DFW Clay soil: slab or pier and beam?

kgizoSeptember 8, 2012

Hi. We are looking at a lot and getting an initial cost estimate from the builder. We're transplants from the north and have had a P&B since we've moved to Texas so we are most knowledgable about P&B. Most building seems to be done with slabs as it is thousands cheaper, but slabs are much more costly to fix when things shift. If you live in the area, would you pay a premium for a P&B over a slab? If yes, about how much? I'm leaning towards a P&B foundation for the new build, but don't want to get killed when it comes to resale. Thanks for your help.

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galore2112

I'm in Dallas. There's also the option of a pier-beam slab (where the concrete slab sits on concrete beams that sit on concrete piers), which I've seen for several new higher-end builds in the area.

The cheap option here is a post-tension slab, no piers. It sits on soil (often clay here) and is designed to be strong enough to not crack. It works if it's made correctly but if it fails it's difficult to repair.

The better option can be a design with piers. Either crawl-space or slab on beams on piers.

Just make sure that the piers are deep enough (ideally to bedrock). The house I tore down sat on 5ft deep piers that terminated in clay. Totally stupid and made it a "fun"house with severely warped floors.

I'd pay a 20% (of foundation cost) premium for a proper P&B foundation over post tension slab. You will recoup this investment by a wide margin if the other foundation would have failed. You may also recoup this investment in any case if it's marketed correctly.

    Bookmark   September 8, 2012 at 7:46PM
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8mpg

Im from the DFW area as well and I would possibly look into having a geotech drill and evaluate the soil. I can tell you that soil composition has much more to do with the foundation than anything else. Im in an amazing spot in Keller with a very high sand content so I wont have slab problems. Las Colinas area on the other hand has some of the worst clay and a very high PVR. The Geotech engineer will recommend slab styles (post tension, conventional rebar, etc) and also the fill for under the slab and whether to install a plastic lining or not.

Not sure what it costs as mine was already done before we bought the land.

    Bookmark   September 8, 2012 at 11:46PM
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ellenandco

We are way over in Jefferson Co, and it cost $1800 to drill and evaluate two 15' core samples. Money well spent, in my book. Most homes in the neighborhood are pier-beam-slab.

    Bookmark   September 10, 2012 at 2:45PM
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virgilcarter

The foundation is not where one wants to guess or go with the cheapest solution. Best solution, as suggested above, is to get a soils engineer to test the soil and give recommendations for the best type of foundation based on soil conditions.

An important caution is to keep the surface water and drainage always moving positive away from all sides of the house. Water soaked soils cause more foundation damage than almost anything else, so be sure that grading is done to properly ensure run-off and maintain it over the years. Good luck!

    Bookmark   September 10, 2012 at 3:09PM
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galore2112

On cost: I also paid about $1800 for two core samples and the geotech report. The foundation design was done by a structural engineer as part of the whole house structural design for approx. $8000 (small three level house 2250sf).

    Bookmark   September 10, 2012 at 11:16PM
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kgizo

Wow! Thanks for the great responses. This is really helpful and I will get right on the soil / geotech report.

I know next to nothing about the pier and slab option, but have heard concerns that during our flash floods the water can reach the slab. Since it is porous it holds the water longer and can crumble. Have any of you heard this and know if it is true or not?

    Bookmark   September 11, 2012 at 10:34AM
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virgilcarter

You have flash floods in this area? Will you be required to have flood insurance due to being in a flood zone? Do you have a local building department? Do they have restrictions or regulations on floor height above exterior finished grade? I'd really do some serious research on the effects/impacts of building in such a situation before committing to go ahead.

FWIW, a properly designed concrete foundation and slab will not be unduly damaged by a limited period of high water. What might happen, especially with surging water, is that there could be ground erosion away from the concrete foundation. This should not necessarily be a problem for properly designed pier/grade beam foundation unless all of the ground down to the bottom of the piers is eroded. A local soils engineer and structural engineer, familiar with your area, should take all of this into account in their recommendations. Good luck!

    Bookmark   September 11, 2012 at 12:40PM
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kgizo

Our spring rains can get intense. Ive never had water come into my home, but friends/coworkers have had 1-3" in their homes. Sometimes due to a bad neighbor's landscaping and water diversion actions.

    Bookmark   September 11, 2012 at 1:19PM
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