Stem wall vs mono pour

[PWilks]

Hi,

I live in central MN, with a typical temperature swing most years around the range of -30 at the worst to 105 at the max.

I’m in the process of looking for a concrete contractor to pour my slab for my new 24x32 garage.

The big question; should I insist on monolithic or stem wall construction?

The land that it will be going on is virgin dirt, no trees within 20 feet of any side of where the garage will be. I want something that will last. Heaving is a large issue in MN, and there’s codes specifically designed to combat it, but the question of monolithic or stem wall is typically up to the contractor.

Which should I insist on, and what’s typically the difference in cost? (I’m aware that stem wall has higher labor costs)

 

[fourbyford]

Similar climate here... getting ready to help my buddy build an addition to his shop... going with footing/foundation

 

[matt_i]

An earth-formed "trench" foundation is a good economical way to go below frost line. But you or the contractor must have focus and dig + pour quickly in a good window of weather. Of some concern is the earth itself, in pure sand the trenches could collapse while if there's a clay then they will probably hold up. This can be a 1-pour where the slab is also formed and poured at the same time or a 2-pour where the trenches are filled to flat-ground level only with some rebar pins and the slab is poured on top as flatwork.

The three-pour system where a pad footing, stem wall and finally slab is classic and is most proven and also costs the most. The pad is needed when you get 2nd story loads as in many residences. But not as critical for a single-story shop bearing just the roof loads.

The prices are going to be mostly labor. I can't quote costs since I provided the labor for excavation, pad, stem walls, then paid a crew of 2 to place and finish the slab in a 25x40

 

[ConCretin]

If you have reasonably well draining soils, a mono-slab for a detached structure works fine and will save you some money. I have a 28x32 up here in Maine. Although I didn’t, you can protect it from frost movement by placing rigid insulation under it and extending it out or down a distance equivalent to your frost depth and still save money over footing, frost walls and slab. Give my Guide to Floor Slabs in the link below a read for some additional thoughts.

 

[thayer]

Due to the frost concerns in your climate, slab heave is a real issue. Check your code for required footer depth - I’m guessing minim 36”. I’d pour the footers and piers on the first pour, isolate them from the slab with isolation joints, and then pour the slab separately. Isolation joint material can be nailed to the foundation wall prior to the slab pour. Make sure you’ve got 8” of compacted 3/4” cut gravel over compacted undisturbed earth with a vapor barrier on top. Cut control joints into the slab top and consider installing an expansion joint down the center dividing the slab into (2) 12’ x 32’ sections. Good luck!

 

[u2slow]

For increased grade-to-structure separation, you want the stem wall. This would be important for wetter areas, and makes it less attractive for termites and carpenter ants.

It also lets you pour the slab later... which allows the building to keep being built before all the in-floor considerations are figured out. (heat, insulation, piping, conduits, structural piers, etc.)

 

[Kevin54]

A slab vs. a stem wall...you had better go with a stem wall for a garage. If you go with a slab and build a wall on top of it, and you want to have to wash out the garage.....Slab = mop....Stem wall = using a hose. Plus it is easier to finish off on the inside without problems.

 

[Zeke]

Here in CA we place concrete in the footing with forms hanging just below the edge of the excavation for the stem. If a mono placement is favored and there are enough personnel to handle it, we leave the inside forms as high up as the slab thickness is called for and place the whole multi-truck batch all at once. The finishers pull the stakes on the interior forms as soon as they get out on the slab and soon after pull the interior form boards to float the inside of the stem. Like doing steps.

Best of both worlds with no cold joints. Score the next day or use 4" fiber expansion joints, but some don't like that look. Personally, IDT trowel made joints do much for crack control.

 

[NUTTSGT]

Me, I'd prefer a foundation with stem wall.

 

[The Cobbler]

for me it was a trade off. I ended up doing monolithic. Due to grade on my property I had to do 24" deep mono pad. I used a heck of a lot of concrete to do it. I would have used a bit less to do a footing & stem wall, but instead of 18" into the ground I would have had to go to 48" . Would have been more earth to get truck away, less concrte to come in, more forming ,2 or 3 pours instead of 1

 

[ConCretin]

If cost isn’t a concern, frost walls might provide a little peace of mind. A couple things to keep in mind. You are placing your slab on several feet of backfill around the perimeter where the slab is most susceptible to cracking. Even if you achieve 95% compaction, you could easily get additional settlement. Also if the structure is unheated, you still need to protect the slab from frost movement. Insulate the frost walls and under the slab to keep the soils under the slab from frost penetration.

For a detached structure, I prefer a mono-slab. If you want to lift your wood framing above grade inside and out, it’s pretty simple to add a concrete curb. I’m helping a buddy build a 36x28 mono-slab right now. We formed the perimeter with 2’ form panels and placed the slab against the bottom 12”.

We’ll use a 2x12 On the inside to form the curb. No matter how carefully you grade perimeter, the form panels seem to vary a bit so for a perfectly level curb, you are better off using a shorter form of the inside referenced off the slab for grade.

We put 2, #5’s in the bottom of the thickened edge with curb dowels at 18”. The slab itself is #4’s at 18” OC supported at mid slab. To complete the overkill, we’ll run a #4 bar in the curb tied to the top of the dowels.

I wish I’d taken a few pics. It was purty.