First Floor Framing Detail
We’re building a single family residential home. My structural engineer designed our stem wall and first floor as in the attached Stem Wall Detail. I am concerned bc the way designed, the top of the subfloor (3/4″ advantech) will be flush with the top of the stem wall. We’re planning on a Hardwood finish floor, so is there a way to properly install this directly over the stem wall without the risk of moisture wicking up to the flooring/framing? Is there some kind of barrier i can put in bw the stem wall and floor?
It would make more sense (in my humble opinion) to raise the hanging floor joists so their tops are flush with the top of the mudsill and have the subfloor extend over the mudsill as in the 2nd attachment. Appreciate any thoughts/suggestions. Thanks.
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Replies
Ya, not good. why not put the joists on top of the foundation? Solves all your issues. Concrete wicks cold and damp, so that'll be coming directly into your floor.
oren_h,
Yes, you can use an impermeable membrane to separate the wood framing and sub-floor from the concrete. This should extend from the backside of the structural ledger up and under the sill-plate.
There are a number of variations on what you are considering. The one you posted works well. If you go with that detail see if you can convince the engineer to use 2"x4" sill-plates. That allows more room for insulation between the concrete and sub-floor.
Another would be to reduce the width at the top of the stem-walls creating a ledge to hold the floor framing. The advantage of that is being able to run some rigid insulation behind the ledger.
I like to use a ledge detail like the one you're showing, Malcolm. Sometimes I add a pressure treated 2x4 plate on top of the ledge, even if it overhangs a bit to the interior.
Michael,
I've never done it, and in my rough sketch showed some anchorage of the rim-joist back to the concrete. Is that even necessary, especially if you include a sill-plate on the ledge as you suggest?
Malcolm, that's a good question for an engineer. An anchor bolt down through a narrow ledge probably wouldn't provide enough concrete cover. I have used Simpson Titen HD concrete screw anchors horizontally as you're showing.
I like Michel's suggestion of a sill plate. Most codes allow the foundation to be reduced to 3.5" to form this ledge. With 8" foundation, that gives you 4.5" of room for you floor joists. Typically floor joist need 1.5" of bearing plus 1.5" of rim joist, this would give you room for 1.5" of rigid. It also means no joist hangers so it is a pretty quick install.
This setup removes the cold spot in the base of the wall which typical would be a comfort issue and mold magnet. It is also not much more work so well worth the extra effort over the original plans.
Akos,
Since it appears to be some sort of unwritten law that each GBA discussion of a detail contain at least three variations (or thirty if the topic is double-stud walls)... Another option would be to set the floor joists on the interior framed walls of the crawlspace or basement. These would need to be detailed to bear on the footings.
On slab-on-grade houses I use a similar detail as described by Malcolm and Mik, except that I need the space for insulation and a floor ledge. Curious why you'd use this in a home with a wood floor? Is it over a crawl? If it';s a cellar, it'll take away from your headroom. Sorry not to add another entire method!
Yes, I have used this detail over crawlspaces, but also over full foundation walls when the goals included deep floor joists/trusses and keeping the first floor as close to grade as possible. It does require foundation walls taller than the typical 8' nominal (which are really more like 92-94"). Builders in high-end areas usually have 9' or 10' forms that make it relatively painless. Where builders only have 8' forms, it's extra work to pour the walls taller than normal.
Thank you all for the comments, suggestions, and discussion. I'll probably just go with the setup as i uploaded, since in my mild southern california climate zone the small insulation penalty is not too significant.
Malcolm, out of curiosity, can you recommend which impermeable membranes can be used to separate the stem wall from the subfloor if one did decide to go that route? And if those exist do any builders simply use those instead of typical sill seal to obviate the need for PT mudsills?
oren_h,
I use whatever peel & stick membrane my lumberyard stocks. I don't see much difference between them. Currently it is: https://www.resisto.ca/en/product/air-and-vapor-barrier-membrane-redzone-pro-1973/
Yes you can use them instead of sill-sealer, but I don't know enough about your codes to know if that eliminates the requirement for the mud-sills to be pt. It does here.
One other small point you might want to ask your engineer: Whenever I have hung a ledger it has just been 2"x lumber. Any reason for the 4"x10"?
The fastening isn't shown, but perhaps he wanted enough depth for 3" x 0.148" nails that are sometimes used? Just a guess.
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To meet the specified loads listed by Simpson you must use the nails they require. Simpson requires the 3" nails for these hangers. The specifications specifically require the 3" nails for the diagonal nails stating that the 1 1/2" nails can't be used. At one time Simpson derated the load by around 70% if the short nails were used except where specifically specified.
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eust2023,
You are quite right - I don't know what I was thinking. I've deleted my posts.