Any comments on item 2 below in regards to OSB vs Plywood for
reroofing applications in Southern California. I was told that OSB
has good strength properties but may be hard to nail and may be
susceptible to moisture.
Dennis, what are you using on your reroofing project??
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Subject: Reroofing - UPDATE
Author: Tom.Hunt(--nospam--at)fluordaniel.com at fdinet
Date: 4/19/99 1:32 PM
First of all, thanks for all the wonderful replies.
The following are some responses to previous e-mail, some of which were
sent privately (all names have been respectively removed).
1. The Turtle Rock area of Irvine has been designated a "special fire
hazard zone", therefore the City requires a class A roof. Are there
any light weight "class A" roofing materials not made of clay or
2. Our Homeowners Association has hired a roofing consultant
(Associated Roof Management) which is probably a good idea on large
projects since as a minimum they provide continuous inspection during
construction. I talked to the consultant and he seems very
knowledgeable and tries to get a consensus or compromise when needed.
He is also recommending real plywood instead of OSB!! Any comments on
using OSB for roofs in Southern California??
3. The plan is to remove all the existing wood shakes, paper and felt
BUT leave the existing 1x6 slats at 12 inch o/c that are on top of the
2x8 roof joists at 24 inch o/c. Nearly all roofs are sloped 4:12.
4. New plywood will be laid on top of the 1x6 slats then new
multilayered felt, then the roofing tiles. Nailing of the plywood
will obviously (IF done right) be through the plywood, through the
slats, and then the required penetration into the 2x8 joists. I
suspect these will need to be 2 inch minimum length nails.
5. The roofing consultant is recommending that a structural engineer
review the final roof details for "general" comments. He explained
that this is not detailed calculations but a review for obvious mayor
6. Several commented that wind will govern anyway which I agree is
probably the case transversely since four condos end to end makes a
very rectangular building. However dollars to donuts seismic governs
longitudinally. Also, all the good continuous shear walls appear only
to be in the transverse direction due to sound separation between the
units. Longitudinally I doubt there are few if any substantial shear
walls since this is the side with all the plate glass windows, sliding
doors, and garage doors. Over the last 15 years I have punched holes
in every interior wall and I have not yet hit any plywood and the
exterior walls are only stucco, chicken wire and paper, marginally
nailed to the 16 inch o/c studs.
7. I had a comment from a (non-Irvine) building official that if the
cities required calculations on every reroofing project then there
would be a lot more leaking roofs. Personally I tend to agree that
for a boxy looking single family home with slab on grade construction,
detailed calculations might prevent some from replacing leaking roofs
and may not be justified. However I think where you are talking about
165 buildings of over 10,000 sq. ft. each, all two story and split
level, built on a hill with over half the structure elevated 6 feet
above grade then I think doing detailed calculations would be
desirable by the city. Actually out of our 165 buildings there are
only 4 unique types and structurally speaking there are only two
types. If the calculations cost say $5000 (pure guess) then that is
less then $8.00 per family.
8. If I am reading UBC Appendix Chapter 15, REROOFING, correctly you
can reroof Asphalt roofs twice and ALL other roof types only once.
9. The 1997 UBC section 1517 - TILE has very strong language for
using tile material over existing roof system. There is similar but
not as strong language in the 1994 UBC (Irvine will adopt the 1997 UBC
July 1, 1999).
SECTION 1517 - TILE
Tile may be applied to roof with a slop of 4 units vertical in 12
units horizontal or greater over existing roof coverings in accordance
with Table A-15-A. Such installations shall be substantiated by a
report prepared by an engineer or architect licensed by the state to
practice as such, indicating that the existing or modified framing
system is adequate to support the additional tile roof covering.
10. Since the cost is minimal on a per family basis, I am going to
recommend that as a Homeowner's Association we have as part of our
roofing contract detailed structural calculations including seismic
load analysis performed on two building types.
Again, thanks for all your thoughtful suggestions,