Foundation wall tie in to slab question

[Nashty]

Hi - I'm building a small detached garage to do some light work next to my house. I have the garage packaged designed and want to make this a project for my boys and I to build. So we're doing as much of the work as possible.

I've done plenty or remodeling, framing, wiring and plumbing. I'll hang the drywall but will pay someone to finish it. I know my limits.

But I've never poured concrete.

I talked to the zoning man. I need 48" deep foundation, only 6" wide. Rebar at 1' and 3' from bottom of the trench. I'm going to rent a trencher and just pour directly in the trench.

Now my question. How do i tie the slab onto the foundation walls? Do I just frame up around the foundation walls and pour on top of it? Does it need rebar sticking up from the foundation to tie the slab to? Or just pour on top of the foundation?

There are trees nearby, I'm planning on just digging out 6" of the soil, then 2" of insulation board, plastic and some metal "fencing" wire concrete reinforcement. Sound good?

The slab will be 16' X 24' How far apart will any cuts need to be?

I plan to fill the cut slots, then epoxy the floor.

Any help would be appreciated.

Roger

Illinois

 

[Firebrick43]

Six inch foundation wall sounds right but a much wider footer is usually needed unless on bed rock. Doubt you are if your planning on a trencher. Footers here are usually 18" and it pretty dense soil. As far as pouring directly in a trench it won't be possible if a footer is required and even if a footer is not it will turn out poorly. I highly suggest you don't do it. You want the walls to come out of the ground at least 6" above the top of the floor level of not more. 12"+ would be ideal in a garage so drywall or other wall covering is above floor level so it doesn't absorb moisture or get damaged from rolling floor jacks and such. This can be a formed and poured wall, concrete block(pour at least a cell every 2' and around doors full with pea Crete) or icf.

The pour your slab inside be care not to key or tie into the walls. I recommend at least 8" of compacted stone under a slab. 2" foam and plastic great but welded wire reinforcement always ends up on the bottom of a slab when the concrete guys pour. And no lifting it with hooks does not work well when they are standing on it. Every piece of concrete I have tore out has the wire on the bottom unless it was combined with at least some rebar on chairs.

If you do a good prep job that flat and level and not keyed or restrained in any way to the perimeter the slab will not have visible cracks. It also need to be kept wet for a week to reach full strength and prevent curling(plastic underneath is needed as well). Both my 28'x50' basement and 28'x30' garage have remained crack free. As the floor slab shrank it pulled away from the walls about a 1/4 inch.

 

[Elginz]

" I'm going to rent a trencher and just pour directly in the trench." Seems like this will be a dream, not a reality.
Usually the slab does not tie into the foundation, the walls are built on the foundation. The floor will be up against the foundation wall. Bring the foundation up a couple inches above the floor height.

 

[JamesW84]

I agree...you want a floating slab (not attached to the foundation). The footer/foundation wall won't settle the same as the slab, so having a floating slab reduces cracks. I actually had a concrete guy tell me he pours the footer and foundation wall then pours the floor with the edge of the floor over the top of the foundation wall. He said that's how it's commonly done, but he said he would do it however I want. Seems foolish to me, but apparently he has been doing it a while.

Check with your inspector to see if you need a footer. As mentioned, you likely do, so your trench and pour idea won't be possible. I'll do a lot of things, like serious DIY (I bought a mechanics special backhoe and I'm doing my dirt work, but I've researched enough to know that you get one shot at concrete. I've watched videos, researched, and read a lot, but I usually require more than one try to get something right. You get one try with concrete unless you do it in sections.

 

[kelpaso1]

Everything about the OP's post sounds wrong and destined for failure.

 

[matt_i]

The trench foundation is very common in Michigan. I know of a dozen people who have built shops and garages this way with no issues including 2 of my neighbors who I helped prep and pour.

I would try to get 8" wide trenches even though that's more concrete. I would try to get enough time off work to be able to break ground, prep, and pour all in a window of good weather. If you get enough rain your nice trenches will collapse and fill with muck (in the right soil condition) and fixing this is a hateful process.

Typically there is also a concrete form that is at the grade level, which gives you a slight raise from the ground level itself. The slab is monolithic with the footings. Before you break ground its important to strip out grass and topsoil from the center area, and I think its a good idea to dump a pile of crushed washed limestone that will be used later under the floor. I would place & compact the gravel, the insulation, a vapor barrier, and then the concrete, so some study (draw a detail) is necessary to get the elevations correct.

Most people hammer rebar vertically down and then wire-tie a horizontal perimeter as far as they can reach down into the trench, and then another one about a foot lower than the surface. Whether you steel-reinforce the floor is up to you. I recommend it.

Its a very good idea to have your plans to the point where you can identify anchor bolt locations before the pour. Putting little kerfs into the form is best as the grey mud will wash over almost any other marking.

Personally I'd hire a concrete crew to help place and finish your floor. There's a lot of money at stake and not a lot of time to learn on the job, there are special tools you can't just get anywhere. Wash the inside of the forms with a rag soaked in Mazola corn oil the morning of the pour as a release agent. And then focus on how to cure the slab properly, either a curing sealer can be applied (that you bought earlier) or water-flood curing can be done as well but is more maintenance.

I recommend saw cuts the next day after the pour, 10-12' squares is typical, but I'd strike 8' squares off for a 16x24. A good concrete crew should be equipped to handle this and seem like its part of their normal job.

 

[ConCretin]

matt_i is more familiar with earth formed foundations than I am and seems to have given you some good advice. Our soils up here won't stand up long enough to get concrete placed and I'd never want to try to get all that work done between rain days.

I assume you'd want to have your slab base all prepped and anything else you could do beforehand completed before you dig your trench. Normally you wouldn't tie your 'foundation' to your slab but in this case you would. It's kind of like a mono-slab with extra deep haunches.

Matt_i indicates that trench and slab are usually placed monolithically but I don't see why you couldn't fill the tranches first and leave some rebar dowels sticking out to tie in the slab later if time and weather become an issue. I'd place the trench a few inches lower than the slab sub grade and then form the perimeter of your slab and place your slab on top of the trench concrete.

Honestly, I've never really understood the concept. A narrow little trench filled with scarcely reinforced concrete wouldn't seem to have much bearing capacity and without insulation, the concrete won't stop frost from penetrating.

With all due respect to the good folks of Michigan I think I'd place a well constructed mono-slab or regular old frost walls and footings. Apologies to Nashty for the rant. Listen to Matt_i's advice and experience and you'll be fine. Good luck with your project.

 

[GMCGarage]

Why not do away with the footing and just do slab/raft type foundation. can thicken the edges to support the walls. Will be a bit more reinforcing, but should not crack to hell like what you are going to pour.

 

[Nashty]

Thanks Matt I. Just what I wanted to hear.

 

[matt_i]

Good point by LLWillys that the trenches can be done with short pins sticking out, and the slab formed later. My one neighbor did exactly that now that I think about it.

My take on the trench foundation is that its OK for a single story structure of smaller size. Get into a big structure with larger roof loads, 2 story loads, which are placing a lot of bearing on the soil and there could be a problem.

 

[ard]

Read up on a Ufer ground in your foundation. Adds minutes to the job, saves pounding in two ground rods...

 

[Copymutt]

If you decide to go conventional footer, stem wall, slab I second keeping the slab separate from the stem wall. If connected your wasting that insulation under the slab due to conduction. I always insulate right on up,the inside of the stem wall to the very top. Use MDF or plank flooring as a perimeter thermal break to cover the insulation between the slab and stem wall.
Jim

 

[ard]

FWIW....

If you will be using wide lumber- say 2x10s, 2x8s - on the project, you can use them as forms for the concrete, clean them up and build with them later.

Even if for only the top ** inches of the stem wall.

 

[kbs2244]

The OP didn't give his location so all the frost depth comments are pi$$ing in the wind.
That said "detached" most often means a floating slab would be fine.

 

[sierradmax]

Sounds like you're in the northern half CONUS. 6" is a narrow foundation wall. I used 8" but poured 24" wide by 12" deep footings beneath the 48" x 8" foundation wall. I used what we call "hairpin" rebar and w6x6 wire mesh in the floor.