Concrete slab questions

[North Dakota]

I'm getting ready to build a 30x45 detached garage I found out from the local code office that I can do a monolithic slab. Being in the north does anyone have any experience with these? I don't want the foundation to heave and crack all over. Also as far as site preparation goes any advice? It needs to have a inlet for propane / well water/ and exit for septic.. I'm thinking I should ask for quotes with that rigid insulation under the slab with a 10mil plastic... Any other things that anyone can think that I need to mention while asking for quotes? I'm new and trying to wrap my head around this whole building process

Thank you all for all of your help!

 

[gizardlizard]

I built two garages that had monolithic slabs. Never had serious cracks in either. I used zip strips in the concrete and spent the month prior soaking the ground everyday and compacting the soil. Worked great. I didn’t add vapor barrier or insulation but that may be required now days per code.

 

[matt_i]

Imo best way to get your building's foundation setup so it won't heave involves one of two methods which are easier than traditional methods of building a poured wall or a block wall on top of a buried footer.

1) FPSF = frost protected shallow foundation. Google that and a US Government HUD .pdf document comes up and its basically a "recipe" for building an unheated building anywhere in the continental US. You'd have to get your local bldg department to agree to trust the engineering contained therein, but after that its a lot less excavation.

2) You could do a trench foundation and pour it as a monolith. Very common in SE Michigan, the foundation walls are "earth formed" which means you excavate a trench in soil that's stable enough to hold together long enough for it to be filled with concrete, you'd excavate down to a level equal to the depth of your local frost line. The slab surface could be poured at the same time by wood-forming a ~4" deep slab on top of the trenches and continuing to fill past the excavations until the wood-form itself is filled.

Trying to build a 30 x45 on a simple slab right ontop of the ground is like playing a high stakes lotto. It could work and be fine, but if it doesn't: your 5 figure material budget is going to be a 100% loss, all the labor is going to be a loss, the concrete is going to be a total loss = demo it and start over and do it correctly.

Most people don't want to play roulette with ~30-50-100k in the balance (depending on who is doing the work) and so I highly recommend one of the two paths above if you don't want to do a traditional foundation design (block or poured wall).

 

[Don1357]

Next time you go driving around pay attention to every single road surface you are on; they are all monolithic slabs. Count how many you see that has heaved.

I'm in Alaska. Monolithic slabs are very common here. At 30x45 it will crack, but then it is a matter of having control joints to channel them.

 

[Craig Balzer]

North Dakota - you have already received some great advice. I used the identical layout you IDed in your original post. My pour was in the first week of December 2019 -- after 14 months the only cracks are where the control cuts led them to be. I have 3-5 cracks that are from the end of the cut to the edge of the pour (about 4-6 inches): right they are supposed to be. One absolute fact about concrete: it will crack. Proper placement (every ~8 feet) and depth (1/3 of the depth of the pour) of the control cuts will place the cracks where they are supposed to be.

The only difference between our prep is I installed in-floor heat and used some stuff called Crete-Heat. It is designed to serve as both a 2" thick closed-foam insulating layer and the "hockey pucks" are staggered to hold the heating tube (see photo).

Two things I can add to pre-pour prep

1. Look closely at your electrical grid. Where does the power come in and where do you you need it. Where you need it "now" is easy to visualize; but where will you need in 3 years? Cheap insurance is to lay some conduit in the floor that will allow you to easily add electricity to the far sides of the structure. A coupla bucks now in conduit will make your life very easy in the future for that unexpected need for power "over there".

2. With water in your garage -- a floor drain might be prudent. Plan for it now. It'll allow you to wash cars and whatnot in doors in winter.

 

[Stuart in MN]

This is a link to a .pdf file on the City of Minneapolis website that describes the garage slab requirements here. Of course, your local requirements may be different, but it should at least give you an idea of what's required for slabs in cold climates (my garage is built on one, they're very common here and work just fine.) http://www2.minneapolismn.gov/www/groups/public/@regservices/documents/webcontent/convert_264724.pdf

 

[ConCretin]

A monolithic slab works great for a detached structure as long as the soils underneath are stable and drain reasonably well. Up here in Maine, I have a 32 x 28 that is 10 years old and a more recent 14 x 36 addition on monolithic slabs. No issues whatsoever.

A monolithic slab will generally cost considerably less than traditional footing/frost wall/slab construction and will also save money over an earth formed grade beam if those are an option in your area. You can protect the structure from frost movement by using rigid insulation under and around the foundation but I didn't see the need in my case.

Give my Guide to Floor Slabs in the link below a read for some additional thoughts on the subject.

 

[astroracer]

What style building? Post frame or stick built?
A post frame slab is generally a monolithic slab that is poured inside the rat/grade boards using them as the form. The slab can have thickened edges but they are not really necessary because the weight of the structure does not bear on the slab.
A stick built will require a thickened edge or, as mentioned, a short footer/foundation to support the structure.
Here in MI. the standard practice is to peel out the topsoil and do a couple of lifts with hard pack/gravel to prep the site.
Most of this can be left to the concrete contractor as he will know what is required for code to pass inspections. Knowing what the codes are is a good thing as you can talk intelligently with the builders even if it is to just let them know you know what is involved.
I am getting quotes for a 36' x 24' addition to my barn and right now the concrete quote is $4500 for site prep, vapor barrier, insulation and 4" of 4500psi concrete with fiber.
Mark

 

[yeldogt]

Just make sure you have someone that does them on a regular basis ... In the midatlantic I had trouble with the code official and finding contractors.

I had built a house in VT and wanted to use the same foundation type in NJ and ran into the problem ....

It's the same with poured and block .... things are moving to poured obviously -- but, there is nothing wrong with a proper block foundation and most of the best builders around me still go with block.

You always build with what the picked contractor is regularly doing. Mono w/ VB and insulation is an elegant foundation

 

[speedracerfx]

I just did a 28x30 monolithic slab in November for my detached garage. I followed the design for Unheated Buildings (since I don't plan on having the garage heated 24/7) as shown in Figure 7 on Page 17 here : https://www.homeinnovation.com/~/me...de-to-Frost-Protected-Shallow-Foundations.pdf

My local code official was unfamiliar with this design, so I had to show him this pdf, and also point out in the section of the IRC that points to using this design method.

 

[lowrollin70gmc]

It depends which side of NoDak you’re in. The soils of the Red River Valley are much different than out west.

I wouldn’t want a monolithic slab near the Red River, due to the shifting clay. In reality we shouldn’t have basements here either, so I guess pick your poison if you’re in the eastern half.

 

[ericm]

In a cold place like ND I'd want in floor radiant heating. And good insulation under the slab and on the sides.