To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Reroofing
From: Harold Sprague <harold.sprague(--nospam--at)neenan.com>
Date: Wed, 14 Apr 1999 14:16:09 -0600
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.
The Neenan Company
From: Tom.Hunt(--nospam--at)fluordaniel.com [mailto:Tom.Hunt(--nospam--at)fluordaniel.com]
Sent: Wednesday, April 14, 1999 9:35 AM
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
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.