lowering basement floor

wantsadeckJanuary 21, 2006

Has anyone done a major basement dig? I have a 5' crawlspace that i'd like to dig down to get 8'+ ceilings. Surely such a deep dig will go below the current foundation. Ideally, i'd prefer that my house not collapse.

The one contractor who gave a bid suggested that i use a "bench foundation" whereby a new foundation is attached directly inside the current foundation without disturbing the old one. this doesnt seem kosher to me. i would expect that underpinning is the way to go.

anybody have experience here?


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YOur contractor was correct, its done all the time. The existing footing and soil remainds untouched. A new foundation is poured inside the existing one down to the new depth. It does make the basement smaller by several feet.

    Bookmark   January 22, 2006 at 12:41AM
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When digging below existing footings, you have to dig out at a 45-degree angle to the footing. This prevents soil collapse and a possible undermining of the footing.

There are ways to get around this, and the solution your contractor gave you is the easiest and probably the least expensive one.

To dig down directly under the existing and extend the existing walls straight down would require the loads be transferred off the foundation/footing, then excavating underneath, then most likely several courses of block set in place. Conditions will dctate whether block or poured concrete would be better.

This route, while possible, is more difficult and much more expensive, and it may very well be beyond the capabilities of your current contractor from the experience and engineering points-of-view.


    Bookmark   January 22, 2006 at 9:17AM
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There are almost always easier, cheaper, better ways to get more room than digging out a crawl space, and they would be more likely to add value to your home.

    Bookmark   January 22, 2006 at 7:06PM
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thanks everyone for such helpful comments. i have a few comments:

1) the contractor was NOT planning on a 45 degree slope. his proposal was to dig straight down (not under) and pour a new footing and wall. the new 8 inch wall would be directly adjacent to the current wall.

2) mag77, i wish you were right. unfortunately i live in an inner city row home where there is no land to expand and height restrictions. i think the only option is down.

3) does anybody know how to go about finding a competent and experienced person to do a job like this?



    Bookmark   January 22, 2006 at 7:30PM
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Sounds like you might want to couple a structural engineer with an excavation contractor as well as a foundation contractor.As with any trade or sub, Get references from all of the above and check on them.A reputable engineer will probably be able to refer a good excavator and foundation crew, someone they are familiar with, but most importantly, poeple who have tackled this kind of project before and are comfortable doing it.You are dealing with the single most important part of your house, the foundation. Dont skimp and get rookies or go for the lowest bid.You also might want to consider putting in an internal drain system while you are at it. It's one of those "might as well's" that could save you some wet basement problems later on down the road.

    Bookmark   January 22, 2006 at 8:42PM
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I'm not trying to be negative, but a lot of homeowners put themselves through unnecessary grief and expense with major remodels when they really should be thinking about buying a house that better suits their needs, and that's even if they do have room to add an addition or a second story.

If the only option to get more living space is to excavate below the foundation, you, your accountant, and your lawyer need to do some soul searching. That's because, from your remarks, I gather your home and those on either side are old and close together, and that means you have to consider how this work will affect the stability of your neighbors' foundations.

If you decide to go ahead, get an architect, structural engineer, and contractor familiar with this kind of work.

Good luck.

    Bookmark   January 23, 2006 at 7:46PM
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I might try looking up building movers. They pick up buildings and place them on new foundations all the time. If nothing else they should be a good source for engineers, foundation contractor ect.

    Bookmark   January 24, 2006 at 12:57PM
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Yeah, I have to agree with the comment about looking for a house that better suits your needs vs. adding a basement.

The house isn't a free-standing house; if your contractors mess up your neighbors could sue you for destroying their homes.

You may not even be able to dig into the ground if you only own the inside of the house, like most condos.

And do you know if there are pipes running under your building, under your crawl space? Just saw a show where people wanted to make their basement a foot taller, tore into the slab, and punctured a big sewer line and had raw sewage pouring into their basement.

Jeez, man, just move.

1 Like    Bookmark   January 26, 2006 at 8:30AM
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Before you do anything, find out if this is even legal in your city. Chances are it is illegal simply because the house is a row house.
And..if it is legal by some wierd quirk, chances are you will have to get the permission of your neighbors whose homes are attached to yours...and, IMO, few folks in their right mind would agree....all risk..no gain.

    Bookmark   January 31, 2006 at 1:27PM
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Can someone provide some insight on this project.

I'd like to lower a section of our basement floor to allow more ceiling height for a gym space. The basement is poured concrete, poured footings and walls. The home is 30 years old. A footprint of 1300sq ft. I would just want to lower a section - 800sq ft - of the basement. This section does not need to butt against an outside wall. Would this project be possible and what estimates (range)could you provide.

Niagara Falls, ON

    Bookmark   March 23, 2007 at 7:44PM
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I live within the city of Washington, DC in a 100+ y/o rowhouse. Digging down into a basement to add more usable space is very common practice. There are dozens of companies that do this kind of work. Some small, many not. Digging down is much cheaper and "easier" than adding up or out. In DC you can only go up one floor if you can step it back from the street the same height so if it goes up 10' then you step it back 10'. Similarly, you can only occupy 60% of your lot with a structure. If you house currently takes up more than that, it is grandfathered in and you need not worry about replacing an existing structure with a like-sized structure.

As for signing off with your neighbors. In DC, you do not need to have them sign off. It is a couretesy but not a requirement. It is best to get a structural engineer to look at the project along with the builder and hopefully architect. Make sure that they are licensed, bonded and insured. Check your home owner's policy as well to see what you covered for. You do not want to upset your neighbors.

Underpinning is the best option but adding a "bench foundation" is what is usually done the way you described it. I have seen dozens of basement dig outs and they hold up very well with no damage to neighbors.

Finally, many people who do not live in urban areas do not understand what a rowhouse is. Many people have more space around their house so the concept of digging down as last resort is not understood. It is cheaper to go down than up and out when you are in a dense urban environment like, NYC, Philadelphia, Baltimore, Washington, DC, Boston, and other northeast cities. Buying a bigger place costs more and may not be feasable. I have a 30 minute walk to work that I love, driving to work is as crazy a thought as adding up a floor in my house.

Some local companies to consider are Case Design in MD; Horizon Builders in MD.

Here is a link that might be useful: Top DC Area Remodelers - Washingtonian

    Bookmark   May 18, 2009 at 11:51AM
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...many people who do not live in urban areas do not understand what a rowhouse is.

I'd say the typical reader here is either on acreage or in a small town and hasn't a clue to the space constraints and value of downtown land. Try building a new home one foot from the lot line with another house one foot away from the lot line on the other side!

I'm in Toronto, where deepening basements on old homes in the downtown areas is common. Personally, I've done it once, on a three storey attached home on an 18x80 ft. lot. We used the bench method. No permit was required but the work was in accord with the plan of a structural engineer. Lots and lots of rebar around the "bench" and back down and under the new concrete floor. We also changed the drains.

I'd run from any so-called contractor who doesn't know the basics of cutting the slope at 45Ë. Not only does this risk a collapse, but it doesn't provide for a proper bearing surface for the bench.

    Bookmark   May 19, 2009 at 2:16PM
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if you have a 20 X 20 foot wide crawl space and have to cut at 45 degrees, what are you left with for living space?

    Bookmark   May 19, 2009 at 2:22PM
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"if you have a 20 X 20 foot wide crawl space and have to cut at 45 degrees, what are you left with for living space?"

About 11 x 11 if the 5 foot crawl is dug to 8 feet, with perimeter benches all around about 3 feet high.

A decent amount of storage space, not as useful for living space.

    Bookmark   May 20, 2009 at 10:38AM
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I only lowered the floor about 8". And the main purpose of the bench was to support the foundation that had been undermined by the po. For three feet, better to underpin a section at a time.

    Bookmark   May 20, 2009 at 2:41PM
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