Roofing: 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!
Phone
708-478-1221
15255 S 94th Ave
15255 S 94th Ave
Orland Park,
Il
Stormaster
is a Division of Modern Kustoms
Contractor, Inc.
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