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OSB vs Plywood

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     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??
     
     Thomas Hunt

______________________________ Forward Header __________________________________
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 
     concrete??
     
     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 
     problems.
     
     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,
     
     Thomas Hunt