Detail Needed: Load bearing knee wall for 1-1/2 story house
I would like to build a 1-1/2 story house with a 1-ft high knee wall that supports the rafters. I’m concerned with creating a hinge effect from the outward thrust of the rafters as the floor joists will be below the knee wall. (ie. sill plate, main fllor studs, top plates, joists, 1 ft knee wall, rafters. The roof pitch is 12/12 and a ridge beam will be used. However, the knee wall would still take 1/4 of the roof load. I’m hoping to stay away from collar ties to facilitate the installation of the roof vapor barrier I’m thinking of a balloon framed wall,with the appropriate firestop blocking. However, 1) letting in a 1X will not give the required bearing area for the joists. 2) letting in a 2X will excede the notch allowance for even a 2 X 6 stud 3) using bolts to attach a 2X ledger to 2 X 4 studs, as per Michael McKinley’s suggestion, appears to excede the knot hole allowance for studs (1-1/2″ hole per foot on wide face or equivalent [approx 5/8″ per foot on the edge 4) I could use a pair of 1/4″ carriage bolts and meet the knot hole allowance Any ideas on how to create a load bearing knee wall that is not supported atthe top by floor joists? Michael McKinley,Fine Homebuilding 125, pp. 114-119, suggests: “…bolt a 2×8 ledger to the studs to carry the ends of the second-floor joists. I used a pair of 3/8-in. carriage bolts, countersunk on the outside of the building, at every other stud. In between, the ledger is nailed. In addition, the 2×10 joists are fastened to the studs with 16d nails.”
Replies
Unless the building is a very long one, I doubt the "hinge effect" of a kneewall would be a problem.
The roof planes for a diaphragm due to the plywood on the roof. And that ties into the end wall, which also has the diaphragm of the sheathing on the end.
But - Opinions posted on here don't really mean a whole lot. You're gonna have to run this by your local code officials and get their approval.
If you have a ridge beam, seems like you have little horizontal thrust from the rafters. I thought you could notch a 2x into a 2x6 w/ no problems. I've seen it done for window headers w/ no raised eyebrows from the BI. Other options might be Simpson connectors to connect a face mounted rim joist to the wall framing. Certainly would be good energy wise. Another would be to skip the rim joist ... connect joists to the face of the studs and simply face nail. You'd need fire blocking, but you may need that anyway w/ balloon framing.
I'm not an expert, but those are my thoughts. The BO should give you a yay or nay on your suggestions, so maybe arm yourself w/ a bunch and 'have a chat'.
if your ridge is truly a load bearing ridge beam and your rafters are sized properly your knee wall is just a regular old wall
only shorter
and if you are convinced the wall is going to hinge you could balloon frame to rafter bearing and sister jack studs, all the way to bearing, under your floor joists-firestop at the sheathing butts (two birds one stone) and be done with it.
The engineers I've used
The engineers I've used always consider the rafters as a vertical load when a structural ridge beam is in place. It would be a good detail to run the sheating so that the horizontal joint is well below the floor line, you could even center a sheet at that point.
If it's still a concern, you could put interior kickers like someone said. What you gain in height, you loose in width though.
I don't know what your
I don't know what your 'half-story' habitable requirements are but we have a condition like this and the top plate is tied back in the (attic) area above (behind knee walls) with angled 2 x 4s ('kickers') from near the top plate to the ceiling joists (say 4' out). No problems resulting from this.
Our old place has 1x let-ins 3-4 feet below the top plate. It lasted 100 years before a renovator put kickers in. The studs are 3.5" deep, but we never have snow, so they haven't been stressed much.
I'm curious why the 1x is considered insufficient bearing width. Is is actually specified as insufficient somewhere, or is it just the lack of lookup tables for 1x that's the problem?
For example, our place is about 24' span with 2" FD joists, so the 50 psf load on each joist is 24*4/3*50 = 1600, or 800 lbs. on each end. Design value for Douglas Fir is 625 psi, so the 2 square inches should be able to hold up to 1350 lbs. Add a few hundred pounds for nails as well.
http://www.awc.org/technical/spantables/tutorial.htm