knee wall question

Started by steff503, December 05, 2017, 09:08:55 AM

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steff503

I am hoping to build a small 1 1/2 story house. I would not need any more than 24' x 24'. The thing that is bugging me is the upstairs.
If I have a 4' high knee wall with a 12/12 pitched roof, what am I looking at for usable interior space on that second floor?

azgreg



ChugiakTinkerer

Livable space is defined in the code as having a ceiling height of at least 7'.  To get that with a 4' knee wall you're looking at an upstairs livable space of 18' x 24'.  Useful space will be greater, and you could have furniture against the knee walls to maximize the free livable space.

Edit to add: Greg's picture above illustrates the fact that the walls have some thickness.  You're probably looking at about 17' x 23' of actual livable space.

Second edit: I think I'm interpreting knee wall differently than azgreg.  I'm thinking of a 1.5 story structure, where the knee wall is an extension of the first floor wall, such that the eaves are at about 12.5' height.  In that case you would have 17' livable width under a 12:12 roof.  For an attic living space as azgreg shows, the knee wall is already 4' inboard of the outer walls.  In that case your livable width upstairs is only 11' or so.
My cabin build thread: Alaskan remote 16x28 1.5 story

azgreg

Quote from: ChugiakTinkerer on December 05, 2017, 12:30:59 PM
Livable space is defined in the code as having a ceiling height of at least 7'.  To get that with a 4' knee wall you're looking at an upstairs livable space of 18' x 24'.  Useful space will be greater, and you could have furniture against the knee walls to maximize the free livable space.

Edit to add: Greg's picture above illustrates the fact that the walls have some thickness.  You're probably looking at about 17' x 23' of actual livable space.

Second edit: I think I'm interpreting knee wall differently than azgreg.  I'm thinking of a 1.5 story structure, where the knee wall is an extension of the first floor wall, such that the eaves are at about 12.5' height.  In that case you would have 17' livable width under a 12:12 roof.  For an attic living space as azgreg shows, the knee wall is already 4' inboard of the outer walls.  In that case your livable width upstairs is only 11' or so.

When I think of knee wall I think of the way I drew it like the old Cape Cod style. The other way would look something like this.


MountainDon

#4
The first image shown is the better method, because the rafter ties / ceiling joists, which are floor joists for the upper area, connect the tops of the lower wall framing. Having a 4 foot high upper story side wall can be done,  when the rafter ties are located in the lower third of the roof triangle. There will still be some sloped wall/ceiling. I will see if I can find and post an image later. 

Or build the roof with a ridge BEAM, not board. That will need supports for the beam though.
Just because something has been done and has not failed, doesn't mean it is good design.