new shop construction questions

[Brian White]

I am getting ready to build a garage with floating slab with stem walls and have a question about the stem walls.The stem walls will be about a foot above the slab.Should I block out for the overhead doors and let the slab sit ON top of a stem wall at the garage door opening or should I leave the stem wall and footing out of the areas where the overhead doors will be? Cost difference is not an issue just seems as though it would not be needed and would allow the entire slab to be independent of the stem walls. Also would make the pour much easier to leave them out as I could back the truck inside the foundation forms to make the pour

[George wilmore]

I have a stem wall with openings for the garage overhead doors and side door. I think you will be happy if the openings are in the wall.

[Thom Porterfield]

I am getting ready to build a garage with floating slab with stem walls and have a question about the stem walls. The stem walls will be about a foot above the slab. Should I block out for the overhead doors and let the slab sit ON top of a stem wall at the garage door opening or should I leave the stem wall and footing out of the areas where the overhead doors will be? Cost difference is not an issue just seems as though it would not be needed and would allow the entire slab to be independent of the stem walls. Also would make the pour much easier to leave them out as I could back the truck inside the foundation forms to make the pour

Hi Brian.
I do not recommend that you cast your floor slab so that it extends over the top of walls at door block-outs UNLESS that block out is about a foot below the top of the finished slab AND that area of the floor slab is well reinforced. That way, you can reinforce the slab there to avoid some really crummy cracking that can occur if the slab is not thickened.

Of course, as you point out, the slab so configured is not independent of the walls, so it's very important to ensure the fill below the slab is very well compacted. Even then, however, you can't guarantee that the slab will not crack where there are any possibilities of differential settlement.

On the other hand, your stem walls are no doubt thicker than the width of standard door thresholds. Even if you purchase extended door saddles, you will inevitably experience some degree of change in floor elevation at the door. In my shop, I did run the slab through the opening, thickened to 12", with #4 bar reinforcing doweled into the wall below and at the jambs. This reinforcing extends into the floor a minimum of four feet. The building is just about ten years old, and so far, the only cracks I have are hair-line ones, which are virtually impossible to avoid.

[Jim Andrew]

Thom pretty well covered it. I like to frame out the openings for overhead doors plus 3", so just a 2" board on each side goes down to the slab. For walk in doors, frame just 2" oversize of a door, for example, using a 36" door frame 38". As for walk in doors, I like to frame the slab out a 2" board, so the slab extends out under the door sill. Just put a board between the foundation on each side of the door before putting your form across the doorway. I like stem walls much better than forming slab and foundation all together. Concrete is guaranteed to crack. I also like fiber mesh in floors, the metal mesh usually winds up on the bottom of the slab, but the fiber is mixed into the concrete uniformly, seems to hold cracks tightly together.

[jay st clair]

Hi Brain, in oregon building code requires a continuous footing around the perimeter of the building, are you going to use compacted fill under the slab or round rock? Fiber is a great product , also how are you doing your expansion joints?

[Ken Fitzgerald]

Brian,
The contractor that built the empty shell for my shop did as Thom suggested.

If you live in an area prone to a lot of rain or snow, I would highly recommend making the stem wall higher than your poured slab floor and the only place where I would have it even would be at the doors.

I have learned valuable lessons at each of the homes I have bought over the years.

My first home had an added on....family room. The family room was slab construction. That winter we had a lot of snow and it piled up on the west side of the house where the family room was. The house had electric baseboard heat except for a wood stove in the family room. We could heat the entire house with the wood stove. Well, the snow melted due to the heat from the wood stove escaping through the waqll and the resultant water ran in between the wall and the slab. So I am out there shoveling 2' of snow back from the wall..putting up a temporary bisquine lean-to. Then with a couple of borrowed electric heaters, heated the exterior of the wall, allowed it to dry and then I caulked the gap. I would recommend making your stem wall at least 12" or what ever local code requires above the finished grade level to prevent flooding.

[Don Bullock]

Check your local building code. In many cases it will dictate stem wall/slab peramters. My slab, for example, is on top of the stem walls and eight inches above grade as required by code. Unlike you and Brian, I don't have to deal with melting snow so his advise, if allowed by code, sounds like it would work.

Good luck with your build. I'm sure you're vert excited.

[Thom Porterfield]

When I built my shop, I set the slab almost flush with exterior grade, figuring I would later make a paved apron of some kind. I didn't want to have to deal with ramps to get things in and out of it. Well, the door faces south, and guess where all the winter weather comes from? I didn't have a LOT of leakage, because there was a four-foot overhanging upper story. But there was some.

And when, two summers ago, I decided to remodel, that four-foot overhang got sucked up by the expansion. Now there is only the 32-inch roof overhang (an eave, no less, but with a gutter), that is some 9 feet above the bottom of the door. I was seriously worried now, about winter leakage, mainly from melting snow.

Here's my solution (drawn in SketchUp, after the fact).
SLAB-AT-DOOR.JPG.jpg

The concrete finisher looked at me askance when I showed him the drawings. It could be because here, in the central Oregon high desert country, people pretty much ignore drainage systems, as there isn't a great deal of precipitation. But I come from wet country, and I really do NOT like mopping up snow melt from to keep my finer attempts at furniture from water damage. (I don't like mopping it up to protect my lousy attempts either.) Well, he said, in effect, "You build the forms and bend the rebar, and we'll cast the concrete."

So I did.

So far, no leakey. And, amazingly, no cracks! Keeping my fingers crossed on the last one, but as you can see, I put plenty of steel in the thing and we used fiber-reinforced concrete. A big +1 to whoever mentioned that above. It definitely keeps down the mini-cracks. But if you do use it, don't count on a fine acid-etched finish. The guys who do that, say no-no to fibers in the concrete.

[Jim Andrew]

When I built my daughters house, I took the slab out under the patio door, and got a big crack that started at the offset on one side. Looked like what was needed was a piece of expansion joint against the wall to help prevent that. Not all around the building, but just the sides of the door opening. And where you have a garage door, form an offset in the floor, dropping down 3/4" to stop rain from splashing into the shop from under the door. Just be sure to get the offset far enough into the building for the door to set down in the offset.

[Thom Porterfield]

Yeah, I always specify a 1/2" expansion joint at all entrapped edges of a concrete slab with a dimension of 10 feet or larger. Why? The walls are generally always the same temperature, cool, and the slab is randomly warm or cool. The slab will expand and contract sufficiently that if you don't have the expansion space, it will, and I repeat, WILL crack. If the walls aren't sufficiently reinforced, the expanding slab can crack them, too. You really don't need to ask me how I know this, do you?