# Flashing A Wall Sill Plate On Slab On Grade



## philcav7 (Jan 15, 2009)

Repeat client of mine wants to use his existing attached 2 car garage as a home office. The garage doors will be walled off with a swing door placed in one. I can only assume there is no moisture barrier below the slab, but he claims no moisture issue during his tenure in the home. 

The slab extends slightly past the existing exterior wall and meets a blacktop driveway with minimal grade. The wall area is covered by a 24" soffit. 

Any best practices or precautions to take when setting the sill and framing the wall? It seems quite vulnerable to moisture. Would sill seal be adequate isolation from the slab?


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## Tinstaafl (Jan 6, 2008)

philcav7 said:


> Would sill seal be adequate isolation from the slab?


Well, I've seen it hold water _inside_ an unroofed structure for at least 24 hours. Probably not best practice, but for a retrofit home office that's likely to be repurposed in a decade or two (being optimistic), it ought to serve the purpose.


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## philcav7 (Jan 15, 2009)

The garage has been in place 30+ years. There was a ton of clutter/storage in there, but I didn't see any indications of moisture damage. However, it still worries me in the back of my mind. 

I saw the damaging effects of capillary action rotting out the studs on another building before. The PT sill was sitting on a slab with no seal, it sucked water up like a sponge into the spf studs. The bottom 12" or so of the studs were rotten and covered with mold. That image has stuck in my mind. 

I've been trying to think of an improved method of creating a water resistant barrier between the wood framing and concrete. SOG isn't that common around here, I'm used to framing from a damproofed foundation wall with drain tile at the footer. Would a layer of vycor added in with the sill seal be adequate?

I've attached a sketch of what I'm dealing with.


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## Tinstaafl (Jan 6, 2008)

I get it, and framing up a whole new wall I might go another route, like setting the plate on top of a row of CMUs. But no point in building your section of wall better than what's surrounding it (assuming the adjacent wall sections are sitting on the slab).


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## philcav7 (Jan 15, 2009)

Pretty much. I just don't want anything to bite me in the @ss 2 years down the road with a "there's mold on our drywall" phone call. Did I mention he's an attorney? 

Grade is higher on the back wall. The back wall is sitting on a cmu knee wall along with part of the side wall (it is stepped down to slab).

BTW, where in central PA are you?


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## JAH (Jul 27, 2014)

I've been in a comparable situation. A 3/8" - 1/2" deep cut was made in the slab, flush with the sheathing for the new wall. I put a piece of 2' coil stock in the cut line, tacked the top to the wall. Then sealed the coil stock in the cut line. 
It may not be the best solution but it has provided that piece of mind.


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## illbuildit.dd (Jan 7, 2015)

I did one a couple of months ago and poured a small stem wall to put the plate on. Sloped concrete up for a smooth walk into and out of the room and installed as slider. IMO it's a big mistake to put the wall directly to the slab in these situations.


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## Seven-Delta-FortyOne (Mar 5, 2011)

illbuildit.dd said:


> I did one a couple of months ago and poured a small stem wall to put the plate on. Sloped concrete up for a smooth walk into and out of the room and installed as slider. IMO it's a big mistake to put the wall directly to the slab in these situations.


That's what I was gonna say. That's how I've done it.



Delta


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## Rio (Oct 13, 2009)

Ditto on the curb...........


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## smpcarpentry (Aug 13, 2014)

Be sure that u ha have gutters on it to get all that water away


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## Tinstaafl (Jan 6, 2008)

philcav7 said:


> BTW, where in central PA are you?


Across the river from Harrisburg.


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## TimberlineMD (Jan 15, 2008)

Around here, those kinds of conversions require a 6" stem wall all the way around or bring the slab up 6" above grade.


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## Builders Inc. (Feb 24, 2015)

Same with the stem wall. You could need a footer too. The garage is sloped right? We gutted one. Dug a footer and poured several yards into the garage floor all the while adding a 6mm poly over the existing slopes slab before we poured new concrete and created a vapor barrier. Then the new wall was framed back up. Even though the garage door header was left in and it carried the weight of the roof we still added a footer for j bolts. 

But the reglet cut into the slab with coil stock would probably work. I'd still opt for the stem wall. It's only right like that plus you can do a poly vapor barrier that way. 


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## superseal (Feb 4, 2009)

I always cut them out and excavate down to frostline...pour a curb, slope your door threshold cutout and pan flash.


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## antonioooooooo (Mar 8, 2010)

Doesn't the footing usually run through anyways?


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## illbuildit.dd (Jan 7, 2015)

antonioooooooo said:


> Doesn't the footing usually run through anyways?


And it's already made to hold the weight of a vehicle.


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## Builders Inc. (Feb 24, 2015)

illbuildit.dd said:


> And it's already made to hold the weight of a vehicle.




Not the one we ripped out. It's a 50/50 chance I guess. 


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## hdavis (Feb 14, 2012)

If you want to see if there is a vapor barrier under the slab, put some plastic sheeting down over it and see what you get for condensation.

Sometimes the easiest thing to do is frame the whole thing with PT.


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## brhokel606 (Mar 7, 2014)

I am with hdavis, PT lumber for bottom plate, heck I would double it and use a foam sheet (the name escapes me right now) as a break between the concrete and PT lumber. The plastic down on the concrete for a day or 2 will really show a potential moisture problem wicking up through the slab.


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