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Building Homes or Extensions/Exterior construction for 2nd floor addition

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Question
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?

thanks,
Kevin

Answer
Kevin,

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.    

Steve Major

Building Homes or Extensions

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Stephen Major (Owner--Major Design Group)

Expertise

I can answer any questions regarding the design and construction of homes and additions. This includes trade-specific questions (how-to) in all major building trades: framing, foundations, site prep, electrical, HVAC, plumbing, water treatment, interior finishing, trim & cabinetwork, exterior finishing, roofing, siding. PLEASE indicate your state or region, so I can provide the best possible answer. PLEASE provide photos whenever possible.

Experience

30 years experience in building design and construction, all hands-on, including the construction of dozens of single-family homes and hundreds of remodeling projects in the northeastern US.

Publications
Author: "Architectural Woodwork - Details for Construction" published by Van Nostrand Reinhold (now Wiley).

Education/Credentials
BS Cornell University.

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