Monoslab Garage Build, Should Wall Sheathing Overhang Concrete at all?

charmin35 · Mar 21, 2019

Should wall sheathing extend slightly past the sill plate<->concrete joint? Or not at all? Whether extending 1" over the concrete or not, should I use a air/water barrier tape made for that kind of interface? Or paint on a gooey sealant? Or just a bead of exterior caulk of some kind?

By using OSB for wall sheathing under vinyl siding, I am under the impression the bottom edge would be vulnerable to moisture if not treated with some kind of moisture barrier. Concrete will be 4-6" above grade. I plan to put a perimeter of gravel or stone landscaping around the slab as well.

Background:
I'm building a 16x24 detached garage on a monolithic slab, no stem wall, just framing right on perimeter of the flat & level slab (with anchor bolts). My treated sill plate will be on top of the foam seal to prevent capillary movement of water, and I plan to at least seal both the outside and inside of that sill-concrete joint with either a bead of an appropriate sealant on the exterior and interior, or a type of tape made for that purpose (I think you are supposed prime both surfaces if doing tape on the exterior).

I'm going with vinyl siding, and I'm planning on using 7/16" OSB with radiant barrier facing inside to give a little bit of heat reduction during the summer, since the garage will not be insulated. I used this same sheathing for a smaller tool shed I built last summer.

Thanks for any advice, I'm a long time lurker. I've got a ton more questions to come for this build :beer:

Edit:
Started garage build thread: https://www.garagejournal.com/forum/showthread.php?t=417954

Stuart in MN · Mar 21, 2019

My garage is on a slab, but it also has one row of concrete block around the perimeter so the wood is held above grade level. Unless your grade around the garage is sloped well away from the structure I'd recommend doing the same thing.

finn · Mar 21, 2019

All of my garages that have a poured rat wall or a couple of courses of block have always been dry.

Both the garage and my shop with no block or poured rat wall have water intrusion under the sill, even with good grading. Snow melt is the biggest issue.

I would never build new without a lip.

spudley · Mar 21, 2019

I'd go with a lip or a course of block. On my build, I ran the sealed factory edged OSB even with the bottom of the sill plate, caulked inside and out. I'm at least 6" above grade over a 30" wide gravel along the perimeter. Went through a -30 degree winter and heavy snow (still melting) almost without any issues.
l say almost because I did get a small puddle near one overhead door where some of the three feet of snow and ice melted in while I was staying warm in FLA. Just had too much ice build up preventing the melt from flowing away.
I don't believe you'll have that problem in NC.

The Cobbler · Mar 21, 2019

yes, I would let the wall sheathing overhang the pad. and I would consider a stem wall on the pad, but that wasn't your question

Stuart in MN · Mar 21, 2019

The Cobbler said:
yes, I would let the wall sheathing overhang the pad. and I would consider a stem wall on the pad, but that wasn't your question

He wants to avoid capillary action of water coming up into the sheathing. The best way to do that is to keep the sheathing elevated so water can't reach it in the first place.

As far as that goes, building practices will vary depending on where one lives but in my area the face of the sheathing is generally kept flush with the outside edge of the slab or wall. The siding then overhangs it.

James-W · Mar 21, 2019

If it were my garage, I would want at least one row of concrete blocks and preferably two rows. That way the sheathing is above the ground level high enough so that water would have a difficult time getting under the sheathing. As an added bonus, if you set an 8 foot wall on top of the concrete blocks, the walls are now taller than 8 feet.

charmin35 · Mar 22, 2019

Thank you all, I guess I'm hearing you loud and clear that adding a raised wall for the framing has a lot of benefits. But otherwise in general, the sheathing is flush with the sill plate?

I was thinking that building on the base slab was going to be very straightforward, so I need to do some more research into what it would take to have a perimeter of the slab extruding upwards.

Is this a standard thing to ask for in a monolithic slab pouring? What are the various official terms for this wall feature if I was to do some Google-fu? When I hear stem wall I think of my house foundation where it actually has a big buried footer way down in the nether regions all around the perimeter, with the vertical stem wall of masonry blocks going up and the floor joists rest on that perimeter and on piers within the crawl space.

Is this usually something can that be formed along with the slab and poured all at the same time, or is it usually done with masonry blocks after the fact? Any idea how much this might cost? My quotes for the slab are hovering around the $4,000 ballpark with my current specs. I'm already pushing the limits of my hopeful budget regarding the foundation, but I also don't want to have major regrets and water problems later.

However... if the slab is indeed 4" to 6" above grade all around (I'm doing 12"x12" footer), I guess the real issue as finn described is for snow accumulating and melting at that level. I can't remember the last time we had more than like 2-3 inches around here in the Raleigh area lol, so maybe the relative danger of snow is low in my particular case?

charmin35 · Mar 22, 2019

I don't know if it's relevant to this discussion but for context... the reason for the slab being level is that this is a primarily a workshop space for me, rather than a car space.

matt_i · Mar 22, 2019

Here's how I would roll. I would place the ply/osb flush with the concrete edge, paint the bottom 6" inside and outside, and all edges with an exterior latex, prior to nailing. But then lap the bottom edge of your siding 1" lower than that interface, and again don't seal the bottom edge.

What I don't like about overlapping the concrete edge is that the bottom wall plate has to be exactly flush with the concrete. If there are deviations it either causes the sheathing to be tight to the concrete but loose to the wood framing or tight to the wood framing but loose to the concrete, latter would be preferred for insulation reasons. Its highly dependent on the finish and alignment that was achieved by the concrete form.

Its a conundrum between allowing any water to release by gravity and keeping the space tight for critter control.

In my own build I went with flush, and overlapped the vinyl siding starter strip over the interface. I didn't paint the edges due to a wintertime install but its something I wish I would have done.

Two other suggestions, one is for gutters so you don't get extra splash, and the other is for a rub-brick to tune the edge of the slab when its got bumps or warts, etc, so you get a tight seal.

charmin35 · Mar 22, 2019

Matt, thank you for those suggestions, the 1" overhang for vinyl makes sense. Have you ever had issues with water intrusion in your case?

That's the first time I've heard of rub-a-brick! Sounds like a good thing to have on hand.

Does anyone have an opinion on wood-to-concrete water/wind seal with tape, like those shown in this video?

mmb617 · Mar 22, 2019

I built my garage 15 years ago and it's on a concrete slab which is only a couple inches above grade. I was also concerned about water damage along the bottom of the sheathing, so I used treated 1/2" plywood for the first 4' the whole way around then OSB above that.

While I could have probably just used treated sill plates I instead used treated 2x6's for all my plates and studs so I didn't have to worry about water damage to the bottom of the studs in the future either as I've seen how quickly water will wick into the end grain of a stud.

I sided with vinyl and let the starter strip hang a bit below the seam between the slab and sill plate.

The only place I ever get water into the garage is under the doors in the winter sometimes when the snow melts off the roof then refreezes into an ice dam in front of a door thus channeling water to the inside instead of running down the driveway. I could have probably prevented this by installing gutters but I have no easy place to dump them without causing ice problems in the driveway.

I'm not saying my way is the best way, I'm just throwing out some ideas you might want to consider.

I take it you are doing all or most of the construction yourself. I did mine completely with just me and the wife and I'll have to say it was probably the most satisfying project I've ever done.

charmin35 · Mar 22, 2019

Thanks mmb617 that gives me great hope! I will be constructing it all myself and I'm really excited to get started, although it may be like 4-6 weeks or more before I get the slab poured and can start rolling. Still trying to figure out various details like in this thread here. This community is so awesome.

OccupantRJ · Mar 22, 2019

In my area of NC I was told by an inspector that 8” from siding to ground was required due to rot issues. I did a concrete block stem wall and have been glad I did. I painted the stem wall with block sealer because moisture would creep through during heavy rains. I then let the osb overhang the stem wall by 1/2”, and the vinyl siding starter strip by another 1/2”.

charmin35 · Mar 22, 2019

OccupantRJ said:
In my area of NC I was told by an inspector that 8” from siding to ground was required due to rot issues.

The inspector here said 6" above grade would be code. Since my structure is under 400 sqft I actually don't have to build the foundation to code, but I still want to be close since it all makes sense.

TLCObsession · Mar 22, 2019

A lot of it really depends on your overhangs. Here in the PNW, we have deep overhangs. I have built several mono-poured garages and other outbuildings. I like to have the sheathing overlap the slab - but - here are a few caveats and tips:
If you use sheet good siding (like T-111 or Hardi panel), then you need to trim your studs so that you get the overlap.

On my own garage I did that anyway because I bolted down the treated sill plate (over the foam sill sealer) and then framed up walls on the ground so that I ended up with a double bottom plate. I also used 5/4" x 4" bottom trim below the siding.

On other projects, I have applied 4" colored aluminum flashing to cover the interface between the sill plate and the concrete.

Lastly on radiant heated slabs, I have had good luck covering the exposed XPS foam (above and below grade) with pre-painted smooth Hardi which stands up to weather and string trimmers and capping it with flashing.

bullnerd · Mar 22, 2019

When working by myself, which is always, I drive a few nails in below the plate, above the concrete, set the sheathing on the nails, remove after nailed off.

Overhang the starter strip like mentioned above.

kasander · Mar 22, 2019

I put a 6" stem wall on my monolithic pour, it should only add $2-300 or so to the cost for the extra framing and concrete. Here's a time lapse of the pour, missing a bit because of dead battery, but you get the picture.

I put a strip of sill plate gasket on top of the concrete, used treated wood for sill plate, and put OSB flush with the bottom of the sill plate.

https://www.lowes.com/pd/Dow-STYROFOAM-R-0-22-9-sq-ft-Unfaced-Polystyrene-Roll-Insulation-5-5-in-W-x-50-ft-L/50071521

I used fiber cement siding and let it hang down 1" below sill plate. It is approximately 10-12" above grade.

charmin35 · Mar 22, 2019

Welp I just found out that the slab does have to be to code, but luckily that's what I was spec'ing anyway, top at 6" above finished grade, and footing 12" below grade (frost line is at grade here in Central NC).

kasander, that video is great, that looks like it would be pretty straightforward. Except you'd have to make sure those rough openings for doorways are exact?

bullnerd, driving nails under the sill plate sounds like it would create a bit of a tunnel that water would like to find, but maybe if it's caulked up after the fact that's no big deal? Seems otherwise like that's a good tip to get it pretty flush all the way across.

TLCObsession, I'm planning on having my overhangs at about 1.5 feet all around. How much is a deep overhang considered to be in your area?

Just submitted the permit. Also I had a concrete guy out here today to check out the site who gave me a lot of confidence in their work based on the clarity of the information and conversation we had. He says he can do it for $3,700-ish. I'll have to ask about the potential of adding a stem wall.

TLCObsession · Mar 22, 2019

charmin35 said:
TLCObsession, I'm planning on having my overhangs at about 1.5 feet all around. How much is a deep overhang considered to be in your area?

Most homes are between 20" and 30". IIRC, my personal residence is 30" on the eaves. I can't remember the gable ends - maybe only 24". I'll measure when I get home. It really helps eliminate the rain splash on the lowed parts of the house. Your plan of having gravel/stone/rock around the outside is excellent.

In2toys · Mar 24, 2019

Make sure you use some kind of rat guard / flashing at the bottom of the siding

charmin35 · Mar 24, 2019

In2toys said:
Make sure you use some kind of rat guard / flashing at the bottom of the siding

Thanks In2toys, could you provide a link to a product or image of what you are thinking of for a rat guard? I'm not sure what that would look like.

Ch3No2 · Mar 24, 2019

Two ways in my professional opinion...

1. 6" stem wall above finish floor, 6" wide and outside of bottom plates inset 1/2" from outside to allow for Shear Paneling / sheathing to be flush with the outside of stem wall for mud sill sheet metal. This will allow you to hose out garage as well.

2. Inset bottom plates 1/2" to allow for shear panel/ sheathing to be flush with outside of concrete and run sheet metal mudsill above and below finish floor

JamesW84 · Mar 25, 2019

Sounds like this is stick built. What siding are you using? If stick built, you'll probably want to run a 2x4 horizontally every 2 feet or so across your framing to attach your siding to.

That's how I did mine, so I had an 8" wide foundation wall poured that raises 6" above the floor for the "stem wall" everyone is talking about. I set my bottom plate 1/2" in to allow my inside sheathing to be flush with the concrete; the outside has 2 inches to allow for the horizontal and then the metal panel siding. At the bottom of the panel siding, you use a gasket and also I'm using rat guard like this:

mcbane · Mar 26, 2019

The rat guard is interesting - never seen one before. Local codes in CA typically want flashing installed to prevent any contact between sheathing and concrete. Their concern is that concrete can hold a lot of moisture and it can cause non-pressure treated siding to rot.

charmin35 · Mar 26, 2019

It's happening my dudes. Excavation and forming is starting today. Decided not to do a stem wall, I'm going to be 6" above grade and 12" footer below grade. I'm going to start a garage build thread and you can follow my slow progress, and say "I told you so" in the future if I have any moisture issues. Thanks for all your advice, info, and discussion. The saga begins...

https://imgur.com/a/HB9FkU5

OccupantRJ · Mar 26, 2019

One piece of advise. I would be there when the concrete is poured and finished. That is all.

charmin35 · Mar 26, 2019

OccupantRJ said:
One piece of advise. I would be there when the concrete is poured and finished. That is all.

You bet, got cleared to take off work to be there. Not only am I fascinated by it but I have a few details I need to make sure are correct -- the anchor bolt locations, and the 9' sloped lip in front of garage door. Going to try to get some good video of it, I've really enjoyed and learned from watching videos of pours.

JamesW84 · Mar 26, 2019

good luck getting the anchor bolts the correct heights. Mine are pretty much where I wanted them along the lengths of the wall, but some are farther in and varying heights above the concrete.

All I can think of is maybe you could tape the end of the anchors where you want it above concrete so they have a visual. I'm also less than confident that the J bolts are hooked under the rebar in the wall, but I'll hopefully never find out.

charmin35 · Mar 26, 2019

JamesW84 said:
good luck getting the anchor bolts the correct heights. Mine are pretty much where I wanted them along the lengths of the wall, but some are farther in and varying heights above the concrete.

All I can think of is maybe you could tape the end of the anchors where you want it above concrete so they have a visual. I'm also less than confident that the J bolts are hooked under the rebar in the wall, but I'll hopefully never find out.

Thank you for that tip, I tried looking up what an appropriate height would be but I'm not sure if that's a clear answer. Would it make sense to have it 2.5" above slab so that 1.5" of sill plate will leave a final 1" sticking above the wood? Or would half an inch be better? I'm not sure how long the threading usually is at the end of those j-bolts.

OccupantRJ · Mar 27, 2019

charmin35 said:
Thank you for that tip, I tried looking up what an appropriate height would be but I'm not sure if that's a clear answer. Would it make sense to have it 2.5" above slab so that 1.5" of sill plate will leave a final 1" sticking above the wood? Or would half an inch be better? I'm not sure how long the threading usually is at the end of those j-bolts.

Yes. You want room for foam seal, washer, lock washer, and nut, with a bit of excess. You can always use a few extra washers if the bolt sticks up a bit too far. Harder to stretch a bolt if it is too short. Try to position the bolts to miss the stud locations by marking their locations on top of the forms for reference. Good on you for being there when the pour occurs. You will not regret that decision. Be prepared for yard ruts, and do not let them drive over any septic system or leach field lines if that applies to your situation.

JamesW84 · Mar 27, 2019

check minimum embedment to make sure you're good there. For example, my minimum embedment in my county code is 7" (more is ok), so I had 10" bolts with ideally 7" embedment and 3 inches out. Check the specs from the manufacturer of the bolts you get also. They have minimum and maximum embedment with ratings listed.

Also, you need one bolt within 12" of each end of a bottom plate, and my codes are (minimum) 6 feet on center after that. I measured out where my studs would be. You can always cut bolts off if you have too many or bad location (stud location, etc); you can most likely put in new bolts if you need later on. My inspector is ok with epoxying in new bolts.

Also, you might look into bearing plates for the bolts. My j-bolts came with small approx 1" washers. I'm going to use 3" bearing plates instead to give it more hold-down force on the bottom plate.

Not sure how the bolt would stick up too far since you're threading a nut on it.

Some concrete guys are crusty and hard to deal with. He fought me on using my 15 mil vapor barrier, even after I bought it. I told him I had it and that's what I wanted. I put it down with help from my brother (good thing he came, it was windy and the wind was making it a circus act) and asked them to please not poke holes in it.

JamesW84 · Mar 27, 2019

mcbane said:
The rat guard is interesting - never seen one before. Local codes in CA typically want flashing installed to prevent any contact between sheathing and concrete. Their concern is that concrete can hold a lot of moisture and it can cause non-pressure treated siding to rot.

The rat guard is metal flashing I'd assume mostly used on metal panel siding since it has ridges that create a gap at the top and bottom. It just bends back toward the concrete to keep mice/rats out. The metal would sit on top of that little shelf and you'd have a gasket behind the metal to also keep bugs and air out.

If you're doing anything other than metal panel siding with ridges, I'd assume a regular flashing would be sufficient.

kbs2244 · Mar 27, 2019

It sounds like I may be a bit late to this party.
But around here you can ask for a garage slab with a curb around the outside.
You spec the height and width.
It does add to the cost since the finishing is inside a box.

Monoslab Garage Build, Should Wall Sheathing Overhang Concrete at all?

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