Existing concrete slab/patio question

[Jimmyspeach]

the rear of my residence is a solid concrete patio I would like to build a garage my question is

If you were in my situation would you rip out the concrete and just do a monolithic slab or would you cut through the existing concrete patio build a thickened Edge and use the remaining concrete for the floor of the garage

Secondarily if you were to attempt to make use of the concrete that is already there any tips or thoughts on that I was thinking Drilling in to it and connecting it with rebar to the new pour

The existing patio is in very good condition it's not cracked and broken up it's probably less than 10 years old

I took a shovel and dug down along the edge of the existing slab it appears to be about 4 in thick

additionally from an expense standpoint it would be quite costly to rip everything out both in terms of time and labor disposing of concrete in the city is a pain

I attached a quick rough sketch the pink area would be what I cut out to pour the foundation the foundation would be a thickened edge 8 in wide and would be a foot below grade the top of the New Foundation would sit about 4 to 6 in above grade

Again any thoughts are appreciated

 

[billconner]

I think you're in Wisconsin, so frost is an issue. I don't know how large the garage is so seems to be less than 600 SF so by code you do not have to frost protect this for an accessory structure.

Not sure what you want to do in garage or on slab left exterior but assuming mostly park cars inside and walk maybe lounge outside, I'd consider some sort of curb - I'd probably use block - and just stick frame on top of that.

If you don't want that curb and can stand to add an apron, you might just pour another 3 1/2" slab atop existing, and an apron/ramp, and just stick frame on that but I'd like more than 3 1/2" above slab for protecting framing and siding.

I like the block curb better, plus easier. A few rebar towels, grout the course, j-bolts for sill. Consider surface bonding cement rather than mortar - easier diy and very strong.

I see no way you can cut that and insert a footing. It would be too narrow and support less than the 4" slab.

 

[ConCretin]

Assuming everything drains where you need it to, there's no need to remove what's there. I'd cut a trench in the existing slab, excavate and place a footer flush with the existing slab. Take a little time finishing the top of the footer where it will show and you're done. Normally I wouldn't suggest pinning the perimeter of a slab to anything but in this case, I'd be tempted to pin the existing slab to the footer just to keep everything flush. #3 bars every foot or two would work.

If you don't like the looks of that, you could always place a 2-3" concrete overly over the whole thing, which would provide the added benefit of raising your garage floor above existing grade. While we're on that subject. I'd place a row of block between the footer and the garage framing to get the wood up and off the existing patio.

 

[Fav Onefour]

Interesting idea to keep the slab.

I'm in a similar temp zone. I have some exterior stuff that is poured on a thin base over undisturbed ground with good drainage. Those slabs move a lot with frost. This winter with deeper frost there was more movement. Slab on grade moved 1/4 - 1/2" upward and are now dropping. Interestingly enough, the 20+ year old slabs are not cracking apart, they just move as a unit.

I like the plan to keep the existing pad. I'd be concerned about the true stability of the pad. It's hard to know if the base is truly stable or if the concrete is a consistent thickness. In my case, I'd probably think of it as a separate floating slab.

Both suggestions are pretty good. Maybe a little modification with the idea that your slab is moving.

I have another G.J. method.
Build the new garage larger than the slab. Put in traditional footings and use the existing concrete pad as the new interior floor. You might get by with a narrow pour next to the footing.

 

[RMW1]

Interesting idea to keep the slab.

I'm in a similar temp zone. I have some exterior stuff that is poured on a thin base over undisturbed ground with good drainage. Those slabs move a lot with frost. This winter with deeper frost there was more movement. Slab on grade moved 1/4 - 1/2" upward and are now dropping. Interestingly enough, the 20+ year old slabs are not cracking apart, they just move as a unit.

I like the plan to keep the existing pad. I'd be concerned about the true stability of the pad. It's hard to know if the base is truly stable or if the concrete is a consistent thickness. In my case, I'd probably think of it as a separate floating slab.

Both suggestions are pretty good. Maybe a little modification with the idea that your slab is moving.

I have another G.J. method.
Build the new garage larger than the slab. Put in traditional footings and use the existing concrete pad as the new interior floor. You might get by with a narrow pour next to the footing.

The house I bought had a detached carport with a simple flat 3.5" concrete slab with a post/beam covering but no walls. Done without permits.
I wanted to put an enclosed garage there and started by taking down the wood structure. City building inspector said I had to have a 24" deep footing under the new 2x4 stud walls. So I cut a trench around the perimeter and built forms with the top edge at 3.5" above the old slab. Installed rebar. Then hired a guy to pour it with concrete and make an apron sloping down to the driveway. Building inspector was happy.

 

[Fav Onefour]

The house I bought had a detached carport with a simple flat 3.5" concrete slab with a post/beam covering but no walls. Done without permits.
I wanted to put an enclosed garage there and started by taking down the wood structure. City building inspector said I had to have a 24" deep footing under the new 2x4 stud walls. So I cut a trench around the perimeter and built forms with the top edge at 3.5" above the old slab. Installed rebar. Then hired a guy to pour it with concrete and make an apron sloping down to the driveway. Building inspector was happy.

I'm glad you chimed in with your project.
I like reusing existing concrete if possible. It didn't seem viable to go right on top without proper footings. My thinking was along the lines of what you ended up doing.

One item I'll add. @Jimmyspeach , I know it's tempting to assume the pad is 4" based on the excavation you did on the edge. I myself wouldn't trust that method as a true measure of concrete pad thickness. It would be too amazing if they did a patio pour on perfectly flat sub base. It's fairly common to see nonstructural pours thicker around the edge of fill.

 

[Skooterj]

Easy, dig around slab, pour footers, put stem wall and building up on new footers, keep existing slab as floating floor inside, new, bigger building. Done.

 

[racecougar]

Is there any concern about the existing surface finish? Typically a patio will have a much rougher finish than a garage floor. Depending on the planned use for the building, it may/may not matter to you. Additionally, will the existing pad slope work out? Will you have to have the top of your footer at differing heights relative to the pad to place the framing level?

When I built my latest shop, I strongly considered building it over an existing 30'x50' pad (basketball court), but better senses prevailed in the end and I built next to the existing pad. I certainly do not regret doing so.

 

[d300]

You might consider the value of the finished project and compare it to the cost of removing the existing and doing a FPSF. If there is any risk of frost heave damaging the new structure then....