Building on concrete slap question?

[Chase77]

Any contractors out there? My parents want to do an addition on to their place over top a 6.5" thick slab. I wanted to see if it was as simple as footers having to be somehow anchored and poured underneath the existing slab or are they lookin at tearing out the 6.5" slab and starting from scratch with an 8-10" slab w/ footers?

Thanks for any response on this,

Chase

 

[tea LC]

how big is the addition? what climate do you live in? how old is the current slab?

 

[Chase77]

how big is the addition? what climate do you live in? how old is the current slab?

The current slab is 1 year old and is 28 wide and 12 deep. The idea is to use the existing and pour another 12 x 28 behind it to make a good sized master suite. We're in Cincinnati.

Thanks,

Chase

 

[fj40crusher]

Always better off starting from scratch. Especially if you don't know how that slab was poured (ie:compaction, select fill etc...)

Personally, I'd tear it out. Slab's are a bitch anyway. Try plumbing in a toilet in an exisiting slab.

 

[82-SOA-60]

Always better off starting from scratch. Especially if you don't know how that slab was poured (ie:compaction, select fill etc...)

Personally, I'd tear it out. Slab's are a bitch anyway. Try plumbing in a toilet in an exisiting slab.

I know what you mean. The house we bought is 50 years old and is built on a slab. Good luck trying to replace pipes!

 

[Deep South Cruisers]

I suppose you could saw cut and investigate the existing slab re-bar and sug-grade.

Hell, if its 6" thick concrete all be damned if I'de tear all that out just to build living space on top of it

 

[flintknapper]

Have a local contractor look at it.

He will be familiar with all of the local codes and such.

It may well be possible to pour another slab that joins the other, but this results in a "cold joint" and could present a problem later if a footing or piers are not incorporated.

If the existing slab is not Pre-tensioned then I like to cut a keyway into the existing slab, drill holes in the keyway to accept 3/4" rebar and dig a footing underneath the existing slab at the site of the joint.

All of this assumes a proper substrate and personally, I would require core samples of the existing slab to ensure its worthiness.

Any plumbing will need to be considered as well.

 

[TJDIV]

Have a local contractor look at it.

He will be familiar with all of the local codes and such.

It may well be possible to pour another slab that joins the other, but this results in a "cold joint" and could present a problem later if a footing or piers are not incorporated.

If the existing slab is not Pre-tensioned then I like to cut a keyway into the existing slab, drill holes in the keyway to accept 3/4" rebar and dig a footing underneath the existing slab at the site of the joint.

All of this assumes a proper substrate and personally, I would require core samples of the existing slab to ensure its worthiness.

Any plumbing will need to be considered as well.

And that, folks, is the end of this thread.

 

[Steve C]

Check the code for moisture barrier underneath. If this was a patio i doubt that it has it, but for living area, at least in my neck of the woods, it is required.

 

[Arya Ebrahimi]

If the existing slab was placed as a patio or the like, I highly doubt the subgrade was prepared properly to living space standards. I agree 100% w/ flintknapper's comments. I would look at the flipside of the coin for a second. How much is it going to cost to have it demoed and removed so you can start from scratch? Probably 2-3k MAX(including hauloff). To me it would be worth it for the piece of mind and ease of construction than anything else. Hell you could probably find a couple of mexicans w/ an F-250 who will do it for $500 D). Concrete is cheap....with the amount you'd be replacing I don't think it would add a significant cost to project as a whole.

Then again, if it's only a one story addition, this may all be overkill....

So, like flintknapper said, have a local contractor come look at it(I would have at least 2-3 come look, but that's me).

Ary

 

[Cookiemonster]

Have a few local guys take a look at it, to get varying views. Different standards throughout the nation, all based though on ICBO (www.icbo.org) codes. Unknown how Ohio has it set up but you may need to look for a specific contractor such as a foundation contractor-vs-general contractor (i.e. general contractors are three or more trades for CA). Good luck........

 

[White Shark]

The current slab is a hair under 6.75 yards. Not a lot of concrete, but worth looking into as a salvagable option. If the house were mine, I'd get the bobcat with a breaker or a 90 lb. hammer and go to town. A lot of quarries take clean concrete with no organics, i.e. wood, for free as they crush it to reuse as aggregate. Other places will use it to stabilize creek beds, slowing erosion. If you get a lot of freezing, frost heaving can also be a factor. Being a General Contractor as well as other things, (all legal I assure you...), I would tear out the concrete, set some footings below the frost line, consider a pretensioned system, and look into the in slab radiant heating systems available. The concrete is cheap, the labor is not. If I were building a 20-40k addition however, I would seriously consider a straight pour using 6-7 sack mix with plenty of #5 rebar.

If you do reuse the current slab, I'd stick with local code, hire a structural engineer with a background in soils, and follow his advice. You would most likely need to drill holes in the current slab, set rebar in the holes with Simpson Set-Pac Epoxy, and tie into the new pour rebar grid layout.

Like I said, concrete is cheap. Especially when it comes to fixing buckling sheetrock, warped siding, and a fat swale in the roof trusses. Estimate the job both ways. Trust the engineer and stay on the budget you created when you worked up the estimate. Get permits. If anything bad happens, it's much easiler to argue with the insurance company when your bases are covered. Good luck.

 

[critterhunter]

Remove the slab, 1st if the house is on footings and the slab is not slab will move around with weather changes csuch as freezing, this will shift the addition on the permentant structure of the house. Not to mention the floor will always be cold all winter long.

 

[trainrech]

Is the slab level? If it isn't and you plan to lay carpet directly on it, than this is a no-brainer.