As roofing contractors in Seattle we are often asked, why do we need plywood over our old solid sheathing?
The keyword here is “solid.” It used to be standard to use shiplap, 1″ boards, tongue & groove, or other boards for the roof deck. These 8 to 12-inch wide boards were nailed together tight.
When the roof was first nailed on, of course all the nails were in solid wood. But the lumber was not usually kiln dried; as wood dries it shrinks. As shrinkage is from 5 to 10 percent, gaps of ½-inch or more are common.
Which homes have board sheathing? Exterior grade plywood was not available until the 1940’s. As the boards to do the roof deck were manufactured here in the Northwest it was still more economical than plywood through the ‘60s. If your home was built prior to 1970 there is a good chance it will have board sheathing.
Recent changes. When the roofing material wore out, new composition shingles would be applied over the original roofing system. As more layers of roofing were added there would be little chance that water would work its way through to the roof deck.
It is common to find older homes with 3 or 4 layers of roofing over the original roof. The Building Code changed in July 2004 and does not allow over-roofing when there are two or more layers of existing roofing. This law applies to all areas and cities of Washington State.
If the original roof deck is boards; when the old roofs are torn off there are now open spaces between the boards. Heavier underlayment can be put down first but over time it will sag into the spaces (as shown by the black lines in the drawing). When a nail ends up in the “void” it will usually leak within 5 years.
What if some of the new synthetic underlayments are used instead of the normal “organic” underlayments? Some may insist the polypropylene used in most of these will seal the nails. These sheets may delay failure, but fail they will.
Some contractors may insist the manufacturer will warranty the product even if laid on “solid” board sheathing. But the manufacturers’ warranties will not cover leaks; all they warrant is manufacturing defects.
The cure. If you want new asphalt shingles installed then solid sheathing must be applied first. This can be exterior grade plywood or oriented strand board (OSB). Be sure that the roofer uses rated sheathing; most is rated by the APA – The Engineered Wood Association base right here in the Puget Sound Region. To prevent buckling; be sure the applicator uses adequate plywood clips.
Exceptions. Some roofing materials can be installed on boards. It is acceptable to apply wood shingles or shakes as well as some standing seam metal roofing systems.
So, anytime your board-decked roof is torn off and you want to install composition shingles be sure to lay down solid sheathing first!