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Converting crawl space to basement

| Posted in Construction Techniques on May 7, 2002 10:17am

We’re planning to convert a crawl space under a one-story house into a full height basement for a workshop. (House is on a slope and already has an opening on the low side big enough to get in with a bobcat to dig the dirt out.) This is North Carolina. Foundation is brick; goes down about 4 feet; no concrete footing, just a sort of soldier course of bricks laid crosswise.

Wondering about best way to excavate and extend foundation wall down far enough to get the desired height. Obviously can’t undermine the existing foundation, so we’re thinking to a poured concrete wall that sits just inboard of the existing foundation and extends up to overlap the existing foundation by a foot or two.

Main worry is stability of the final foundation wall. As it is now, with the crawl space there is as much dirt on the inside as there is on the outside, so the wall is stable. Once we dig away the inside, we want to be sure the pressure from the earth outside doesn’t make the wall buckle.

Anybody ever done this? Sure would appreciate some ideas from someone who has already been down this road.

Thanks, Jay

 

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  1. Gabe | May 07, 2002 10:44pm | #1

    Are the exterior walls of the house brick as well or was the brick used as a filler for a masonry foundation for the house to sit on?

    Is everything else conventional on the house?

    Gabe

    1. JayBeaumont | May 08, 2002 02:39am | #5

      In response to Gabe's question, it is only the foundation that is brick, the rest is wood frame and clapboards. And as a matter of fact the mortar holding the bricks together is pretty crumbly which is why we're leery about how stable it is.

      I suppose our main concern is to excavate as much as possible so that the new wall inside the old foundation is not so far inboard that it takes away too much from the available space, but not excavate so much that the old wall collapses before we can set up the forms and pour concrete.

      We're trying to do this on a tight budget, so jacking up the house and doing everything the way we wish we could is probably out of the question.

      Jay

      1. Gabe | May 08, 2002 03:59am | #6

        I would encourage to re-consider your options.

        Putting the house up on cribs to do it right can be the cheapest and best way to maximize your return if your were to sell.

        What you do is excavate for the cribs that will support your house first. That way, the work doesn't have to be redone.

        After the house is supported. You can use bigger equipment to remove everything, including the old foundation right down to final grade. You will have the room to work safely and will be able to install proper drainage and waterproofing to your new full height foundation.

        Then when alls done, you lower the house onto it's first class foundation that will last several generations and will require no excuses or explainations.

        Give it a thought. Sometimes doing it right is actually cheaper and faster than trying to work around it.

        Gabe

        1. JohnSprung | May 08, 2002 09:17pm | #13

          They did a complete foundation replacement on a building near me recently, and they had a house mover lift it up a couple feet on steel and cribbing.  It could have been done in sections, but this was a lot faster because they had good access to everything at once. They did put it back down to its original height. Another advantage to having a house mover lif the place is that you can pick a new height for the whole house.  By going higher, you reduce the amount of digging you have to do, and what you save on excavation may offset the cost of the house mover.  Displacing air instead of dirt could be the way to go.

          -- J.S.

          1. Gabe | May 09, 2002 01:30am | #14

            "Displacing air instead of dirt could be the way to go".

            I like that, a lot.

            Gabe

      2. slash | May 08, 2002 04:07am | #7

        Read up on tuck pointing masonry walls. This is a repair you can do yourself. While it can be labor intensive, the material is not very costly. Practice inside the crawl space. Also you could use a fiber reinforced mortar mix that is easily applied but costs more than standard mortar mix and the finished look is more like a stuccoed wall. If your foundation walls consist of short thickened walls supporting wood beams separated by single width ( wythe ) brick walls you have a pier and curtain wall foundation. This is a common foundation for older homes in High Point. What this means is the short thickened walls are supporting the house and the single wythe walls are nonstructural. By the way, I take back the idea of adding a story. Is an addition out of the question?

        1. junkhound | May 08, 2002 05:26am | #8

          1. You are very fortunate to be able to get in a bobcat, I've helped with one and did one by myself, with buckets up the stairs on one and wheelbarrow out the back door on the other (did lose 24 # doing it).

          2. Do not try to do the whole house at once, try to stay to 1/2 a wall or even 6 feet at a time, you should be OK that way. If you get a bobcat for just 1 day, dig everything out down to the footing depth but leave a 3 foot berm - remove that by wheelbarrow a few feet at a time as you pour concrete.

          3. Budget or not, don't skrimp on concrete.  The one I did 3 years ago used $2000 worth of concrete for 900 sq ft basement. Poured wall 1 foot thick and left 1 foot dirt, used space above for 2 ft deep shelves, cabinets.  Survived the 6.5 earthquake last year with no movement.

          My dad got away with a 6 inch concrete wall cut down flush with existing  brick wall as you describe in the '50s in IL, he did totally tuckpoint first. Central IL has even 100% clay to 10 ft down, parents still live in the house and there have been no problems, but it still scares me after I'd learned a few things about soil dynamics.

          4. BEWARE of water, put in exra drain to daylight inside whatever berm you leave and be sure to vibrate your mix to avoid voids.

          1. JayBeaumont | May 08, 2002 05:36am | #10

            For junkhound

            Thanks for the encouraging suggestions. We are coming around to thinking of doing this in short segments.

            By the way, around here we can rent the bobcat for only $800 for a whole month, which seems like a pretty good deal. Took a while to get the hang of it, but only a couple of days to get most of the rough excavation done. Will use it again later in the month to bring in the gravel base to go under the floor slab.

            Jay

          2. ClevelandEd | May 08, 2002 06:36am | #11

            It is quite common around here to find basements that were built from crawl spaces using the "short segment" approach.  Based on what I've seen it is a way for a capable individual to get the basement built without incurring those higher costs. 

        2. JayBeaumont | May 08, 2002 05:29am | #9

          For Slash

          Thanks for the reminder about fiber reinforced mortar. Will keep that in mind. There are probably a few spots where we will have to reset bricks.

          As for an addition instead of this crawl space project, we're on a rather tight lot, so it's either go up or go down, but not out. This is going to be a workshop with heavy woodworking and metal working equipment, and the on-grade, walk-out access will be ideal for dragging lumber and metal stuff in and out.

          For Gabe; yeah I know what you mean, but it just isn't in the budget. Besides, this is not a house that will last another 100 years. We're trying to save enough money now so some day we can afford to buy the 100-year house, but in the meantime we need more workshop space.

          Jay

        3. larryfeir | May 08, 2002 04:03pm | #12

          Hey Slash,

          If the "curtain" portion of the wall is truly non structual than this could make the job much less risky. The focus would be on the "piers".

          Do you know of any resourses where I could read up on this Pier and curtain construction technique?

          Thanks,

          Larry

  2. slash | May 07, 2002 11:20pm | #2

    Hello from High Point. The residential code book only addresses foundation walls holding back unbalanced fill, not forces applied from footings. If you want to maximize your space you will have to get an architect or engineer to size the interior foundation walls. Otherwise think about how far down the slab on grade will be below the brick footing ( is that pier and curtain wall ? ). This distance will be how far your interior foundation wall will have to be from the existing foundation wall. In general, this is not an easy operation. Going up a story will increase the value of your house more than adding a basement. But if you really want a basement..............it would be a good idea to check the condition of the existing foundation. Maybe it would be better replacing your existing foundation walls with basement walls.

  3. UncleDunc | May 08, 2002 12:22am | #3

    Considering the amount of space you may have to give up to keep solid ground under the existing foundation and the amount of hassle involved, I would think about jacking the house up a few inches and putting a regular foundation under it.

    1. JCarroll38 | May 08, 2002 12:51am | #4

      Jay, I live in Durham, NC and also have a brick foundation. Like you, I intend to make a portion of my crawlspace into a basement. Right now, it's six feet high at the lower end. The most common way to do this is to build a wall a few feet inside of the existing foundation wall. You could shore up the existing foundation and build a new one up to it but doing so would be more difficult and expensive than building a wall inside the existing foundation. The basic premise of your plan is sound and it is not uncommon around here. 

      Edited 5/7/2002 5:53:10 PM ET by John Carroll

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