Foundation System - Crawl Space vs. Slab-on-grade?

INTRODUCTION

Decision on which system to use is generally driven by local climate.  Cold winter temperatures freeze top layer of soil.  Moisture in soil expands and exerts pressure on foundation systems.  If the footing, the lower load bearing component of a foundation system, sits above depth, it will likely move and damage or compromise the entire structure.  A condition known as 'frost heave' (figure 1).  Contact your local building department for more information on frost depth and soil bearing capacity.

Figure 1 - Foundation damage caused by 'frost heave'.

CRAWL SPACE

Rationale: Generally used in colder climates requiring footings 18" or more below adjacent finished grade.

Pros: Allows underfloor access to in-floor electrical, plumbing and mechanical components.

Cons: Deeper footings means more excavation and taller foundation stem walls (figure 2).

Figure 2 - Crawl space floor system.

SLAB-ON-GRADE

Rationale: Generally used in warm climates where footings may be within 12" of adjacent finished grade.

Pros: When used in warm environments, less excavation and shorter concrete stem walls are required.  In some cases, footings and stem walls can be poured together to shorten and simplify concrete placement process.

Cons: Doesn't allow access to in-floor electrical, plumbing and mechanical components.

Figure 3 - Concrete slab-on-grade floor system.