Crawl space foundations provide an elevated floor structure with an accessible space underneath for mechanical systems, electrical service, and plumbing. The typical height is 18 to 48 inches above grade, sufficient for workers to move through the space while maintaining lines. Perimeter foundation walls, typically 2 to 4 feet tall above grade, are constructed of concrete blocks, concrete, or treated wood posts supporting the building above. Grade beams or concrete strips distribute loads from the structure to the supporting soil or piles. A below-ground footing excavation typically extends below frost line to prevent frost heave. Footing depth varies by climate but is typically 3 to 4 feet deep in northern climates. A gravel base provides drainage and prevents moisture accumulation. The ground surface of the crawl space is typically covered with a vapor barrier of 6-mil polyethylene to reduce moisture and radon gas entry. Support posts under the structure are typically 4-by-4 or similar timber or concrete piers, spaced 6 to 10 feet apart depending on load and span. Posts are supported on concrete pads or continuous foundations. Diagonal bracing or shear walls provide lateral stability and prevent racking (parallelogram distortion). Ventilation openings or mechanical ventilation systems control moisture in the crawl space. In high-moisture areas, dehumidifiers or conditioning systems maintain acceptable humidity levels. Access is provided through a removable panel or access door. Plumbing, electrical, and HVAC systems installed in the crawl space require proper support and protection from damage.