Subscribe or Renew Membership Subscribe Renew

Adding a Basement Room under a slab

RandyR's picture

HI, I would like to talk to anyone who has added a basement room under an existing slab, by sawing an opening in the thin part of the slab and excavating underneath, then added a floor and walls, etc. It seems possible, but some questions I have: is the thin part of the slab strong enough to stand without the support of the soil underneath, or would it have to be reinforced/supported somehow? Will the excavation, even though it would not extend to the slab perimeter footings, disturb the support of the footings enough to cause foundation movement? Seems to me a separate exit to the outside would be needed in the interests of safety (I intend to use the space as storm protectionas well as storage).

(post #97121, reply #1 of 33)


Welcome to Breaktime!!

If you click on your name above (where it says "To: RandyR"), you will open your profile.

We would appreciate it if you could fill some of that in, especially where you are located!!


Also, you posted this thread in the wrong folder. This should probably be in Construction Techniques (but I don't think anyone would complain if it was in General Discussion. Please feel free to post again in one of those folders.

I'm not qualified to comment on what you are doing...but I suspect that someone will be asking if in fact we are talking about the slab your house sits on (sounds like it, but your not specific).

Rich Beckman

Another day, another tool.

(post #97121, reply #2 of 33)

you cannot do it, without the extra rebar in the slab itself to control brending and the extra thickness needed, it would fall in on you, also the footer would not be deep enough to suport the building. Now if you could jack the house and change the foundation that would work.

(post #97121, reply #3 of 33)

Unless you want to join the ranks of the Darwin Award recipients or just want to make people pancakes.




(post #97121, reply #4 of 33)

I disagree. I think it is possible to add a basement under a slab on grade house, and do it safely, and end up with a properly engineered structure. But it would be fabulously expensive, probably more than tearing the existing house down and starting over.

(post #97121, reply #5 of 33)

Just what correctional facility are you enrolled in? We may need to look at some soil samples. I understand Alcatraz is solid rock. Would'nt it be easier if someone baked you a cake with a file in it?

seriously though, it would be way more expensive than moving the house out of the way or building a new foundation next to the house then move the house onto it.

I do know someone who had a full 8 foot high cellar dug under half of his house. The finished area was about 24x30. The slab has to go to begin with. They excavated enough of the perimeter to pour a footing and lay about 10 feet of block wall at a time. After they had their outside perimeter foundation system in place  they erected another block retaining wall down the center of the house and poured the basement floor. it was a lot of work back when labor was cheap!

(post #97121, reply #6 of 33)

I've put basements under "regular" houses but never one built on a slab.  I'm sure it can be done but the cost would probably be astronomical.  If you're really serious about this talk to a good structural engineer.

(post #97121, reply #7 of 33)

HI, I saw recently a web site where a man described a prefab storm shelter which was installed in his garage by sawing a rectangle out of the slab and excavating enough soil to set the shelter into the hole; it sat underneath his car when he parked it in the garage; sort of like a "grease pit" with a steel lid. This gave me the idea. I wondered, though, if they realized: if the car was in the garage and a storm hit, the car would have to be moved out of the garage before the shelter could be used.

Thanks for all the replies and ideas. I know this would be expensive. People in my area don't typically have basements, and few of the builders know how to build them, so I was afraid to try to build one. I sure wish I had one now, though.

(post #97121, reply #8 of 33)

See there is no profile info yet?

For what you want to do location is everything, soil type, drainage, etc.

As others mentioned having done, I've added basements under frame houses with crawl space, which is 'easy' compared to what you are suggesting. Your BIGGEST problem is that you are not just needing a 3 to 4 ft extra wall in the basement, you have the full 7 ft depth to go.  If you do not know how to design for soil shear characteristics and retaining wall design (or unwilling to learn), I'd shy away from a full basement attempt.  Without knowing your soil type, even a hole with new wood floor over the  slab removed above to within 4ft of wall, and leaving 4 ft dirt embankment underneath during new basement wall pour may be very risky, as in sink hole. 

(post #97121, reply #10 of 33)

What I was thinking of was removing the slab inside the under-stairs closet, where the basement stairs would have been anyway. I only needed about 8'X10' which would fit well within the space under the den; there would not be outside access, though. Also, I added profile info.

(post #97121, reply #11 of 33)

Oh well hell yer Cajun, go ahead and do it! 8)








(post #97121, reply #13 of 33)

Wrong, I may be redneck, but not Cajun (north Louisiana). Besides, anyone knows a Cajun can't have a basement, it would be full of water.


(post #97121, reply #14 of 33)





(post #97121, reply #15 of 33)

but if a cajun had a basement full of water, he be making money as a crawfish farm

(post #97121, reply #16 of 33)

What's the difference between a coona$$ and a dumba$$ ?  The Sabine River. 

Ok, for those of you who don't get it, the Sabine separates Texas from Louisiana.


I'm sorry, I thought you wanted it done the right way.

I'm sorry, I thought you wanted it done the right way.

(post #97121, reply #12 of 33)

Just a thought for you. It might be possible to hire a house mover to slide beams at critical points under the slab. The by supporting the slab you could dig out from under. You should consider the lumpiness of the slabs belly. Depending on  the age of the house,the underbelly might be somewhat smooth due to insulation.

There is still a lot of caveats. Check with your city inspectors and see if anyone has tried the same. They may have some helpful ideas if approached on a good day. bring donuts.




(post #97121, reply #19 of 33)

Since nobody elese said it, I will, "Welcome to BT."

I'm thinking that your idea has some soundness, but not the intended execution.  You will not enjoy excavating by 5 gallon bucket through the stairwell.  No way no how (not having any stairs in the stairwell being amongst the least of your problems.

If we use your 8 x10 space as an example, you're stiull looking at 9' to 9'-6" of excavation.  That's 720 to 760 cubic feet (about 27 yards of material), or more than 350 bucket-fulls.  This does not include any backcutting or "angle of repose" cuttting required. 

So, we get this hole made, now it will need about 2 yards of concrete for footings and slab, about 30-35 buckets worth, back down the hole.  That's before getting to the walls.  Oh, and we'll be needing rebar in the hole, too.  Wrangling #4 & #5 bar in 120" lengths through the house and down in the hole does not sound fun.

Then, we have to get wall up.  They could be block (probably have to be, to get back fill behind them, and to get pilasters to support the slab above).  That mean geting mortar in quantity into the hole, down the narrow stair case opening.

Mind you, this is all happening in the very dark, likely damp (even in NawLous'ana) soil, in cramped muggy conditions.  Oh, and because Murphy requires it, you will have a soil pipe, the main house water line, and at least one other utility right in the way to cope with, too.

That's why you may have gooten some "noise" in this thread.  If you are really keen on this, do as was posted before, cut the entire "ceiling" of the proposed room out ahead of time.  Probably ought to wall off the room the work is in from the rest of the house while you are at it.  A "temporary" door for construction access will not go amiss, either.

That's sounding like a lot of work, though, isn't it?  Yeah, a couple of week's worth--better than ten or more tunneling like it's the Great Escape, though.

Storm shelter does not need an exterior egress, per se.  It ought to have it's own air supply, though.

Occupational hazard of my occupation not being around (sorry Bubba)
I may not be able to help you Occupational hazard of my occupation not being around (sorry Bubba)

(post #97121, reply #20 of 33)

"Since nobody elese said it, I will, 'Welcome to BT.'"

So what am I?? Chopped liver???

Maybe you have me on ignore!!

Rich Beckman

Another day, another tool.

(post #97121, reply #21 of 33)

close guess...

Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming

WOW!!!   What a Ride!

"Some days it's just not worth chewing through the restraints"

(post #97121, reply #22 of 33)

"close guess..."

OK, but if it's all the same to you, I'd rather not know how close.



Rich Beckman

Another day, another tool.

(post #97121, reply #23 of 33)

Probably never read my signature line either



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #97121, reply #25 of 33)

Maybe you have me on ignore!!

Nope, I don't; just clean missed it in your post.  Mea culpa.

Occupational hazard of my occupation not being around (sorry Bubba)
I may not be able to help you Occupational hazard of my occupation not being around (sorry Bubba)

(post #97121, reply #24 of 33)

You really need to learn some things about the strengths (and weaknesses) of concrete before you do something really foolish there.

Very generally, concrete is great for compression loading but is very poor in tension.  That's why slabs have rebar - the steel provides the tensile strength that the concrete doesn't have.

If you undercut an 8'x10' area under your slab, that portion of the slab has no underlying support (from the soil) and is totally dependent on the rebar to keep it from collapsing.  In the words of Clint Eastwood......."Do you feel lucky?"

You should talk this over with a structural engineer and let them explain the hazards of your plan - as well as ways to safely do this.  I'm sure that it can be done, but it will probably be expensive.

(post #97121, reply #26 of 33)

Maybe just save yourself some headache (literally) and a whole bunch of money by building an addition to your house and putting the safe room basement under that. Working from the bottom up is so much easier than working in the dark all hunched over waiting for the trickle of earth to become an avalanche.


(post #97121, reply #27 of 33)

Shhhh, He doesn't want Col Klink to know he's building it!



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #97121, reply #28 of 33)


So that's why there was mention of 350 buckets of earth to move. One at a time, shaken out through the pants leg and shuffled into the dirt of the parade grounds.

(post #97121, reply #29 of 33)

If the basement must be under the existing slab, then rather than excavate through the slab and attempt to keep the weight above from falling and the earth from trickling down the sides of the pit, an external approach might be more feasible.

Pick a spot on the perimeter closest to the basement location and tunnel inward, building your shoring and then your footings and foundation walls as you go. Any weight above can be successfully supported if done incrementally as can preventing sideways collapsing of the tunnel walls. Think of it as a horizontal application of vertical slip forming.

(post #97121, reply #9 of 33)

Randy, just dig the room under the slab. Then, cut the slab the exact size of the room. Leave the four corners intact and get three buddies to simultaneously finish the corner cuts.

KABLAM! The slab falls into the hole and you have an instant poured floor on your new lower level roof!

Fill in the top with advanteck.


Just because you can, doesn't mean you should!

Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!


keep looking for customers who want to hire  YOU.. all the rest are looking for commodities.. are you  a commodity ?... if you get sucked into "free estimates" and  "soliciting bids"... then you are a commodity... if your operation is set up to compete as a commodity, then have at it..... but be prepared to keep your margins low and your overhead  high...."

From the best of TauntonU.

(post #97121, reply #17 of 33)

Water and soils types would be first on my list of concerns to check out before starting. But if you have a lot of time on your hands, I don't see that it wouold necessarily cost you that much - but now if I were doing the work - it would cost an arm and a leg.

What kind of engineering? Doesn't sound liek a PE.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #97121, reply #18 of 33)

Not civil; mechanical.


your spot on (post #97121, reply #30 of 33)

you can do this easy as long as your not under a load berring wall simply excavate the dirt it should be decent dirt or your house is on piles second put a beam of wood or metal in the center of your room just for gigles your floor or celing for the basment slab should have rebar if it dosnt its old but no matter concreat does have some tinsle strength and shoring will eleviate this.

all these asshats claiming to be enginers and such need to go back to the basics of physics its not like you building under cardboard my bigest concern that deep is water but a simple sump pumped into your existing floor drain should work just dont over complicate it miners have been digging caves for a very long time so enginers hehehehe go figure out how to build a floating house

and just for giggles lets assume that the ground is baby crap why not just build shoring walls and shot creat using the dirt wall as your back forum not pretty on the back side but hey it works
........been there done that