Hello:
If I have a double wall construction, and both walls are directly supported by the foundation, can both walls be considered 'bearing walls'?

if a floor or roof bears on that wall, than yes. Both walls are potential bearing walls.

Thanks Bob. :)

Posted By robert.thompson on 16 Feb 2013 05:45 PM
Hello:
If I have a double wall construction, and both walls are directly supported by the foundation, can both walls be considered 'bearing walls'?

How do you envision this being erected?  Have you thought this through?  What gets installed first?  The flooring system or the outer 2x4 wall?  If the flooring system will be put in first, how do you expect the carpenters to drop that outer wall down to the concrete foundation?  If the outer wall gets built first, what are the carpenters going to stand on while the wall is erected?

Rob,
There are several reasons to designate only one of the walls as load bearing, here are a few: 1. Headers are only required in the bearing wall eliminating significant lumber in the other. 2. The load will NEVER be divided equally so if one is not designated as bearing wall both will need to be able to carry the full load whereas if one is designated as bearing the other can be constructed much lighter. 3. Shrinkage! Wood shrinks more 'cross grain' so the wall with the most 'cross grain' wood will shrink more. By designating the wall with more cross grained wood ( usually the inner) as the bearing wall and adding a "crush zone" to the other wall, the effects of shrinkage over time can be effectively eliminated.

Hi arkie6:
Thank you for your input. :)
The 'plan', at this point, is:
1) Bolt both sill plates to foundation, over gaskets.
2) Frame & apply sheathing to floor.
3) Attach temporary 2x blocking to outside edge of outer sill plate to prevent outer wall from slipping off the sill plate when erecting.
4) Frame outer walls on floor & erect.
I imagine that the whole process will be a real pain in the butt - I will be doing it with a couple of friends.
The good news is that the top of the foundation is only about 2 feet above grade for 75% of its circumference.

On our house, the crew did the outer walls first. I wasn't right there with the camera to see just how they constrained the bottom of the outer wall as they tilted it up from the floor deck onto the the outer sill a foot lower. I'm imagining that you would push the wall frame out over the edge of the platform by 16-17", with metal straps under the bottom plate of the outer wall and nailed to the platform, so that the wall frame would hinge on the edge of the platform until the bottom plate hit the outer sill, then would just tilt up on the outer sill in the normal way. Then the straps would be cut at the bottom plate and the bulk of it removed from the platform. To attach 2x temporary blocking outside of the outer sill to keep the wall from sliding off to the ground, I think you'd have to use a lot of blocking, attached with lag screws to be strong enough. To me, metal strapping seems simpler. I've asked the builder if he remembers how they did it.

BTW, our concrete contractor did a superb job of smoothing and leveling the top of the concrete wall. Still, I had the framers use a double layer of that foam sill sealer. There are other products with more thickness than that thin stuff usually used, specifically for better air sealing at the sill. Also, I should point out that the concrete foundation was, for structural reasons, 10" thick. This fit well with the double wall design, as it let me have the two framed walls sitting on it, with the outer wall (2x6) and sill overhanging the concrete to capture the top of the exterior 2" foam layer. If you do an 8" concrete wall, then you have to consider bearing loads, particularly if your outer frame is 2x4 and it overhangs exterior insulation. I used 2x6, as a good part of the outer wall has a deck structure attached to it. I can provide sketches of those areas if you wish. I'm wondering if, to do the whole thing again, I ought to have just had the outer walls flush with the outside of the concrete and just put all of the insulation on the inside. As it is, I have a second 2" layer inside, covered with framing on the flat and sheetrock.

Hi Dick:
Regarding your drawings, I'd love to see whatever you wish to share as I think that they are great! I'd be particularly interested in your drawings that show the 'deck to wall' detail as I have yet to deal with that on my house. If you have drawings on the eave details, that would be helpful too.
To get the width for my double-wall, at the foundation, I plan to just 'reverse install' the top most course of ICF's which include a re-bar reinforced brick-ledge.
Thanks for the 'metal strapping' tip - it sounds good to me and a lot safer than what I had planned.
Rob.

OK, here are a few as .pdf files; I guess the resolution is better. In the case of deck attachment, I didn't want to use a ledger, as getting that right in terms of leak proofing it for the long haul. With various hardware bolted through it or nailed into it, many ledgers fail over time. Then replacing it is major surgery on the house. I went with Maine Deck Brackets, which are like sections of aluminum I-beam. They hold the ledger out 4" from the shell of the house. Flashing the base plate of the bracket, where it bolts through the rim, is easy to do so as to be leakproof. Also, if the deck ledger has to be replaced or repaired, it's just unbolted from the outer flange. Structurally, whether a ledger is bolted or lagged directly to a "rim" board or brackets are used, things are different in the case of a double wall where floor joists don't extend to the outer wall. In my case, I needed an outer  2x board, fastened with structural screws into the tops of the lower wall studs on my downhill side and into the bottoms of the outer wall studs on the uphill side where there is no lower level framed wall. To keep the face of the board flush with the studs, it is let into the studs. Now, if you do this with a 2x4 stud, you'd got just 2" of stick left, which isn't an awful lot of meat there. That's one reason why my outer wall is 2x6. It could be done differently, with 2x4 studs and the board not let into the studs, and with siding adjusted and the board flashed accordingly.

Thanks Dick, great drawings!

I might have to reconsider my choice of 2x4's for outer wall.

Rob.

I should have added that you could always make the deck free-standing, and not attached to the house at all. Bracing against sway is a little more complicated, but it does eliminate flashing issues at the house wall, and you could keep the 2x4s for outer wall. The inner wall could be used for load bearing, so that the outer wall holds up only itself and the windows. I think I checked the bearing loads for 2x4 walls 16"OC, with only 1.5" directly bearing on the sill and foundation and found it to be ok for up to two stories, but I wouldn't bet on that meeting code or the BI's approval. I'm not sure I would have been comfortable with that either. Another thought on the deck support I have, if your concrete foundation wall comes sufficiently close to deck framing level, is to bolt brackets for support beams to the concrete and mount the deck framing on those. I did that once for a deck I built on another house I had, and it worked well. A steel bracket bolted to a concrete wall is an awful thermal bridge, though, and would be another reason to put all the foundation wall insulation on the inside.

Yes