a structure bare? You don’t want a circus on your home. Most experts agree that three layers of roofing is the safe and sane maximum. Adding layers of roofing on top of existing shingles is called an “overlay”. But in reality, a “tear off “ keeping your roof to one layer of shingles is best. The reason for this is that a good roofer will want to strip the roof down to the wood decking to check for rotten, warped or otherwise damaged roof boards or sheathing and even note any obvious issues with the trusses before he lays a new roof down on the properly repaired flat surface. Think about it. If you add an additional layer of shingles onto a damaged roof, the damage is still there. As with everything in life, there are exceptions. Like, say a tree fell and damaged a little bit of your house while your roof was still in excellent shape, this would definitely be a time to consider an overlay. But don’t choose an overlay out of cheapness or laziness. Oh, and just to be clear here, we are strictly talking about layering shingles on top of shingles. Metal roofing is an entirely different animal and we will talk about that another time.
For now, lets stick to the subject of overlays. Obviously, it is easier and cheaper to just do an overlay, just putting more shingles on top of the existing shingles. This is because there is no labor involved in the removal of the shingles. There is no cost for the hauling off or disposal of the removed shingles. But in the long run, it will cost more. This is because of inflation. When it becomes necessary to remove the shingles, hopefully before there are six layers, time has passed and the cost of hauling and disposing will be higher, plus, now there are more layers to remove and they are going to be in very poor condition causing extra labor costs. There are also life expectancy drawbacks to layering roofing shingle. A new layer of shingles placed on an uneven surface is not going to experience peak performance. The shingles may curl or cup, a prime cause of the shortened lifespan. Of course we already covered the weight issue! That extra layer may be all it takes to cause the roof to collapse! And if you were considering just putting more shingles on because of a leak, well, guess what? The source of the water damage is still there! So, nope, not a great idea after all.