Many people often wonder what the point is of installing plywood over solid wood sheathing. They think what’s the point of it? Isn’t it a waste of time, energy and money? However, that isn’t the case. Remember, the keyword is solid. At one point in time, it was standard to use shiplap, tongue and groove, one-inch boards or other boards for your roofing deck. These wider boards were all nailed tightly together.
Whenever your roof was first installed, the nails were put into the solid wood. Lumber wasn’t kiln dried, so it would shrink upon drying. With shrinkage of 5-10 percent, gaps of another a half-inch begin appearing all over your roof.
How do you know if your home has board sheathing? Well, here is one way to figure it out. Exterior plywood wasn’t actually available until the 1940s. With the boards to do the decking being manufactured in the Northwest, it was a lot more economical to use these boards than it was plywood, at least through the 1960s anyway. If your home was built before 1970, there is an excellent chance that your roof is going to have board sheathing in place.
Once this roofing material started to wear out, composition shingles began being applied over the original sheathing. As more and more layers of the roofing kept being added, there was a minimal chance that any water was going to be able to work its way through your roofing deck.
Actually, it isn’t all that uncommon to find an older home that has three to four layers of roofing placed over the main roof. In July of 2004, the building codes were changed in that they don’t allow you to have over-roofing when you already have a couple of roofing layers installed on your roof. Make sure to check the laws for your specific city and state to see whether this applies to you or not.
If your original roofing is boards, you will end up with wide open spaces between your boards whenever the old roofing is torn off. You can have a heavier underlayment put down, but it is going to begin sagging in all of those spaces over time. Whenever a nail ends up being put in that voided spot, it is going to leak within a few years.
To help prevent your roof from leaking and causing more problems than what it is worth, you need a trained roofer who can come out and examine your roof to see what you are dealing with and how it can be fixed. Don’t waste time and money trying to do it yourself when there is someone who can give you an informed opinion about how to approach it.