Pour over existing slab

[Jimmyspeach]

Hello all I'm new to the Forum and have a question with regards to pouring over an existing slab

my garage was built in the 1940s and is 16 by 20 with a small apron that is approximately 8 feet by 6 feet

The slab and apron are in very good condition with no cracking the only problem is currently they sit approximately 6 in below grade therefore any rainfall or in the spring when the snow melts it absolutely floods the garage

Currently there is a solid inch of mud covering the slab and apron

my plan is to demolish the existing garage which I'm sure you can imagine the sill plates are 100% rotted from sitting in water for many years

The new garage slab would be 30 by 30

The new slab with completely incapsulate the old slab and apron

I'm in Wisconsin and would like to have the floor heated so I will be running PEX

Option a

I would have 2 inches of gravel on top of the old slab and then I would top that with 2 in of foam insulation and finally on top of that I would pour 4in of concrete

This would bring me an inch or 2 above grade I've talked to a couple of concrete guys that said the only way to do this is to remove the existing slab

Option B
If we remove the existing slab

The formula with change
3 inches of stone and or fill dirt then on top of that 3 in of gravel next the insulation and finally four inches of concrete


Not sure on this what do you guys think I'm really leaning towards option A just pouring over the existing slab

 

[ConCretin]

Option A. There is no reason to remove the existing slab. I would however construct an 8 to 12" deep thickened haunch around the perimeter of your new 30x30 slab. I like to use rebar in monolithic slabs, which I assume yours is. It helps hold everything together if you happen to get some differential movement. Give my Guide to Floor Slabs in the link below a read for some additional thoughts. Good luck with your project.

 

[ford33]

Is "This would bring me an inch or 2 above grade" sufficient? That seems low for the perimeter foundation wall above grade. Over time, soil will collect around the slab footing and you will soon be in dirt again. Maybe I am reading your post incorrectly.

 

[Toomanytools?]

No reason to tear up old slab since your adding gravel on top and the old slab is not cracked.

 

[joes169]

I agree with LLwillysfan, and also being from WI, can tell you that you will need a thickened edge by code here, with a minimum of two rebar continuous in the periemter. Also, while discussing code, it requires any wood framing to be a min. of 2" above any paved area, and 6" above any soil. You may need to raise your new slab higher and use a curb on the perimeter to elevate the framing.

As far as the old slab is concerned, it's hard to give advice without seeing it.

 

[The Cobbler]

the authorities may have a comment on whether or not they will permit pouring over-top of existing.

 

[Jimmyspeach]

The thickened edge will be 12 inches

2 inches above grade would give us enough pitch for the drive way

But we dont want the grade of the garage higher than the house

City is ok with pouring over slab

Imho the fact there were 3 huge oak trees that we have since removed really elevated the grade

Yes i will be using a gas curb

 

[brownbagg]

no,any cracks in the old slab will migrate to the new slab, you dont know the sun surface conditions so the extra weight could make the slab tilt, settle or crack.

 

[mcbane]

If you dont want to see cracks that coincide with the edges of the old slab and any large cracks in the old slab, you might want to turn the old slab into something approximating aggregate. No need to remove it. Just rent a skid steer with hydraulic breaker and work it over till there isnt an uncracked piece bigger than 6" across.

 

[homelessdespot]

Just curious, are you going to regrade the land around it and put in drainage also? Was just fixing the drainage issue one of your options or no because you wanted to go bigger anyways?

 

[Jimmyspeach]

Yes i will regrade the lot

I wanted a bigger garage that suited my needs