Expert: Richard Taylor, AIA - 1/24/2011

Question
Richard,

I am remodeling an existing house with standard 8 foot walls.  I want to add some addition space and make the entire house 9 or 10 walls.  How do I frame on top of existing walls?  Do I build knee walls on top of the existing walls?

Answer
William - thanks for your question!

The simple answer is no, you can't just build knee walls on top.

Here's why.  Although the added wall height might support the vertical loads, they won't support the horizontal loads.  When the wind pushes on the wall, the connection between the upper and lower walls acts like a joint - and the wall will buckle.

The studs in the wall have to resist the horizontal load by flexing, and that (usually) takes a continuous member from plate to plate to accomplish.

Typically, when we raise wall heights we add new studs alongside the existing studs up to the new height.  Often, these taller studs also have to be wider to resist the horizontal loads; so a 2 x 4 wall sometimes has to become a 2 x 6 wall.

There are other ways to do this, for example you can do the knee wall option IF you brace the joint properly.  That usually means metal strapping and/or bracing on one or both sides of the wall, or other methods to resist the moment forces at the joint.

You also might build new full-height walls inside the existing - which would give you additional insulation space.

The only way to be sure of the appropriate solution is to consult with a structural engineer.  There's likely one in your area who can help you with this, and help you determine the most cost-effective solution.

Hope this helps,

Richard Taylor, AIA
www.rtastudio.blogspot.com
www.rtastudio.com