lowering basement floor

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  #1  
Old 01-28-06, 05:49 PM
dsw
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lowering basement floor

my concrete basement floor is about 6'6" below the 1st floor joists. we'd like to finish the basement but realize we'll need to lower the floor to meet code and make the space comfortable.

can anyone offer advice on cost (about 700 sq feet worth), process, and preparation? i don't expect to be able to do this myself...
 
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  #2  
Old 01-29-06, 04:31 PM
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You may not be able to lower the basement floor at all.

Codes prohibit excavating lower than to the bottom of the existing footer.

For example if your footer thickness is 8", you can only dig down 8" maximum and that's it. Otherwise you disturb the bearing capacity of the foundation walls and compromise the structure.

8" may not give you ample headroom to meet minimum finished ceiling heights for a basement which is normally 7'.

The reason is that you will need at least 4" of washed stone base under 4" of concrete floor. If you had a 4" slab on top of the existing footer, dug out 8" then installed the necessary new 8" of stone and concrete, you only have increased your ceiling height by a total of 4" to 6'10"...still too short to be issued a permit in most locations.

Unless your footer exceeds 10-12" in thickness, you won't be able to excavate deep enough to meet minimum headroom requirements for habitable basements.
 
  #3  
Old 01-29-06, 07:28 PM
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Underpinning

DSW,

Underpinning is allowed ALMOST anywhere. Issues that usually arise are existing water tables. Check with your City for verification of this. In some cases, the width of your new footing would have to be the entire width of the underpinning. This has to be certified by a structural engineer prior to the undertaking of such a task. In cases where an 8 inch block was used for a basement, the underpinning would have to consist of using a 12 inch block with vertical rebar and horizontal reinforcement. How this is applied, such as leaving the 4 inch to the outside exposed means that the cores must be filled and waterproofed. Inside exposure means a larger gap between newly framed walls and the 8 block, both of which are not a problem. Existing soil conditions and overall height of the new wall will determine this.

With a basement this low in ceiling height, that may be an issue unless others around your area have deeper basements.

Underpinning is not cheap and I have had to do many of them. In my area, it can run $250 - $400 per lineal foot depending on the depth below existing footing. The labor cost is high for this type of work. However, the end result can mean a very livable and enjoyable space.

Process involves excavating as required under existing footing in alternate 4-foot sections. This is once you have removed portions of the existing slab. As you can see, this can be a long process. Alternative ways are to literally support the existing home with beams and jacks to do more of the work in less time. This allows for more machine digging rather than hand digging and only if access is available for a small backhoe.

Pour reinforced concrete under for new footing and build new block wall from top of new footing to bottom of old footing.

Break off existing footing projection.

Installing a drain tile system with sump pump is highly recommended. This will include the p-gravel, 4" mesh covered piping, sump basket and sump pump.

Then pour new slab over good substrate.

The cost I mention does not include making this inside block look good, especially at the break off of the existing footing.

Here is one example of before and after,

http://dougaphs.smugmug.com/gallery/1165373

Hope this helps!
 

Last edited by Doug Aleshire; 01-29-06 at 08:23 PM. Reason: Example of underpinning given
  #4  
Old 01-30-06, 10:41 AM
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Doug is correct in that just about 'anything' is possible in construction....even having foundation walls engineered to extend below an existing footer...given enough money.

But the real issue is cost to benefit ratio.

One can dig to footing grade with no major issues, but once you need to go deeper than the existing footing, you need to have engineers involved to assess the soil conditions and assess the foundation and loads and to design the necessary changes just to be able to get the permits.

The costs will exponentially rise from that point turning what can be an already pricey job to simply lower the basement a few inches to one where you may end up needing to reconstruct the entire foundation depending on soil conditions, unbalanced backfill heights, seismic zones and need for structural reinforcement.

That is why once you need to dig past the existing footing, all bets are off.

Sure, it CAN be done.

Doug gives a good example that it can be.

The real question is can it be done at an affordable price and one that will return you something for your investment.

In most cases it won't and it would be cheaper to add 'up' or 'out' not 'down' to get more valuable living space.

That's whay I say, if you can't get the space you want by simply digging to the existing footing depth, you probably won't be able to turn this into habitable space given the costs involved.

But Doug is correct.

Digging below the footer IS 'possible'.

The real question is: is it 'practical'.

And I don't think anyone can answer that question for you here.
 
  #5  
Old 01-30-06, 10:53 AM
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manhattan42,

Right on!

Practical is the best word to describe it. The issue of the expense compared to real value it will provide is the issue between DSW and the banker. Also a good real estate agent could provide some insight before shelling out lots of money.

For those projects that I have done, it has increased their value as they made more "habitable space" which met Code. The duration of the payback to recoup the expense is not immediate.

I agree that the money for this venture could be better spend going out or up. The issues that this would involve would be a need for a more efficient and larger heating system.

The final issue is, what can DSW really afford?

Thanks for the followup!
 
  #6  
Old 02-03-06, 05:11 PM
dsw
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thanks for all the posts. very insightful!

as for what we can afford, given what you guys have said, it sounds like it may be out of our budget just for a basement. i guess it all depends on whether or not we would have to reinforce the foundation.

is there a simple way to figure out how deep the foundation runs? is it just a matter of taking a sledgehammer to a corner of my basement, clearing the concrete and then digging down a few inches? (actually, hopefully it would be digging down about a foot.)
 
  #7  
Old 02-03-06, 05:15 PM
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dsw,

Sledgehammer and then you can start shoveling and see what you come up with.

Hope this helps!
 
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