Raised Concrete Slab Foundations for High Water Tables

Summary: A raised concrete slab foundation is the way to build if you can't build your new home on a full basement foundation.

"Carl, first off, you have a great site, thank you for sharing your knowledge with us.

Now for my question:

I live in a coastal town in Texas, that has very high humidly, and I am planning on building my own home very soon.

I am almost ready to take the numbers to the bank but the foundation plans are holding me up.

My current plans are to build the house with a crawl space, about 3 feet due to raising water concerns; it will also have a porch all the way around it that I plan to pour like you would a slab (even with the house).

My question is: how do I vent the crawl space under the house?

I have thought about placing some kind of tube or building a box below the slab before the pour that would allow air to flow under the house but I am not sure if this would work.

Any ideas would be helpful.

Thank you, Bryan"

Hi Bryan, I would strongly recommend that you don’t use a crawl space if at all possible.

Use a 3 foot high (or higher) raised slab for the entire structure, including the porch.

The cost difference should be negligible in that floor joists are eliminated, offsetting the extra cost of raising the slab.

You can find out by getting bids from foundation contractors.

That should only take a couple of days in this slow housing market.

A crawl space in a coastal area, or in an area oh high humidity is asking for trouble.

Post Katrina Louisiana is considering making raised slabs mandatory.

Coastal Texas has similar weather, flooding, and humidity, I believe, as Coastal LA.

If you end up using a crawl space anyway, not only can you vent it, you should use power vents.

See my page on "Crawl Space Vents". It is a very thorough article.

I have seen crawl spaces where I live in NC (non coastal) that are solid mold and mildew due to humidity after just a short time. This is not only a health hazard but a structural problem too (rot). It is also a magnet for termites.

I believe, based on my 30 plus years in the business, that if you can't build on a full basement foundation, a raised slab should be the next choice.

Raised concrete slabs are not to be confused with concrete slabs "on grade", which, in my opinion, should be limited to patios and garages.

Courtesy: Concrete Network

Use a poured concrete "T" wall off the footings (as shown above) instead of concrete block, as blocks will trap water and moisture, and through the process of osmosis, create a damp concrete slab. With the T wall, the wall and slab (and fill) can be raised high enough, in most cases, to be above potential water problems.

Waterproofing and footing drains as always, as well as slab insulation, are critical. With this type of foundation you should not only be high and dry, but termite free as well!

Add radiant heat for a nice comfort level year round, a solar water heater for the radiant heat and personal use, an energy efficient central air system, and raised concrete slab construction is the way to go in any state!

By the way Bryan, solar water heaters are not that expensive, are eligible in many ares for tax credits, and have a relatively short payback.

Be sure to read Radiant Heating.

Good luck, Carl Heldmann