Section R405 Foundation Drainage
Drains shall be provided around concrete or masonry foundations that retain earth and enclose habitable or usable spaces located below grade. Drainage tiles, gravel or crushed stone drains, perforated pipe or other approved systems or materials shall be installed at or below the top of the footing or below the bottom of the slab and shall discharge by gravity or mechanical means into an approved drainage system. Gravel or crushed stone drains shall extend not less than 1 foot (305 mm) beyond the outside edge of the footing and 6 inches (152 mm) above the top of the footing and be covered with an approved filter membrane material. The top of open joints of drain tiles shall be protected with strips of building paper. Except where otherwise recommended by the drain manufacturer, perforated drains shall be surrounded with an approved filter membrane or the filter membrane shall cover the washed gravel or crushed rock covering the drain. Drainage tiles or perforated pipe shall be placed on not less than 2 inches (51 mm) of washed gravel or crushed rock not less than one sieve size larger than the tile joint opening or perforation and covered with not less than 6 inches (152 mm) of the same material.
Exception: A drainage system is not required where the foundation is installed on well-drained ground or sand-gravel mixture soils according to the Unified Soil Classification System, Group I soils, as detailed in Table R405.1.
PROPERTIES OF SOILS CLASSIFIED ACCORDING TO THE UNIFIED SOIL CLASSIFICATION SYSTEM
| SOIL GROUP | UNIFIED SOIL CLASSIFICATION SYSTEM SYMBOL | SOIL DESCRIPTION | DRAINAGE CHARACTERISTICSa | FROST HEAVE POTENTIAL | VOLUME CHANGE POTENTIAL EXPANSIONb |
| Group I | GW | Well-graded gravels, gravel sand mixtures, little or no fines | Good | Low | Low |
| GP | Poorly graded gravels or gravel sand mixtures, little or no fines | Good | Low | Low | |
| SW | Well-graded sands, gravelly sands, little or no fines | Good | Low | Low | |
| SP | Poorly graded sands or gravelly sands, little or no fines | Good | Low | Low | |
| GM | Silty gravels, gravel-sand-silt mixtures | Good | Medium | Low | |
| SM | Silty sand, sand-silt mixtures | Good | Medium | Low | |
| Group II | GC | Clayey gravels, gravel-sand-clay mixtures | Medium | Medium | Low |
| SC | Clayey sands, sand-clay mixture | Medium | Medium | Low | |
| ML | Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity | Medium | High | Low | |
| CL | Inorganic clays of low to medium plasticity, gravelly | Medium | Medium | Medium to Low | |
| Group III | CH | Inorganic clays of high plasticity, fat clays | Poor | Medium | High |
| MH | Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts | Poor | High | High | |
| Group IV | OL | Organic silts and organic silty clays of low plasticity | Poor | Medium | Medium |
| OH | Organic clays of medium to high plasticity, organic silts | Unsatisfactory | Medium | High | |
| Pt | Peat and other highly organic soils | Unsatisfactory | Medium | High |
For SI: 1 inch = 25.4 mm.
- The percolation rate for good drainage is over 4 inches per hour, medium drainage is 2 inches to 4 inches per hour, and poor is less than 2 inches per hour.
- Soils with a low potential expansion typically have a plasticity index (PI) of 0 to 15, soils with a medium potential expansion have a PI of 10 to 35 and soils with a high potential expansion have a PI greater than 20.