Pier and beam foundations, also called post and beam or post and pier, involve supporting the structure on isolated piers (footings) connected by beams that support the floor framing above. Each pier is excavated below the frost line with a concrete footing pad at the base that distributes the load to the soil below. Typical pier depth is 3 to 4 feet in northern climates; deeper in areas with severe frost. Footing pads are typically 24 inches square and 12 inches thick for residential construction. The concrete in piers and footings must be designed for the soil conditions and local frost line. Posts are typically 4-by-4 or 6-by-6 dimensional lumber or concrete piers set on the footing pads. The posts are spaced 6 to 10 feet apart depending on load and beam size. Concrete piers provide durability and resistance to rot and insect damage. Wooden posts require treatment with water-repellent preservatives or must be isolated from contact with soil and moisture. Beams running between piers support the floor joists above. Beams may be single dimensional members, built-up beams (multiple boards bolted together), or engineered I-joists. Beam size and spacing depend on load, span, and soil-bearing capacity. Lateral bracing between posts prevents racking (shifting into a parallelogram shape) under lateral loads such as wind. Diagonal bracing or shear walls provide stability. The crawl space under the floor allows access to plumbing, electrical, and mechanical systems. Vapor barrier installation on the ground reduces moisture and radon gas. Ventilation or conditioned air in the crawl space controls humidity. The height of the crawl space is typically 18 to 48 inches.