re-roofing underlay?

Old 12-12-99, 03:52 AM
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I am about to install a fibergals shingle roof to replace my decaying cedar shake roof in a very wet area of Vancouver, BC (Canada). The roof consists of shiplap with a standard strapping over top to which the shakes are nailed.
To replace with fiberglass shingles, can I apply the shingles directly to the strapping (with a felt underlay) or is a 3/8 plywood base application over the strapping necessary, as many roofers have suggested to me?
Old 12-14-99, 12:40 AM
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I take it that the "strapping is nailed over the existing
sheathing of the roof. If that is the case, remove the
"strapping" and begin with the original shiplap sheathing.
Nail the 3/8 plywood over the ship lap because the shiplap
sheathing may have spaces where the wood has shrunk over
the years and come apart a little. It is also necessary
because I have seen shiplap material get bows in it over

Now after you have applied the 3/8 plywood (and be sure
that the plywood you are putting on is made with exterior
glue.) be sure to put ice and water protection material on
the lower portion of the roof and in the valleys. This is
a material that is sticky on one side and glues itself down
on the wood. You should have at least enough to cover the
roof area that is two feet "inside" the inside wall of the
house. For example, if you have three (3) foot eaves, you
should have a minimum of five (5) foot of ice and water
protection applied to the roof. Another thing to note is
that the ice and water protection does not stick when the
wood to which it is being applied is cold. You are in
British Columbia and I know that your winters are not as
harsh as Central Canada but if it is below 45 degrees (F)
you may have problems making it stick.

Another thing to remember is that the fiberglass shingles
generally have a good "gluing" strip under the shingles
but that does not adhere until the weather is warmer. So
the bottom line is if your roof is not leaking, you may
want to wait until warmer weather befoe you do the work.

Also, before you start the work, contact your local
building inspector to be sure your roof rafters or trusses
are designed to carry the extra weight of the plywood and
you may also need a building permit to do the work.

Clifford A. Olson, Home Inspector
AC Home Inspector - *Free* ezine Subject=DIY
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