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How deep should a footing be for a garage?

thespruce.com - Building Code Foundation Requirements - The Spruce

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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 footing is 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).

Spread Footings

Spread footings help distribute the 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. This summarized guide is intended to give you a general sense of code requirements for foundation footings. The IBC encompasses 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.

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