Building Homes or Extensions/Exterior construction for 2nd floor addition
Hello Mr. Major,
I own a ranch home in Massachusetts and we are planning to build a second floor addition over the existing foot print of the home creating a two level colonial home.
My question is with the exterior walls of the home and the sheathing. My home was built in 1958 using boards (planks) for the exterior walls (which I'm told was the norm for the time period). An engineer has recommended replacing these boards with plywood on the corners of the home to prevent "lift".
Is this standard practice? Or something that is common? Having to replace exterior walls with plywood is adding to my total cost (demolition, replacing siding, extra construction, etc.) and will most likely put me over budget.
Is replacing exterior walls with plywood a necessity here?
If the 1950s sheathing boards are installed diagonally, then they should be OK as is. Diagonal boards are super-effective at resisting the tendency of rectangular walls to "rack" or fail due to lack of adequate bracing or triangulation. Plywood sheathing acts like diagonal sheathing, in fact it's even better in some regards.
If your original walls are sheathed with horizontal boards, then you could check for the presence of "let-in" corner bracing, which is a diagonal brace, often a 1x4, cut into the studs, usually at the corners. It may be on the exterior or interior face of the studs. If you have this, you may be OK, but remember that you are setting a very heavy additional floor (and contents and people)on these walls, so you need to have an engineer verify the structural adequacy of any such existing bracing.
See this image: http://www.sweethaven02.com/BldgConst/Bldg02/fig0836.jpg
I'm not sure what you engineer is referring to as "lift". Can you elaborate?
An alternative to tearing off siding is to install some plywood on the inside face of select portions of exterior walls. You would still have to mess around with new drywall, etc., but it could end up being cheaper.