A good foundation is essential for every house. While poorly installed gutters or gappedhardwood floors are merely annoying, improperly built foundation footings bring down the house.
So, building codes naturally have a few things to say about foundation footings. Like any code recommendations, these are not how-to guides to buildingfootings so much as they are parameters that you need to observe and should be adjusted according to your local code requirements.
| Topic | Requirement or Recommendation |
| Site grading | Slope ground at 10-percent away from the foundation for 10 feet or more. |
| Site grading exceptions | Ground may be sloped at 5-percent in combination with water diversion methods. |
| Soil load-bearing value | Presumptive undisturbed soil or disturbed soil of a minimum LBV per square foot. |
| Footing depth | 12 inches minimum depth or below frost line. |
| Footing width | 12 inches minimum. |
| Footing level, top | Always level. |
| Footing level, bottom | Up to 10-percent slope. |
| Spread footing | 6 inches thick minimum. |
| Concrete strength | 2,500 psi minimum, but up to 3,000 psi may be required. |
Site Grading
The ground immediately adjacent to the foundation footings—on the exterior of the foundation—should slope down at a ten-percent minimum slope. This must continue for at least ten feet.
If obstructions do not permit the preferred 10-percent grading, 5-percent grading may be approved, but an alternative method of diverting water from the foundation must be provided. When there is a ramp or a door landing, an impervious surface may be installed and sloped at a 2-percent minimum.
Impervious Surface
An impervious surface may include walkways, patios, driveways, formal planters, parking lots or storage areas, and concrete or asphalt paving surfaces.
Load-Bearing Value of Soil
Foundations must rest on firm, undisturbed soil. Undisturbed soil is soil that has never been turned over, tilled, graded, hoed, dumped, dug, scraped, compacted, amended, or anything of that nature, by human by or machine. Undisturbed soil is significantly stronger than soil that has been disturbed.
Code refers toload-bearing values (LBVs) as "presumptive."This means that asoil test is the only way to really know the load-bearing value (LBV) of the soil for the footings on a given site.
Disturbed soil can eventually be classified as undisturbed after enough time; a soil test can determine this.
| Soil Type | LBV Per Square Foot |
| Bedrock | 12,000 |
| Sedimentary Rock | 4,000 |
| Sandy Gravel or Gravel | 3,000 |
| Sand, Silty Sand, Clayey Sand, Silty Gravel, Clayey Gravel | 2,000 |
| Clay, Sandy Clay, Silty Clay, Clayey Silt | 1,500 |
Depth, Width, and Thickness of Footings
Depth of Footings
Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings also must extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or must be frost-protected.
These rules may not apply to accessory buildings (such as sheds) if they are below a certain square footage specified by your local building code. Also, deck footings may have a different depth requirement. Some decks, such as those not attached to the house, may not have the same depth requirements.
Tip
Areas with expansive clay soil will require caissons (or concrete columns) to be drilled down to bedrock at pre-determined locations, with some sort of expansion material between caissons to allow for soil expansion and contraction. The foundation walls will then be poured on top of them.
Width of Footings
Footing width can vary according to the structure, site, and conditions. Under code, generally, one-story buildings with footings on undisturbed soil with LBVs between 1,500 and 4,000 should have a minimum width of 12 inches.
Two-story buildings require a minimum of 15-inch wide footings for 1,500 LBV soil. For 2,000 LBV soil or greater, two-story buildings may have 12-inch minimum wide footings.
Levelness
Requirements for levelness are different for the top and bottom of the footing:
Top of the Footings
The top of the footings must be level, with no exceptions.
Bottom of the Footings
The preference for the bottom of the footingis that it is level. Exception: If building on a sloped grade, you can step the footing by as much as one unit vertical per 10 units horizontal (or a 10-percent slope).
Tip
The term "unit" is used in codes to refer to any type of measurement that is being used during building, whether Imperial or metric.
Spread Footings
Spread footings help distributethe load carried by the footings over a wider area. The spread part is a base that looks like an upside-down "T" and transfers the weight across its area.
The spread footing should be no less than six inches thick. It should project, on both sides, no less than two inches.
Strength of Footing Concrete
For foundations and slabs, except for garages, the minimum strength of the concrete should be 2,500 psi. Vertical walls exposed to the weather may be required to have 2,500 psi concrete minimum strength, on up to 3,000 psi, in some severe weathering conditions.
Building Code Basics
These code specifications are derived from the International Building Code (IBC) for 1- and 2-story residences. Thissummarized guide is intended to provide a general sense of code requirements for foundation footings.
The IBCencompasses the International Residential Code (IRC) but includes provisions for commercial buildings as well as for residential buildings. The IRC is equally suitable for residential buildings. In the IRC, the chapter relating to foundations and footings is chapter 4.
Keep in mind that each construction project is unique. For example, the soil is different from place to place, and thus the load-bearing value of the soil will change. Also, code rules are set and enforced at the local level, typically through each city's building department.
Most local code authorities use the IBC and/or IRC as their model codes but may omit, modify, or expand any code specification to suit local conditions and legal requirements. While the IBC and IRC are merely suggested guidelines, the local code is the law of the land that you will need to observe.
I have a solid background in construction and building codes, particularly regarding foundation footings. The requirements and recommendations you've mentioned align with the fundamental principles of constructing a stable and durable foundation. Here's a comprehensive breakdown of the concepts covered in the article:
Site Grading
- Requirement: Ground adjacent to foundation footings should slope away at a 10-percent minimum for at least 10 feet.
- Exception: 5-percent grading may be allowed with alternative water diversion methods or impervious surfaces.
Load-Bearing Value of Soil
- Presumptive Values: Different types of soil have distinct load-bearing values per square foot.
- Undisturbed Soil: Stronger than disturbed soil; foundations should ideally rest on undisturbed soil.
Depth, Width, and Thickness of Footings
- Depth: Minimum 12 inches below undisturbed soil or frost line.
- Width: Varies based on building height and soil type, typically 12 inches for one-story and wider for two-story structures.
- Thickness: Spread footings should be a minimum of six inches thick.
Levelness
- Top of Footings: Must be level without exceptions.
- Bottom of Footings: Preferably level; exceptions for sloped grades up to a 10-percent slope.
Spread Footings
- Distribute load over a wider area, typically at least six inches thick with a two-inch projection on both sides.
Strength of Footing Concrete
- Minimum concrete strength is 2,500 psi, with potential requirements up to 3,000 psi for vertical walls in severe weathering conditions.
Building Code Basics
- Derived from the International Building Code (IBC) and International Residential Code (IRC).
- Local code authorities enforce and modify these guidelines based on specific conditions and legal requirements.
Understanding these principles ensures that foundation footings are appropriately designed and constructed to support the structure. Compliance with local building codes is crucial for safety and structural integrity, considering variations in soil, climate, and specific regional needs.
