We very rarely see a crawl space under a concrete first floor. If the concrete floor is elevated a couple of feet off the ground, it is likely that there is no crawl space and the volume under it contains clean fill soil that was placed after a stem wall was built, and the fill was compacted before the concrete slab was poured over it. If you cannot find an access panel to a crawl space for an elevated concrete floor, then this is likely the case.

    However, we have seen several structural concrete floors elevated just a few feet above ground level in the Florida Keys. The Residential Edition of the Florida Building Code (FBC) and the International Residential Code (IRC) state that “the under-floor space between the bottom of floor joists and the earth under any building (except space occupied by a basement) shall have ventilation openings through foundation walls or exterior walls.”

    While it is possible to interpret that section of the code as only applying to wood floors because of the reference to “floor joists,” no specific material is mentioned, and there are very good reasons to ventilate the crawl space under a concrete floor. The buildup of moisture rising out of the soil into an unventilated crawl space is conducive to “spalling” of concrete, which is where the moisture penetrates the concrete and reaches the reinforcing steel inside, which rusts, and the rust is an expansive process which begins to pop off pieces of concrete. 

    This eventually deteriorates the structural integrity of a structural slab to the point of failure. Mold growth on any organic material in the crawl space will also be more likely in an unventilated crawl space.

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To learn more about exterior walls and structures, see these other blog posts:

• What is the average lifespan of a house foundation?

• What causes vertical cracks in fiber cement siding planks?

• What causes raised white lines of residue on a block wall that are crusty and crumbling? 

• What is the difference between soil subsidence, heave, creep, and settlement? 

• How much ventilation is required for the under-floor crawl space of a home? 

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?

• What causes the surface of old bricks to erode away into sandy powder? 

• What are the pros and cons of concrete block versus wood frame construction?

• Should I buy a house with a crawl space? 

• Why is my stucco cracking?

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What are the common problems of different types of house foundations? 

• What are the warning signs of a dangerous deck?

• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

        Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.