As soon as composition shingles were invented, I’m sure someone decided that it would be ok to lay new composition roofing over the old wood shake shingles, rather than start over and re-roof the right way.
The previous owners of our house were in that category…
I am holding my breath that my composition-over-wood roof will last until I am able
to have it re-roofed the right way.
I understand that a roof is the largest and most expensive replacement item in a home. It’s tough for many people to pay for it and consequently, they put it off. When a roofer suggests the much cheaper route of using the wood shingles as the “decking” (for those who aren’t familiar, plywood decking is what is under all composition roofing) it sounds like an acceptable option.
It is not...and I think any reputable roofer would agree.
Here’s why….when you lay composition shingles over wood shingles, you are doing that because
the wood needs to be replaced in the first place…. it is old, brittle and cracked and probably leaking.
When composition is put over it, water will invariably find a way through. Believe me, I’ve got proof above my head every day.
However, a correctly installed composition roof has the layer of plywood decking, THEN a layer of roofing felt. Both of these layers are crucial as a water barrier. Check out the photo above…the “slats” are the only thing under a wood shingle roof.
Relying on the old wood shingles as your water barrier is risky business...if you’re lucky, you
will escape trouble for while…but not nearly as long as the life of a properly installed comp roof.
AND…here’s where I come in….when water comes through the comp, it goes right through the wood shake into your attic. Many leaks you may never see. A skylight is a means to notice a leak…
not because the skylight caused it, but because that is how the water found it’s way to show up on your ceiling.
I see this problem so often that my first advice for anyone needing new roof is
DO NOT EVEN CONSIDER THE OPTION OF COMPOSITION OVER WOOD…
and be leary of the roofer who offers it up.
It may seem like the least expensive route now, but in the long run, it’s a sure road to headaches and extra expenses that we all like to avoid!
photo of composition on top of shake roof from arwoodroof.com