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Old 09-27-2011, 11:48 PM   #1
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Exterior wall sill plate not sitting on slab


We have a slab house, that at one point was added onto by pouring an additional slab.. The addition portion of the house was always colder than the original, and we attributed it to longer duct runs, cheap insulation, etc. We recently found ourselves gutting the carpeting in the addition due to a pipe rupture, and decided to go ahead and fix the insulation issues once and for all..

So down came the drywall, and to my shock and horror I look at the sill plate of the exterior wall hovering about a 2x4 height above the concrete slab.. At first I thought the slab had sunk, but I looked closer and I see the bottom 2x4 is actually sitting on shims! a 2x4 in the corner, some plywood in the middle, etc etc.. The air flow under that board is clearly the cause of the cold air, as just standing in front of it you feel the consistent airflow..

So my question is.. Who would do this and why?? And any suggestions on how to fix it? My current plan is to remove the existing sill plate, shorting the studs, and putting in a double sill.. (I need to go double since the sill is so warped in order to get it true, I'll need to shorten the studs that much)

Does this make sense? Any suggestions on how to go about doing this without dropping the whole wall?? I've attached a picture of the bottom of the wall for reference.

Thanks!
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Old 09-28-2011, 12:07 AM   #2
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Exterior wall sill plate not sitting on slab


kind of hard to say with that close up pic as to why they would have done it, my guess would be that the slab was out so they shimmed the wall level, and to line up with the other walls.

I think your idea is a good one. There are a few ways to shore it depending on what is above. One that i've seen done and done myself quite a bit is to get a long 2x6, and screw across the studs horizontally, making sure you use plenty of screws. You can cut some vertical pieces to fit between this piece and the concrete.

now you can remove your plates (do one section completely at a time, install new before taking out the next section) making sure the 2x6 is well secured to studs that still have plate under them as well, or have been shored well by the vertical pieces.)mark and cut the studs with a sawzall, bang the new plates under, and secure. PT or with sill gasket.

if the existing sill is warped, take it right out, and replace with new strait pieces. Chalk a line for reference if needed.

plan on some drywall touch ups doing this stuff.


Last edited by chrisBC; 09-28-2011 at 12:40 AM.
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Old 09-28-2011, 05:47 AM   #3
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Exterior wall sill plate not sitting on slab


If the addition has been there for years and no cracking of drywall, I'd leave it as is.
How will you attach the exterior sheathing to the new sill plate without removing siding. The wall is a system and as such needs the sheathing to be attached to the sill for structural reinforcement.
What does it look like from outside? Is this gap visible? I wonder why pressure treated lumber wasn't used since it is in contact with the concrete?
I'd perhaps try to shim better and then use expanding foam sealant to seal the gap.
Maybe this isn't a good idea; someone will say so if it isn't I'm sure.
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Old 09-28-2011, 02:48 PM   #4
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Exterior wall sill plate not sitting on slab


@chrisBC: Really like the 2x6 suggestion.. I think I have some long 2x10's in the garage I can use in place of the 2x6 going across.. The wall is about 12 feet long, I think I can do it in 2 6' sections, with the 2x10 running the full length of the wall.. The studs look in good shape, but there is a big double window right in the middle, of course.. (essentially 2 double-hung windows) My biggest concern is cracking them..

I would love to not fix it and just spray foam the heck out of it, but it's my bedroom, and every time I got a chill I would assume the foam wasn't working, heh.. plus I'd feel bad when it comes time to sell it..

Thanks for the advice!
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Old 09-28-2011, 03:20 PM   #5
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Exterior wall sill plate not sitting on slab


Does the concrete extend out under the sill?
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Old 09-28-2011, 04:15 PM   #6
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Exterior wall sill plate not sitting on slab


Don't know how far you've gotten on this, but I would do it differently. Get a bag of pre-mix mortar, mix it up and push it in between the floor and the plate. The gap looks to be about 1/2 an inch, so it wouldn't take much mortar and you could use a thin piece of wood or something to push with. Also mortar made to be used in thin applications and it doesn't contract when it sets up. Good luck.
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Old 09-29-2011, 12:01 AM   #7
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Exterior wall sill plate not sitting on slab


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Originally Posted by johnnyconcrete View Post
Don't know how far you've gotten on this, but I would do it differently. Get a bag of pre-mix mortar, mix it up and push it in between the floor and the plate. The gap looks to be about 1/2 an inch, so it wouldn't take much mortar and you could use a thin piece of wood or something to push with. Also mortar made to be used in thin applications and it doesn't contract when it sets up. Good luck.
Im with Johnny on this one. But I'd sure non-shrinkable grout. That is what they use out here in 4 story multi family buildings when the concrete is out by a country mile and shims are used.

Why doesn't anyone string line the wall and cut each stud to order anymore? Or better yet, DO DECENT CONCRETE!!
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Old 09-29-2011, 06:21 AM   #8
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Exterior wall sill plate not sitting on slab


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Im with Johnny on this one. But I'd sure non-shrinkable grout. That is what they use out here in 4 story multi family buildings when the concrete is out by a country mile and shims are used.

Why doesn't anyone string line the wall and cut each stud to order anymore? Or better yet, DO DECENT CONCRETE!!
I second that non shrinkable grout the way to go.
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Old 09-29-2011, 04:51 PM   #9
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Exterior wall sill plate not sitting on slab


I'm looking at about an 1 1/2" of gap at many points on the wall, and I'm looking at doing this for about 50 linear feet of wall, but I like the sounds of the grout.. It may help with the height of the slab in relation to ground level, which is only about 3 inches, not nearly enough.. Really the goals are to stop the airflow, keep the critters out, and shore up any structural issues.. We do have some bad drywall in that part of the house, I've been blaming poor workmanship by the drywallers but now I'm not so sure..

Thanks for everyone's input, a couple great ideas here
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Old 09-29-2011, 08:35 PM   #10
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Exterior wall sill plate not sitting on slab


Although NS grout is an outstanding product, I wouldn't necessarily use it inthis case. You don't need a 10000+ psi filler in such a lightweight construction application, it's not cheap & you'll need a few bags likely, and the worse is that's i's self leveling & you'll spend alot of time getting a form tight to the concrete floor so it doesn't ooze out everywhere. Not to mention, it could endlessly run outside of the house through any hole. At 1.5", just get some 5000 psi quickrete, mix it fairly stiff, and push it in tight the entire length, cut it off lfush once itsets a little bit.
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Old 09-30-2011, 06:24 AM   #11
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Exterior wall sill plate not sitting on slab


Concrete shrinks, gaps, non support.
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Old 09-30-2011, 06:40 AM   #12
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Exterior wall sill plate not sitting on slab


The 1.5" concrete wouldn't even shrink a 1/64" if he mixed it fairly tight, not to mention how much the wood wall shrinks through the course of a year............
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Old 09-30-2011, 06:42 AM   #13
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Exterior wall sill plate not sitting on slab


Shim under the studs, foam the gaps and be done with it.
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Old 09-30-2011, 06:48 AM   #14
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The 1.5" concrete wouldn't even shrink a 1/64" if he mixed it fairly tight, not to mention how much the wood wall shrinks through the course of a year............
Wood already shrunk because it's been up for years , key words in your statement shrink a 1/64" , shrink means shrink.
Don't know how tight mix will be, don't assume installer will mix tight even if he does still 1/64"???
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Old 09-30-2011, 06:49 AM   #15
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Exterior wall sill plate not sitting on slab


Do they sell pressure treated shims???

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