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RE: sill plate

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For a sill plate in the desert I wouldn't worry too much about dry rot. You can ask Joe Venuti in Palm Springs, CA for confirmation of that idea. If the building is 20+ years old I would look for signs of moisture intrusion. Also check the height of the sill above exterior grade and drainage away from the building. If there is landscaping or sprinklers look for signs that water is hitting the building. The wood siding will not weather well either if it's getting wet. Look for any indications that water might be a problem around the base of the walls and correct it.
For the metal flashing between the T1-11 siding and the sill plate, it sounds like the flashing sits flat. That should not inhibit the nail embedment or the nail slip in sill or plywood. Maybe others here have test data or experience with this. I've not heard of any shear wall tests regarding the flashing.
Are you adding nails for the shear wall loads? Are the sills split at this time? If not, I would leave well enough alone. If you're adding nails then the added blocking is a good idea. But that will require nailing the blocking down to the existing sill plate. Predrill the holes to avoid splitting, or predrill and use screws.

From: Pinyon Engineering [mailto:Pinyonengineering(--nospam--at)]
Sent: Monday, July 14, 2008 10:41 AM
To: seaint(--nospam--at)
Subject: sill plate

I am working on a wood framed fire station built in the mid 1980's in a dry desert climate in California (used to be seismic zone 4)- they poured a 1.5" curb for the walls to get them above the concrete floor.  they have a z type sheet metal flashing that covers the exterior slab insulation and is flush to the sill plate with the T1-11 plywood siding nailed thru the flashing to the sill plate. 
Problem # 1 - the sill plate is not redwood or pressure treated - should we replace it or can I have them inject asphalt under the plate to limit moisture from the concrete. or apply some other wonder chemical to prevent rot. the concern here would be dry rotting of the sill (the typical failure mode here)
Problem # 2 - is the sheet metal ok between the sill plate and the plywood T1-11 shear panel.  the shear to the wall in the addition is 493 plf - I prefer 3x6 sill plate for the 10d nailing  I could add blocking on top of the sill for the nailing
I will be adding holddowns and all new epoxy sill plate anchor bolts as part of the project
any ideas are welcomed
Tim Rudolph
Pinyon Engineering
Bishop CA