2×4 Cripple, Sill and Header in a 2×6 Double Stud Wall?
CZ6A, Ottawa ON
Putting the finishing touches on our construction details before Engineering takes place. The plan is a two story double stud wall, 2×6 wall load bearing, 2×4 wall non load bearing.
My question is, can I use narrower lumber for the non load-bearing components around the window boxes?
What I’m proposing is to use 2×4 cripples, sills and headers around the window boxes in the 2×6 wall. With a 12″ thick double stud wall using 2x6s and 2x4s, there’s a 3″ gap between the exterior and interior studs. Replacing the non load bearing 2×6 components around the window with 2×4 would locally increase that gap to 5″. The rest of the wall assembly would have a 3″ gap, this would just be around the windows.
I know this detail won’t amount to much from a whole-wall R value perspective, but if the only purpose of those wall members is to support the window and sheething I don’t see why this couldn’t be done? Heck, why not go to 2×3 on both inside and outside walls and increase the gap to 7″?
Has anyone seen this done before? Is there verbiage in the Code that prohibits this?
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Replies
Lance,
Wouldn't it be simpler to just use two 2"x4" walls?
Malcolm, if I remember correctly the Ontario Building Code requires 12" centers for a two story 2x4 wall bearing the load of a floor plus a roof (two story). I think 16" or 24" centers requires 2x6 walls for two story builds. I'm building on 24" centers.
Lance, I don't know about the Canadian code, but structurally (and in the IRC codes) there is no reason to make headers the full depth of the wall. 2x3s don't have much structural integrity and don't stay very straight, in my experience, so they aren't really appropriate for wall framing.
Michael: Finger jointed 2x3s can be had in various structural grades, some suitable for vertical oriented applications (but not headers), others completely interchangable with milled 2x3s in all applications. Finger jointed lumber (including 2x3s) are generally straighter and more stable than milled 2x4s. Using less expensive lesser structural grades of finger jointed 2x3s for the non-structural wall can result in a flatter wall than standard milled 2x4 walls, and less thermal bridging.
Finger jointed lumber meeting Canadian & US standards are available from many sources.
http://cwc.ca/wood-products/lumber/finger-joined/
http://www.lamcoewp.com/en/products/finger-jointed-studs-sps-3/
https://miskaswoodproducts.com/manufacturing/
Dana, good to know. My answer was qualified. I have used other Lamco products and found them to be high quality.
Lance,
Like all code advice given over the internet, it's worth confirming yourself, but table 9.23.10.1 allows 2"x4" at 16" for exterior walls picking up a roof and a second storey. If you are trying to get a larger gap between the two walls, I wonder whether the move from 24" to a 16" spacing, if it allows you to eliminate the 2"x6" wall, might be worth considering?
As to using 2"x4" infill near windows. it looks like a bit of a grey area. the code mandates all studs be the "required width of the stud wall". If you are building a wall you think requires 2"x6" framing, it appears all the studs have to be this width. You are really only talking about the area under the window aren't you? The area on both sides and above is part of the load-bearing structure.
I'd stay away from any 2"x3" framing. Both because it deflects so easily and because you can't get the required setbacks from the stud face to any penetrations for electrical and plumbing.
Good luck with your build. It's a climate where energy efficient construction really pays a dividend in terms of payback and comfort. When we lived there we alternately froze and baked.
Lance,
Since you mentioned "before Engineering takes place", I assume this means you are having an engineer sign off on your plans. If so...
A double-stud wall has double the stud lumber of the walls cited in the structural code. IF both 2x4 stud walls structurally support the joists and the rafters or trusses, and these load-bearing members are aligned vertically, then an engineer could sign off on a 2x4@24"o.c. double stud wall. Essentially you have a 2x7.5"@24" wall (with all the assumptions met), which is more stud lumber than a typical 2x6 wall.
They might require more bracing than drywall to beef up racking resistance on the interior wall.
For fire code, you are required to provide 3/4" ply or 2x every 10' minimum, across the stud walls, to prevent flame spread through the wall interior, as well as a common top plate spanning both walls. That helps structurally too. Window bucks plywood boxes for window and door openings tie together the two walls as well.
Robert,
You make a good point about the possibility of having both walls load-bearing, although it does mean some redundancy of headers and posts, means the possibility of keeping the floor structure in from the exterior wall disappears., and can involve a much thicker foundation.
Worth remembering that Canadian building codes have different fire-blocking requirements than the ones you cite.
Malcolm,
Yes I'm guilty of being a typical American who thinks Canada is just like the US! Sorry! (Although that's probably Canadian...)
If its easy to summarize other fire-blocking or structural suggestions, I'd be interested. Even if not required, anything that improves the structure is worth considering. I do like double stud wall designs.
Robert,
The odds are generally in your favour, i just remembered the OP was in Ottawa.
Perhaps surprisingly, our code doesn't have fire-blocking requirements for exterior walls as long as they are filled with insulation, and the horizontal limits are a generous 20M for those that don't.
Whichever wall you choose as load-bearing for double-walls brings with it complications. In a two storey house it's difficult to line up your vertical loads, as typically the floors will be 16" or 19" oc and the roof structure 24".
Choosing the interior wall means the exterior one is likely to be cantilevered over a typical width foundation. it also makes supporting decks and porch roofs harder, and out here in seismic-land, you end up with the sheathing necessary for shear on the non-loadbearing one.
Choosing the exterior one means bringing the floor structure all the way to the outside, negating the advantage of a thermal break if it could be kept in line with the interior wall.
I think your suggestion to build both as potentially load-bearinging walls makes a lot of sense. It negates all the problems I've listed, and makes the structure both more understandable and adaptable for future occupants doing work. There is a redundancy of posts and lintels, but none bridge the gap between the two walls. I think the energy penalty might well be worth the benefits.
Again, this is all speculation. I haven't built that way to try it out.
A huge thanks to everyone for chiming in!
I asked a city official about double load bearing 2x4 walls and he scared me off the idea simply because it wasn't part of the code. I hadn't considered getting an Engineer's approval for it. He did make what I thought was a good point in that the inner wall would likely end up carrying most of the load since any deflection in the truss would result in droop that would load the inner more than the outer.
Since then I've settled on a 2x6 load bearing interior wall with a second floor supported by ledgers to eliminate the thermal bridge having all that lumber inside the insulated wall cavity. This still needs Engineering approval, but that's the direction I'm headed.
I think most got what I was trying to say. I attached a picture that uses the naming I was referring to for the components. My approach would be a 24" OC double stud creation that mimics the advanced framing approach with only King Studs on either side of the window box (3/4" plywood) with cripple, sill and header. The traditional load bearing header would be replaced by the ledger boards supporting the floors and trusses, once again to keep the lumber inside of the insulated cavity.
Since the cripple, sill and header are only really supporting the sheething, drywall and window boxes, I thought replacing the 2x6s with 2x4s would be acceptable. Maybe I'll toss this at my Engineer and see what he thinks.
The FJ 2x3s sound interesting. I'll have to look into who carries those as I'm planning to use 2x3s for framing my 2.5" deep service cavities.
I'm planning to post up my wall details for review here, I might get to that tonight. It will be interesting to hear your thoughts.
Lance,
Once you are into Advanced Framing, or some variant, you are definitely looking at getting the walls engineered, as the OBC requires jack studs (sometimes two each side). and specifies the size of headers.
I don't want to belabour the issue, but if what you are proposing is outside the code, I'm not sure how much sense it makes to get scared off some assemblies for not conforming to the code when you are then substituting other non-code ones.
What you are proposing sounds a lot like Swedish Framing. GBA has done a couple of articles on it you might find useful.
Malcom, I definitely did end up wasting considerable time bouncing back and forth between several designs, and a large portion of this would have been avoided with less confusing advice from local code officials. If the first guy I spoke with had told me I could do whatever I wanted as long as an Engineer approved the design I would have settled long ago! I must add to this statement that the advice I've gotten directly from people and articles on this site has been extremely helpful!
The design I have come up with is a cross between a fairly traditional dual stud construction and a Swedish framing approach. It was here that I was directed to (or stumbled across, not sure) the idea of Swedish framing.
Lance,
As long as you end up with an assembly you are comfortable with and works, then all is well.
Check with the APA on the single top plates. https://www.apawood.org/ They promote advanced framing but when asked for shearwall test data on single top plate walls they had none to reference. The concept seems good yet all the shearwall info in the code is supported by full scale testing.
Malcolm,
Absolutely, and thanks for your thoughts.
Tim,
Good info, thanks. My design uses double top plates in my exterior shear wall, necessary to minimize wood waste using standard lumber lengths. So for me it shouldn't be an issue.