# To overlap sheathing or not to overlap sheathing on a slab foundation?



## djenks (Oct 23, 2016)

I read some old threads on here about it and the pros and cons weren't weighed very well or certain things weren't covered.

Not all slabs come out perfectly square and adjustments have to be made to keep the walls square. So what is the best practice when squaring things up? Especially on a slab foundation because you only have one bottom plate most of the time so you don't really want to frame your walls, drill your holes, raise the wall, and then find out things aren't square. Would the best thing to do be laying the base plates for each wall around the perimeter and squaring them up, marking your holes, and then assembles the walls and get the holes drilled?

And which method has more pros than cons? Overlapping the sheathing or allowing it to be flush with the wall but hung a little bit above the slab so it doesn't make direct contact?

If you overlap the sheathing and let it hang past the foundation how far would you want it to hang past? Would you use flashing under the sheathing to protect that exposed wood at the bottom?

If you set the sil plate back a half inch to allow for the sheathing wouldn't that mean less exposed wood? I understand it makes it possible to allow water to wick between the sheet and slab, but isn't that what sheathing and siding are for?

To me the overlapping makes more sense because it adds that extra layer of cover and then gives you more to attach siding to. I'm just not sure how you would protect the underside.

There was a thread on this forum where a user said to have the sil plate hang over the slab 1/4 inch and then run the sheathing past the sill. But that makes literally no sense to me. I mean if you need to overhang the sil to keep things square and to dimension that's one thing. But why in the world would you ever hand the sheathing past the sil if it is past the slab? It would just be too much exposed wood and the upside down U would be hard to flash or whatever you chose to do.

Basically, I am asking, what are the actual pros and cons to flush and overlapping sheathing, and how do you protect the underside of the sheathing when it overlaps? I'm assuming some kind of flashing with a drip edge of sorts. And wouldn't you want the sheathing and siding at least 6 inches above grade?

Thanks.


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## joecaption (Nov 30, 2011)

If done right the slab should be at least 6" above finished grade to protect the sheathing from splash back.
A slab on grade is 100% sure way to have future issues.
I always overhang the sheathing at least 1/2" to help keep out wind, water and prevent wicking.
Add sill seal under the bottom plate.
Far better to have the plate overhang the slab a small amount then have the slab stick out past it causing the sheathing bow out and not make 100% contact with the bottom plate.


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## djenks (Oct 23, 2016)

joecaption said:


> If done right the slab should be at least 6" above finished grade to protect the sheathing from splash back.
> A slab on grade is 100% sure way to have future issues.
> I always overhang the sheathing at least 1/2" to help keep out wind, water and prevent wicking.
> Add sill seal under the bottom plate.
> Far better to have the plate overhang the slab a small amount then have the slab stick out past it causing the sheathing bow out and not make 100% contact with the bottom plate.


A slab on grade is that likely to have issues? I mean what can you do to minimize issues when having a slab?

You overlap the sheathing, but do you put flashing anywhere? Won't there be a small gap behind the sheathing and the concrete? So what do you do to protect that?

If you overhang sheathing a half inch on a 6 inch above grade slab then the sheathing and siding will be 5 1/2 inches from the ground. Is that okay?

I don't understand your last sentence. Did you mean "overhand the slab a small amount and then..." or "overhand the slab a small amount rather than"?


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## djenks (Oct 23, 2016)

joecaption said:


> causing the sheathing bow out and not make 100% contact with the bottom plate.


I'm re-reading what you wrote and I don't understand how the sheathing would be more prone to bow whether it is above the concrete or hanging over it? Either way the sheathing isn't touching the concrete, but rather hanging. So how is it more prone to bow inset? And as far as covering the bottom plate entirely and wicking water in - Wouldn't a z trip solve both of those issues when having the sheathing flush? And then with the siding on top of that and the plastic wouldn't that prevent a lot of issues?

If you overlap it the bottom edge of the sheathing will be exposed to the elements and water splashing up onto the edge. Even if it is treated, it's 5 1/2 inches off the ground at the very least (assuming you did a slab with 6 inches above finished grade). So what do you do to protect the sheathing when it is overlapped if it is such a concern to keep that underside protected?


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## 123pugsy (Oct 6, 2012)

I never tried to protect the edge of the sheathing.
Keep it 6" off the ground and it will be fine.
It only gets wet from splash back.

The 1/2" gap is no concern. Little bugs need a home too.
My garage also has a 3/4" air gap between the sheathing and the wood siding. Big bugs have a home too.


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## djenks (Oct 23, 2016)

123pugsy said:


> I never tried to protect the edge of the sheathing.
> Keep it 6" off the ground and it will be fine.
> It only gets wet from splash back.
> 
> ...


So then your plate is over your foundation by 1/4?


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## 123pugsy (Oct 6, 2012)

djenks said:


> So then your plate is over your foundation by 1/4?


Up to 1-1/2 at one corner to square up the walls and at least 1/4", maybe more everywhere else.


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## djenks (Oct 23, 2016)

123pugsy said:


> Up to 1-1/2 at one corner to square up the walls and at least 1/4", maybe more everywhere else.


So if you're doing this is there even a point to overhand the sheathing? Why not have sheathing flush with sil and then a drip edge piece hang down?


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## 123pugsy (Oct 6, 2012)

djenks said:


> So if you're doing this is there even a point to overhand the sheathing? Why not have sheathing flush with sil and then a drip edge piece hang down?


You can do anything you like.
If you're trying to keep the slab lower and the sheathing higher, your idea is valid.

A "Z" channel nailed to the studs and then the sheathing over that will keep water and wind out.


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## Scottomm (9 mo ago)

123pugsy said:


> You can do anything you like.
> If you're trying to keep the slab lower and the sheathing higher, your idea is valid.
> 
> A "Z" channel nailed to the studs and then the sheathing over that will keep water and wind out.


there seems to be an issue with the finished wall extending past the face of the concrete in a urban wilderness fire zone because wind blown embers collecting along the building can ignite the building wall. For that reason I want the exterior face of the sheathing to be flush with concrete face below. Doing so will allow the cement plaster “stucco” to be the fire break.


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## Nealtw (Jun 22, 2017)

Scottomm said:


> there seems to be an issue with the finished wall extending past the face of the concrete in a urban wilderness fire zone because wind blown embers collecting along the building can ignite the building wall. For that reason I want the exterior face of the sheathing to be flush with concrete face below. Doing so will allow the cement plaster “stucco” to be the fire break.


This is a very old thread but you do know that stucco is not sealed tight at the bottom. There will be a drain plane between the stucco and the wall.


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