Crawlspace vs. trench foundation?

Old 01-10-01, 07:00 AM
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I am planning an addition to my house. One contractor uses a technique called "slab overlay" in which a trench is dug for the footings and the interior is excavated only enough to allow for a two-inch layer of concrete and the floor joists. There is virtually no space between the concrete and the joists, except in the middle where a trough is formed to allow for the ductwork to run under the joists.

Another contractor would excavate the whole area down to 42" and provide a crawl space. He says that trench foundations are "bad construction".

Are his concerns about trench foundations valid or is he just bad mouthing the technique because his competitor uses it?

If it matters, I live in the Chicago area so cold is a concern.

Thank you in advance,
Old 01-10-01, 04:28 PM
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Check with your local bldg. dept. and see what they say. Here in CA, if you use a wood floor, there has to be a minimum 18" of clearance below the floor joists.
Old 01-10-01, 05:56 PM
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Cool Just bored.

Hi Homeowner,

I am just sitting here bored and have spent time in Chicago. It seems to me that whether you have 2" or 18" or 42" of clearance is not an important structural issue. But I'll bet it's alot less money for 2" and I would certainly like that.

Meanwhile, my real concern is how deep is the footing for both the crawl space verses the no-space? I have always learned that it is the depth of the footing foundation that makes a structure strong in a freezing climate.

If both methods are common construction depths for footings in freezing climates, then I say it does not matter which method. It sounds like the 42" contractor is accustomed to building a crawl space, for whatever reasons, and I suspect that it IS a superior design, but at a cost to YOU, the homeowner.

So I would automatically be suspicious of the term "BAD construction" without an explanation. Ask him what he means by that. And ask the other guy about it too.

Be bold! From one consumer to another, Ask them!

And please, please, let us know the exciting end to this story!!

Old 01-11-01, 01:32 AM
some help
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What ever happend to plans and specifications, all should bid the same and offer value engineering so you can see the savings and if it works for every one, building deptment and you...your choice....I am from the west and I like to be able to go under the house to make repairs and I need all the room I can get....and in the last sixty years I have never lived in a house that did not want some attention
Old 01-12-01, 03:41 AM
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Trench type of foundation is usally used when you are building a completely new home. You indicated you are building a addition. With the trench type, if there is any setteling, you addition will begin to pull away from your home. Also, I would be very cusious about anything with only 2" of concrete. @'' of concrete can crack very easily. Usually they use 4' with the trench type. The foundation type with crawlspace will be the best, strongest, and you will be the most happy with it. When the crawlspace contractor said bad construction, he did not mean it like you took it. He meant it is not the preferred type, and it can have alot of problems associated with it.
Old 01-15-01, 07:45 AM
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Thanks to all respondees, your input was most helpful.

Regarding some of the points that you made:

The ability to use slab overlay varies from community to community. My community allows it.

Mark Chapman:
The base of the footing would be the same in either case: 42".

I asked the second contractor why he recommends the crawlspace. His reply:

-- With the trench, it is difficult to form the base of the footing, which is wider than the wall. This affects strength.

-- Dirt from the trench wall can fall into the trench as the concrete is being poured. Again, this affects strength.

-- The ability to use trench foundation is subject to soil conditions. You can exavate the trench, find the walls won't hold well and end up doing a formed foundation anyway.

-- Many communities do not allow a second story to be added over a trench foundation. Again, a strength issue.

-- If you ever have a problem with plumbing, HVAC, etc., you have to dig up part of the floor if you don't have a crawl.

He said he has done slab overlay on occasion, but strongly recommends the crawl.

some help:
Plans and specifications do not yet exist. We are planning to use a "design/build" company to do the work, so plans will not exist until after we have chosen a contractor. Of course, we can tell them which type of foundation we want, but we are interested in their recommendation and reasons for it.

Jack the Contractor:
I was in error, the contractor using the trench does pour a four inch slab.

I'm definitely in favor of the crawlspace. In addition to the reasons mentioned by the second contractor and Jack, I like the fact that I can get under the addition to run audio, video, voice and data cables. Without it, I would have to run all my wires (for now and for the future), before the drywall is installed.

The crawlspace will also add valuable storage space for infrequently used items.

Thanks to all.

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