Staggered wall studs on bottom plate
PAUL KUENN
| Posted in Building Code Questions on
Can’t seem to find anything in the codes for amount of stud sitting on top of frost wall above the bottom plate. I’d like to use a 2×12″ with 2x4s sitting on top of an 8″ wide frost wall. 1.5″ would be above cement on one side while 2.5″ would be above on the opposite side. Any recommendations? Thanks!
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Replies
Paul,
I can't find the relevant section in the IRC (it may be there I'm just not that familiar with it).
This excerpt may offer some general guidance. It is from the Wisconsin building code.
(b)Extension beyond the bearing surface. All of the following requirements apply to a sawn-lumber sill plate with uniform loading that is partially extended beyond the load-bearing surface of a foundation wall in order to put the exterior surface of an upper-lying wall flush with or beyond the exterior surface of insulation which is placed on the outside of the foundation wall:
1. The center of any anchor bolt shall be set back from the side edge of the sill plate by a distance of at least 4 times the diameter of the bolt.
2. The thickness of the concrete or mortar cover around any anchor bolt shall comply with ACI 318 Section 7.7.
Note: Under ACI 318 Section 7.7, the minimum cover for a 5/8-inch-diameter or smaller bolt is 1 1/2 inches.
3. Where a stud wall bears directly on a double bottom plate, the plate may not extend more than 1 1/2 inches beyond the load-bearing surface of the foundation wall.
4. Where a stud wall bears directly on a single bottom plate, the plate may not extend more than 1 inch beyond the load-bearing surface of the foundation wall.
I have to ask why you're trying to do this.
Presumably you're building a 12" wall because you want to have lots of insulation. The insulation value of concrete is so low that you really have to treat it as a conductor of heat rather than an insulator. As such, unless you can insulate it on all six sides, you want to treat it as being outside the building envelope and insulate it on the inside. With your proposed construction I don't see how you insulate the concrete properly -- and having only part of the wall insulated largely nullifies having the rest of the wall super-insulated.
What I would propose instead is putting a 2x8 on top of the concrete wall, flush with the exterior, and framing 2x4's flush on the exterior. On the interior, lay a 2x4 on the floor at the base of the concrete wall and go up with 2x4 studs. That will give you a wall that is 11-1/2" thick, slightly thicker than a 2x12. You'll be able to put 3-1/2" of insulation over the concrete. And you'll save a little money.
Items #3 and #4 in Malcolm's post are probably most relevant here, since they deal with how much that bottom plate can overhang the foundation wall. If you drop down from a 2x12 to a 2x10, you're 1.25" wider than that 8" foundation wall, so you can place it so that no more than 1" is overhanging on either side and be good to go -- even with a single bottom plate. You loose 2" of wall thickness this way, but it's easier to build. I wouldn't want to do a double bottom plate, which would stll not quite work (you'd be 1/4" over on one side) even with the 1-1/2" overhang allowance.
If you arrange a 2x10 with 1" overhanging on the exterior side, you could use 1" rigid foam over the concrete to bring that out flush with the exterior edge of the sill plate. Play around with placement until things work out for whatever your finished wall assembly will be, then build it like that.
Bill
The slab is yet to be poured but we had 3" EPS on both sides and thought I could cover it up and be flush with the above wall. Will probably just cut it down 4.5" and take the slab up to the interior wall. The rest of the house has larson truss at 12" with 6" of EPS exterior of frost wall, but this is an addition. Thanks for all the notes.