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RE: Reroofing

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Are they proposing to place the plywood on top of the existing sheathing, or
are they removing the spaced 1x (1x is generally used for wood roofs)
sheathing and placing the plywood on the rafters?

Architectural Graphics Standards lists the weight of wood shingle roofing at
2-3 psf.  I generally assume 5 psf which would account for the heavier shake
roofs.  Sheathing is 3 psf per inch of thickness.  So a wood roof plus
shingles is in the range of 5 to 8 psf.

Your new system weighs 5.8 psf plus about 2 psf for plywood for a total of
about 8 psf.  Even if the weights are off or they put the plywood over the
existing decking, the increased stiffness will engage more shear walls and
more load paths.

I would tend to agree with the City on this one.

Harold Sprague
The Neenan Company

-----Original Message-----
From: Tom.Hunt(--nospam--at) [mailto:Tom.Hunt(--nospam--at)]
Sent: Wednesday, April 14, 1999 9:35 AM
To: SEAINT(--nospam--at)
Subject: Reroofing

     Our homeowners association is planning on reroofing our entire complex 
     of 165 buildings (4 to 5 units per building).  They have narrowed the 
     options down to 5 roof coverings all of which are class A concrete or 
     clay tile.  The most likely choice will be "ClayMax" by US Tile on 
     plywood sheeting.  The existing 20+ year old roofs are cedar shingles 
     on what looks like 1/2 inch by 6 inch planks spaced about 12 inches 
     o/c (you can see 4 to 6 inch of the black paper between the planks).
     The new roof system is obviously heavier than the existing system and 
     therefore will generate more seismic load to the shear walls.  I was 
     shocked to learn that the city of Irvine has a "city policy" that if 
     the clay tiles (EXCLLUDING the addition of plywood) are less than 7.5 
     psf then NO structural calculations are required.  The ClayMax tiles 
     weight 5.8 psf.
     The city official I talked to said that since the plywood would add 
     "extra seismic rigidity" they discount the added weight of the 
     plywood.  I politely explained that with the 3 to 4 psf added to the 
     roofs from the new tiles and plywood the shear walls will be taking 
     more seismic load regardless of the roof rigidity.  His comment was 
     that since the buildings are 20+ years old they would never "calc out" 
     with the added weight and newer codes.  Duh!!
     For those with conventional construction and reroofing experience (I 
     am a concrete/steel guy) is it normal not to require structural 
     calculations when reroofing with a heavier system.  Without structural 
     calculations I would think as a minimum this could void our earthquake 
     Every time we have a good shaker I get cracks around the door openings 
     and it seems this will only make it worse.  Any other comments on 
     reroofing concerns/problems??  
     I should also note that we are located on a slight hill and each unit 
     is half slab on grade and half elevated.  The elevated half is about 6 
     feet above grade.
     Thomas Hunt