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jeffhoward001 01-06-2009 05:34 PM

Cutting out a concrete basement floor
Hello -

We have an 800 sq ft basement that is currently about 60% finished. We have a few remodeling plans on the horizon, some short-term (in the next year) and a bigger long-term plan.

For background, I'll give a quick description of the house and the long-term plan/vision. It's a 1600 sq ft house with 800 on the main floor, and another 800 in a day-light basement. The basement is about 60% finished, with a concrete floor. 1/2 the walls are concrete, and the other half are sheet rock (pretty standard day-light basement config in the side of a shallow hill). The biggest down-side to the basement is that it was built to have 7 ft ceilings. This doesn't bother me, but in Oregon, you need to have *at least* 7 ft of clearance in the basement to call it "livable sq footage", and we'd probably go with 8 ft ceilings when we do the future expansion (more on this below).

With the prospect of having kids in the future, we'd like to push the east and south walls of the house out about 10 ft for the extra sq footage. That's our "long-term" vision, but we don't have the money to do it right now. This presents an issue with the 7 ft ceiling described above as we'd want to do 8 ft ceilings in the new addition, creating a step between the old and new sections of the basement.

That's the long-term vision, so in the short-term, we'd like to finish the basement and add a second bathroom and convert the dry bar to a wet bar (within the existing basement sq footage). This would require cutting into the concrete floor for the plumbing, and doing some rework on the false-walls.

After thinking about cutting into the concrete floors for the plumbing, and the difference in ceiling height between the old and future sections of the house, we starting thinking... Why not just cut out the entire concrete floor now while the basement is mostly unfinished. I've heard of this being done on commercial building renovations, and seems logically possible.

After thinking through the details, I figure we could demo the false-walls and take everything out of the basement (not a big deal because we're not really using the space right now). Hire a concrete cutting company to come out and cut the floor into manageable squares that I could carry out (please insert any advice here if there's a better way than hiring the concrete cutters).

........ and that's where I get stuck... We would probably want to remove 2 ft of soil to provide the extra ft of ceiling, and enough room to pour the new slab. How would we extract the dirt from inside the house? On the commercial building renovation they were using a Bobcat, but that seems a little excessive for 800 sq ft.

Anyone here have any ideas? Is this a ridiculous idea, or common question?


- Jeff

DUDE! 01-06-2009 06:02 PM

Not to bust your bubble but you may want to pay a contractor to come out and give you a ball park figure for what you are wanting to do, let him know ahead of time you don't plan to do the work right now, hense you need to pay him something for his time to come out. You are talking big work here, don't think you want to be carring out concrete peices, that'll get real old real fast. You bought a 800 sq. ft. house and want to convert it to double the size, nothing wrong with that. And by no means is there anything wrong about future planning.

AllanJ 01-06-2009 07:05 PM

Don't forget, when you excavate your basement floor, you will go below the level of your foundation footings. This could cause the soil underneath and also the foundation itself to collapse into the excavation. Not sure how the foundation is shored up to prevent collapse.

And, if you had and needed a perimeter drainage system along the outside of the foundation, you would need to install another such system along the inside of the foundation below the level of your new basement floor.

jeffhoward001 01-06-2009 07:31 PM

Thank you for the replies guys... Agreed it's a formidable task, and we'll definitely have a contractor give us a quote on the different aspects of the project as we start to solidify the details. We're definitely still in the "information gathering" phase right now.

I'm not too concerned about the labor of breaking out the concrete. It's no doubt going to be hard work, but it's something I've done before on an outside slab and I imagine it's the same principle. My main concern is having the concrete cut properly around the edge of the foundation and the footings for the beams. We don't plan to do anything without a contractor's approval on what we can/can't should/shouldn't remove with respect to the concrete floor.

Any manual labor portions of the project aren't a concern since we're not on a tight time line (since we're not using the basement right now). I'm more concerned with doing everything right, making sure that the cost/benefit ratio is there, and that we take all the necessary precautions to protect our house in the process.

Edit: I did some research on the footing, and it sounds like we may need to reinforce while excavation. This is definitely something I would discuss with a professional before removing any soil.

- Jeff

Has anyone heard of this being done?

- Jeff

concretemasonry 01-06-2009 08:14 PM

When you remove the slab from a basement, you are removing a necessary structural element. The slab should be on the footing and prevent the foundation walls from moving inward.

The reason the slab should be on the footing (a floating slab) is because the common practice of some contrectore (mainly poured wall contractors) to not put any reliable connection between the footing and the wall that can resist the soil pressure.

Going below the footing is especially dangerous.

If you are interested enough in the resale, you may want to make sure the replacement slab and modifications are done properly and have some design professional to assume the responsibility and help get the permit that will be required now or when you go to sell.

jeffhoward001 01-06-2009 11:35 PM

We're definitely going to hire a contractor to assess and plan before doing anything, so I'm mainly trying to get a picture of what we're up against before putting the time/money into hiring a contractor.

Thank you for the reply concretemasonry, that definitely helps. It makes sense that structurally the slab would be functional in keeping the footing in place (especially on the daylight side of the basement). It sounds like how we handle the footing is the most important detail. We plan on going down about 24" (2 ft). I've done a lot of reading online, and it sounds like other people have gone 12" down with a small "curb" where they left 6-10" of soil. We definitely would not want a 24" curb, so I would need to do something else.

Is it possible to cut out and excavate in the middle, keeping at least 24" of soil around the parameter, then dig out sections (e.g. 6x6 sections) and pour them individually? In theory, this would keep solid ground around the footing, and allow us to pour new concrete around/under the old footing.

Garasaki 01-07-2009 08:47 AM


Originally Posted by jeffhoward001 (Post 208423)
We're definitely going to hire a contractor to assess and plan before doing anything, so I'm mainly trying to get a picture of what we're up against before putting the time/money into hiring a contractor.

I think what you may be looking for is a structrural engineer. You could schedule a half hour meeting with one to answer specific questions you have. If you have the original plans for your house, it'll make your job a lot simplier. If you don't have the original plans, try to see if you can dig them up thru your local municipality. I'd guess it'll cost 75ish bucks and you'll walk away with real answers to your questions and that should allow you to speak to contractors to price out the work. (FYI it's not really a contractors place to design something. Architects/Engineers serve that role and also assume responsibility if the design fails. Contractors responsibility is to build and they assume responsibility if the worksmanship fails. Contractors will not assume responsibility if they design something and the design fails)

This can be done. Anything can be done to a building if you throw enough money at it. Perhaps you'd also like to replace all your copper wiring with solid gold wiring? It's possible. Wether it's wise (well cost effective, or adds value) or not is a whole different story.

Are there any posts in your basement? Where do the stairs come down? (on the "edge" or right in the middle?)

Chances are, if you go thru with this, you will have a section around the outside that remains at it's original

4just1don 01-07-2009 09:49 AM

Is there ANY way,given the current housing market,,,you can 'buy' a larger house WITH a deeper basement?? Trade up so to speak. Another idea would be to 'buy' another(with not alot down) and rent this one (for as much as your payments are)to a reliable couple that would take good care of it,,till the market improves,,then sell it??

After you do the due diligent homework of propsed work,and analyse THAT cost,,,it would be WAAAAAAY cheaper IMHO!!! The hardest part is going to be supporting center beams while excavating and pouring new footings and center support. As well as the outside ledge left over from footings area!!

Unless your in some sort of different market,,thre are some whale of a deals out there!!! 'Smartest' strategy would be to 'BUY' all the homes you can afford to,for now. Depending how your 'rental' markt is at this point in time!! This housing downturn and economy isnt going to 'last' forever!!! Easiest path to get where you 'wanna' go!!!

yesitsconcrete 01-07-2009 09:57 AM

UNLESS you can find a pe who's also a contractor, those're separate vocations,,, i wouldn't use the former for conflict of interests but would CERTAINLY call in a pe ( structrural ) 1st,,, not sure i'd pick an arch but that's just me & my background.

design/bld contractors are very common.

Garasaki 01-07-2009 10:09 AM


Originally Posted by yesitsconcrete (Post 208608)
design/bld contractors are very common.

This is true.

I think you will find many of the design/build contractors have someone else assuming legal responsibility for the design portion. This fact would be found very deep in the fine print.

losttool 01-07-2009 11:02 PM

This is an engineering job, Civil or structural. If you mess up the foundation it's game over. I liked the suggestion above trade up to a bigger house.:thumbsup:

jeffhoward001 01-08-2009 12:34 PM

It seems people always make the suggestion of buying a bigger house, but (IMO) that discounts so many factors that go into buying a property. We all buy houses as a form of investing... yup, there it is... But on my list of "reasons" for buying our current home, the "as an investment" reason was roughly #6 on the list... We bought in a nice, older area of Portland Or where nearly all the houses are around 50 years old, all the lots are around 10K+ sq ft, and you're less than 10 mins for downtown.

So the theory of "buying up" into a biggest house really doesn't apply unless you either a) aren't really in love with your neighborhood or b) Are will to put yourself another 100K in debt to buy a biggest house in the same area.

I'm not saying it's a *bad* suggestion, I'm just surprised people are so quick to make the suggestion, especially on a DIY forum!!

Maybe my family just gets attached to their houses, or have a stronger stomach for remodels/expansions, but we've expanded/remodeled every house I grew up in and significantly increased the value by doing the majority of the work ourselves (In fact my Dad is adding about 1000 sq feet to a house built in the 1920's right now...).

Most of my friends from college were engineers, so I'll call one of them tonight to recommend a structural engineer to come take a look at our place.

Sounds like concretemasonry is speaking from experience... Is anyone else speaking from experience in trying a project like this, or just throwing in their 2 cents?

- Jeff

yesitsconcrete 01-08-2009 12:52 PM

raising hand - my own 1864 house in upstate ny :thumbup: + did several similar projects in nj & pa

jeffhoward001 01-08-2009 07:06 PM

yesitsconcrete -

Thanks for the reply. So we'll definitely be hiring a structural engineer to make sure all necessary precautions are taken, and that we do the project right. My thought is, if I (or "we", I have plenty of friends to help) do all the back-breaking labor (concrete extraction and excavation) would the project still be expensive? As long as a professional tells me exactly how to reinforce the foundation to prevent damage, it seems like a pretty straight-forward project, albeit ridiculously time-consuming with lots of back-breaking labor. But I'm not in a hurry to get the project done as it's currently unused space in our house, and the pay-off is huge as we'd be doubling our sq footage.

Can you give me a few examples of how a project like this goes? Everyone seems (understandably) cautious about how complicated this can be, but can you give me a few examples of where it gets complicated? Is my idea of doing it in sections ridiculous?


- Jeff

yesitsconcrete 01-09-2009 03:38 AM

my house was easy - 2 friends in competing house-moving biz's split the job & we only lift'd 1/4",,, took time to properly support everything ( fireplace, boiler room, downstairs bathroom, etc ) but actual ' lift ' was easy ( radios, observers, experienced guy on the hydraulic jack manifold ),,, our nj pe partnered w/us,,, his method was steel-rein piles bearing against existing footing,,, wasn't particularly difficult - lots of bull labor, many curses in spanish & english, & we wore out many, MANY 5gal bkts til we got a small bobcat w/tracks :thumbup:

sectioning the work might work but i'd have to think about it,,, our cruise boat leaves tomorrow from fl & there's a bid opening at 2 this afternoon,,, maybe dick's got some ideas that'd help.

even i can't get 5gal of wtr into 4gal bkts :)

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