Slab or Crawl Space? Both Can Provide a Solid Foundation, But There Are Differences

By: , Contributor

Published on: Mar 22, 2020

Slabs fend off cold and possums, but crawl spaces are better for earthquakes and plumbing.

Every home needs to rest on a solid foundation, but which is best: slab or crawl space? Or, what about a basement?

Whether buying or building, that's a choice the prospective buyer faces while shopping around. And while it's not always the first consideration, it's not one to overlook.

There are advantages and disadvantages to each. A basement costs much more to build, but it offers storage, the opportunity to make it into finished, livable space, and storm safety. Meanwhile, the differences between a crawl space and a slab might not be as obvious, so definitions are in order.

What are slabs and crawl spaces?

A slab is just that: a concrete slab, usually about six inches thick, that is poured and that the house is then built upon. Often included are concrete columns serving as footings that extend a couple of feet into the ground to add reinforcement.

A crawl space, likewise, is just that: an area under the house high enough to crawl in. It's created by exterior concrete or brick walls that elevate the house itself, 18 inches or so, typically, off the ground. Along with the walls themselves, there often are support columns underneath the house, too, helping to hold everything up.

Pros and cons of each

Via Home Advisor and BobVila.com, here are some considerations when choosing between slab and crawl-space foundations:

  • Cost: Slabs are usually cheaper, a consideration for construction but not so much when buying an existing home.
  • Grade: Something else to consider when building is that slabs are tough to build on a sloped lot. It's easier to vary the height of the walls in a crawl-space foundation.
  • Energy efficiency: The ground makes a great insulator. The advantage here clearly goes to the slab foundation. The air trapped in a crawl space can make the home harder to heat and cool and can even make insulation necessary.
  • Moisture resistance: Advantage goes to the slab here, too. Moisture under the house can attract mold and rot and can be hard to control. That's why ventilation is important, and some homes have moisture barriers, often in the form of a thick plastic sheet, on the dirt floor of their crawl spaces.
  • Pests: Another slab advantage. Termites love moisture. Raccoons, possums, spiders, snakes, and other insects also find crawl spaces attractive. Keeping them out is an ongoing consideration.
  • Infrastructure and repairs: Here's where the crawl space takes the lead. Plumbing and HVAC -- i.e., ductwork and pipes -- are typically installed in the crawl space, as can be furnaces themselves. You can't do that with a slab foundation. That stuff generally has to go behind a wall or take up space in closets or attics. And accessing piping that does wind up under a slab and then putting things back together again can add thousands to a repair bill.
  • Storage: Crawl spaces are the clear winner here. They can serve as an extra, albeit somewhat flat and horizontal, outside shed.
  • Geography: Crawl spaces take this category, to a certain extent. Some experts advise against slabs in cold regions, because the ground can freeze and shift under them, causing heaving and even in worst cases, buckling. Crawl-space foundations also are more resilient to the shaking of earthquakes.

Raised slabs

There's also a third option: a cross between a slab and a crawl space. Called a raised slab, this option comprises a walled-in foundation filled with soil or rocks for drainage and then capped with a concrete layer. That approach aids in moisture resistance and may be best for that home in a low-lying area or along the coast (if the home isn't so close to the water that it should be on stilts).

So, bottom line, there are good arguments for and against each of the foundation options. Still not sure which way to go? Fortunately, there are certified professionals who can help you make that decision. A major trade group to go to for more information is the Foundation Performance Association.

Experienced builders can also render expert opinions. An arbiter of best practices there is the National Association of Home Builders. And if you're buying rather than building, a trustworthy home inspector can help you determine whether a home has the appropriate foundation. A major trade group for them is the International Association of Certified Home Inspectors.

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