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Posted by Wayne Whitney on July 11, 2007, 3:12 pm
> I would recommend that you use manufactured roof trusses in lieu of
> conventional framing.
A reasonable suggestion, but my addition is quite small, only 50 ft^2
of (projected) roof area and a maximum span of 4'. So trusses seem
like overkill.
> You need to have a detail so that the sheathing can transfer the
> lateral wind/eq force to the sheathing on the walls ( shear walls ).
I see you suggest later on to do this with blocking connected to both
the sheathing and the wall top plate. Would a twist strap face nailed
to both the rafter and the top plate do this, assuming a tight nailing
pattern of the roof sheathing to the rafters? I'm referring to the
new construction, not the existing construction.
> Hopefully the inspector might not recognize a lack of connection of
> the existing roof diaphragm to the exterior walls. But he/she
> might, then, you will have to fix all edges of your roof.
I'm not touching the existing roof and exterior wall top plates, and
my inspector hasn't raised the issue.
> How is the connection between the roof sheathing to the exterior
> walls in the existing construction?
The existing roof seems pretty hopeless so I've not tried to upgrade
it. It consists of (full dimension) 2x4s 32" o.c. spanning 12.5'
plate to ridge. The original skip sheathing has been topped with a
layer of 1/2" plywood sheathing and composite shingles. There' no
blocking at the wall/roof intersection, and as I mentioned there's the
funny detail of the rafters bearing on a flat 1x4 on top of the 2x6
ceiling joists.
> Do you have wall sheathing on the existing walls? Where are you
> located?
I'm in Berkeley, CA, and I've updated the house's lateral force
resisting system up to the exterior wall top plate. There's a new
engineered foundation as of 2004, and I've rebuilt each wall segment
without any openings as a shear wall: hold downs at each end, sheathed
with 1/2" Struct 1 plywood with a 4" 8d perimeter nailing pattern,
blocking between the floor joists, and tension straps connecting
stacked shear wall segments.
> If you are going to need foundations for your project I guarantee
> you that the permit authority will require a structural engineer to
> design foundation and do the seismic and wind load design.
Actually, for my permits for my small addition, the building
department accepted my plans without any engineering. I used the
foundation detail from my engineered 2004 foundation replacement.
Thanks for the detailed response.
Yours, Wayne
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