Deck Footings and Posts
Inspecting the footings, footing sizes, footing depths, and buried components beyond the scope of a home inspection. The home inspector is not required to have knowledge of the local building codes in relation to the house footings and deck footings, frostlines, and soil types. Home inspectors are not required to inspect buried items and components.
Deck footings are sometimes not visible during a home inspection. Decks must be supported on concrete footings or other approved structural systems that can support all of the deck loads. Deck structures that are attached to the house should be supported on concrete footings or other approved structural systems designed to accommodate all loads. Deck footings should be sized to carry the imposed loads from the deck structure to the ground.
Footings are not required for free-standing decks with joists directly supported on the ground over the entire length of the joist. The illustration above shows a free-standing deck that is not attached to a primary structure.
Footings are not required for a free-standing deck that has:
the joists bear directly on precast concrete pier blocks at grade without support by beams or posts, or
a deck area not bigger than 200 square feet (18.6 m2), or
a walking surface is not more than 20 inches (508 mm) above grade at any point within 36 inches (914 mm) measured horizontally from the edge.
Minimum widths and depths of deck footings are based on the deck tributary area (the portion of the deck surface area that bears its mass on top of a footing), snow or deck live load, and soil bearing pressure. Typical deck footings may vary widely. Refer to 2021 IRC R507.3.
The settlement of soils located below decks can pull the deck away from the ledger attached to a house. It is important that a deck footing be sufficiently large to resist soil movement while every effort is made to place a deck’s footings on undisturbed soils.
A free-standing deck is not attached to the house. The illustration below shows a free-standing deck with one of its posts resting on disturbed soil. A deck post should not be installed on disturbed soil. And disturbed soil can often be found next to the foundation of a newly-built house.
Decks not supported by a dwelling (detached decks, free-standing decks) need not be provided with footings that extend below the frost line. Free-standing decks supported by joists that are all directly supported on grade over their entire length (uncommon) do not require footings. Refer to 2021 IRC R507.3.
Required footing depths vary based on local building codes. Deck footings should be placed at least 12 inches (305 mm) below the undisturbed ground surface.
Where decks are attached to a frost-protected structure, deck footings should also be protected from frost by:
Extending below the frost line,
Setting it on solid rock, or
Other approved methods of frost protection.
Refer to 2021 IRC R507.3.
The illustration above shows the bottom of the deck post footing properly located below the frost line.
Disturbed and Undisturbed Soil
Disturbed soils are those which have been altered as a result of grading or construction, etc., giving the soil variable characteristics. Some codes consider soil to be "undisturbed" if it hasn't been disturbed in more than five years. A footing placed in undisturbed soil is desirable because that soil is more compacted and solid. Placing a footing in disturbed soil is less stable and may result in incremental failure of the deck's support, as the footing is likely to sink lower and lower into the poorly-bearing soil.
7-Foot Rule for Steep Ground The illustration below depicts the "7-Foot Rule." On steep properties, the slope of the ground around the footing could affect the footing's stability. The 7-Foot Rule states that there should be a least 7 feet between the bottom of a footing and daylight.
Bottom of Deck Posts
The bottom of deck posts must be restrained to prevent lateral displacement. The lateral restraint may be provided by a minimum post-to-footing connection based on the manufacturer’s requirements for connections supplying lateral restraint or by embedding the post at least 12 inches into the soil or concrete piers.
Lateral restraint can be provided by manufactured connectors or a minimum post embedment of 12 inches (305 mm) in surrounding soils or concrete piers. Other footing systems shall be permitted. Posts in contact with soil should be pressure-treated and oriented, so the cut end is above grade. Refer to 2021 IRC 507.4.1.
The image above depicts a post base that is not attached to its footing. Posts should be connected to their footings so that the posts don't lift or slip off.
The image above depicts a pre-cast concrete pier. Posts can lift out of pre-cast concrete piers, and piers can slide. Posts should be connected to their footings so that the posts don't lift or slip off.
The image above depicts a proper post-to-footing connection. Posts should be connected to their footings so that the posts don't lift or slip off their footings.
The image above depicts an adjustable post-to-footing connection. Posts should be connected to their footings so that the posts don't lift or slip off their footings. Often, the bottoms of the stringer boards for deck stairs have been found to rest on soil, concrete block or rock, as opposed to resting on posts installed below the frost line. Posts set on soil are subject to rot due to moisture. Posts that are set in unsound footings may cause movement and make the deck above unstable. High Decks
The image above depicts a high deck being supported with 4x4-inch posts. Tall 4x4 posts twist under load, and 4x4 posts, even when treated, decay below grade too quickly. As a general rule, for all but the lowest of decks, deck posts should be at least 6x6, and be no higher than 12 feet; 14 feet is acceptable if cross-bracing is used and the posts are 8x8. 4x4 posts can be used for heights up to 14 feet, but only under certain circumstances.
Information used with permission of nachi.org