Expert: Richard Taylor, AIA - 10/25/2007

Question
Hey Mr. Taylor,

My wife and I are considering designing a passive solar ranch with a completely finished walkout basement.  To maximize upper level floor space, I would like to extend the south side an additional 3-5 feet using a cantilevered floor the entire length of the home.  This would serve a two fold purpose by extending all upper level south facing rooms while at the same time protecting the lower level south facing rooms from the summer sun.  Having said all that, is it feasible to build the upper level rooms beyond the traditional 16 foot joist length since less than one third of the 16 feet width (16' + 3-5") would be exceeded, i.e. 16' plus an additional 3-5' cantilever?  Thanks, S. Spencer

Answer
Shawn - thanks for your question!

Typically, the "parent span" should be at least 3 times the length of the cantilever, as a general rule of thumb.

So you're probably OK, although you should have the engineering checked out by a pro - each case has it's own particular situations to consider.

How are you calculating the depth of the overhang?  To maximize the effect, you need to account for the changing sun angles throughout the year, and the size and position of the windows below.

The wrong overhang won't have the effect you want.  There are many books showing how to calculate the proper height and depth, but I recommend Ed Mazria's classic "The Solar Energy Book".

Done properly, solar shading can make a big difference in your heating and cooling costs!

Hope this helps!

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