The roof on my Victorian house has never been fully stripped since the house was built in 1878. It still has wood shakes beneath two, and in places three, layers of asphalt shingles. We’re getting ready to have the house stripped and re-roofed and I’m guessing a fair number of sheathing boards will need to be replaced.
My roofer says if any more than a few boards need to come out it will be quicker and cheaper to lay down a new layer of plywood over everything rather than piece in new pine boards. Any thoughts on his approach?
Many an old roof is groaning under the accumulated debris of half-hearted re-roofing jobs.
It’s common (and acceptable) practice to add a second layer of asphalt or composition shingles over an old roof.
If you look closely, the finished job won’t be quite as neat as it would if the roof were stripped back to the sheathing. But it’s close - and leaving a layer of shingles on the roof cuts costs considerably. Labor costs are lower, and you’ll save the cost of a dumpster for the old shingles.
Yet many homeowners (and we presume builders) take the easy way out by extending this practice indefinitely -- adding a third and even a fourth layer of shingles. Eventually the roof sags under the weight. You’re certainly not alone in facing this problem.
So the first part of your roofer’s plan of attack is right, and long overdue. Getting everything off the roof will relieve a great deal of stress on those old rafters. It also will open up the structure for careful inspection and an opportunity to make repairs.
Replacing old sheathing piece by piece
Your roofer raises a legitimate and potentially complicated question. It’s often true when renovating houses that tearing out old work completely and starting fresh saves both time and money.
Trying to work around bits and pieces of original flooring, plaster, door casing (or in your case roof sheathing) appeals to the preservationist in most old-house owners. But it means that new material has to be worked in around the old and that the builder must mount a search for compatible materials.
In the case of roof sheathing, it may be difficult to find boards of the same thickness. If he patches with conventional milled lumber it will be 3/4 in. thick, but the original sheathing on your roof could easily be a non-standard size: 7/8 in. thick, 15/16 in. thick or some other oddity that made sense at the time.
Widths can vary, too. So every board that comes out might have to be matched with a custom milled mate. That takes time.
In short, it’s a pain in the neck.
Covering old sheathing has some advantages
Adding a layer of plywood over the entire surface without removing every last piece of old sheathing will save time (and therefore money).
There are two other advantages.
The first is that plywood will make the roof much stronger. Plywood resists racking and twisting much better than sheathing made from boards, so under the duress of very heavy weather the roof will be less likely to fail.
Second, the plywood provides a flat, sturdy nail base for the new shingles. Sheathing damaged by old leaks and just plain old age won’t hold nails as securely as new plywood. Plywood also will be a uniform foundation for the adhesive-backed membrane that’s now typically applied along eaves to prevent water leaks from ice dams.
Is there a down side? It would be a mistake to cover up sheathing that’s wet, rotten or infested by carpenter ants or termites. Old problems should never be buried. But assuming the roofer is diligent in pulling those boards out, and uses fasteners for the new plywood that go all the way into the roof framing, you’re better off with plywood.
The other potential for trouble is that the extra layer of sheathing could interfere with trim along the eaves and rakes. If that’s the case, drip edge should solve the problem.
Being true to tradition
Of course none of this makes any difference if you’re a true believer in “restoration†as opposed to “renovation.â€
For many people, there is deep joy in honoring an historic house by using authentic materials and replacing old stuff with identical new stuff.
That’s a wonderful thing and we’re lucky to have people among us who care that much. And you might be smart to check whether you live in an historic district that prohibits unfettered remodeling.
Newer materials (exterior grade plywood and Grace’s Ice and Water Shield come immediately to mind) are often an improvement over what earlier builders used. I’m one of those who believe that if the biscuit joiner had been invented a century earlier the good brothers and sisters at the Hancock Shaker Village in Massachusetts would have been first in line to buy them.
My vote: Cover up that old sheathing and move on.
