To help prevent settling or sagging, the foundation must be constructed on firm, undisturbed soil or fill compacted to at least 90 percent of its maximum relative density. All organic material such as grass, roots, twigs, and wood scraps must be removed in areas where footings are to be placed. After removal of organic material, the home site must be graded or otherwise prepared to ensure adequate drainage, in accordance with § 3285.203.
The soil classification and bearing capacity of the soil must be determined before the foundation is constructed and anchored. The soil classification and bearing capacity must be determined by one or more of the following methods, unless the soil bearing capacity is established as permitted in paragraph (f) of this section:
(a) Soil tests. Soil tests that are in accordance with generally accepted engineering practice; or
(b) Soil records. Soil records of the applicable LAHJ; or
(c) Soil classifications and bearing capacities. If the soil class or bearing capacity cannot be determined by test or soil records, but its type can be identified, the soil classification, allowable pressures, and torque values shown in Table to § 3285.202 may be used.
(d) A pocket penetrometer; or
(e) In lieu of determining the soil bearing capacity by use of the methods shown in the table, an allowable pressure of 1,500 psf may be used, unless the site-specific information requires the use of lower values based on soil classification and type.
(f) If the soil appears to be composed of peat, organic clays, or uncompacted fill, or appears to have unusual conditions, a registered professional geologist, registered professional engineer, or registered architect must determine the soil classification and maximum allowable soil bearing capacity.
Table to § 3285.202
|Soil classification||Soil description||Allowable soil bearing
|Blow count ASTM D 1586-99||Torque probe3 value4
|Classification number||ASTM D 2487-00
or D 2488-00
reference, see § 3285.4)
|1||Rock or hard pan||4000 +|
|2||GW, GP, SW, SP, GM, SM||Sandy gravel and gravel; very than dense and/orcemented sands;coursegravel/cobbles;preloaded silts,clays and coral||2000||40 +||More than 550.|
|3||GC, SC, ML, CL||Sand; silty sand; clayey sand; siltygravel; medium dense course sands; sandygravel; and very stiff silt, sand clays||1500||24-39||351-550.|
|4A||CG, MH2||Loose to medium dense sands; firm to stiff clays and silts; alluvial fills||1000||18-23||276-350.|
|4B||CH, MH2||Loose sands; firm clays; alluvial fills||1000||12-17||175-275.|
|5||OL, OH, PT||Uncompacted fill; peat; organic clays||Refer to 3285.202(e)||0-11||Less than 175.|
(a) Purpose. Drainage must be provided to direct surface water away from the home to protect against erosion of foundation supports and to prevent water build-up under the home, as shown in Figure to § 3285.203.
(b) The home site must be graded as shown in Figure to § 3285.203, or other methods, such as a drain tile and automatic sump pump system, must be provided to remove any water that may collect under the home.
(c) All drainage must be diverted away from the home and must slope a minimum of one-half inch per foot away from the foundation for the first ten feet. Where property lines, walls, slopes, or other physical conditions prohibit this slope, the site must be provided with drains or swales or otherwise graded to drain water away from the structure, as shown in Figure to § 3285.203.
(d) Sloped site considerations. The home, where sited, must be protected from surface runoff from the surrounding area.
(e) Refer to § 3285.902 regarding the use of drainage structures to drain surface runoff.
(f) Gutters and downspouts. Manufacturers must specify in their installation instructions whether the home is suitable for the installation of gutters and downspouts. If suitable, the installation instructions must indicate that when gutters and downspouts are installed, the runoff must be directed away from the home.
(a) Vapor retarder. If the space under the home is to be enclosed with skirting or other materials, a vapor retarder must be installed to cover the ground under the home, unless the home is installed in an arid region with dry soil conditions.
(b) Vapor retarder material. A minimum of six mil polyethylene sheeting or its equivalent must be used.
(c) Proper installation.
(1) The entire area under the home must be covered with the vapor retarder, as noted in § 3285.204(a), except for areas under open porches, decks, and recessed entries. Joints in the vapor retarder must be overlapped at least 12 inches.
(2) The vapor retarder may be placed directly beneath footings, or otherwise installed around or over footings placed at grade, and around anchors or other obstructions.
(3) Any voids or tears in the vapor retarder must be repaired. At least one repair method must be provided in the manufacturer's installation instructions.