Types of At-Grade Footings Explained

Here’s a description and illustrations we’ve created for footing types we frequently reference in our work for sunrooms, pergolas, carports, screen rooms, signs, and similar at-grade building component structures.  Photos of each appear below.

An isolated footing is used to support a single post. Isolated footings are independent footings which are provided for each support. they can be round (cylindrical) or square (rectangular). Isolated footings need to be able to resist a significant amount of lateral (wind) loading as well as gravity and uplift.  Torsional (rotational) and eccengtric forces from unequal loading patterns also affect these footings most significantly.  Saturated soils greatly weaken capacities of singular, isolated footings because there is so little else for the footing to be supported by.

A ribbon footing is another term for a continuous strip footing or one long isolated footing.  Frequently used in signs and patio structures with pavers, ribbon footings suffer less from torsional and eccentric loading due to their profiles, but must still be large enough or have secondary bracing to resist overturning forces from lateral pressures such as wind, especially in saturated soils.

Isolated and ribbon footings can be ‘constrained’ or ‘non-constrained’ meaning that there’s a slab/foundation above them at-grade to ‘lock them into place’.  Constraining a footing offers benefits for lateral and uplift support, and sometimes gravity if the footing is attached to the foundation (see illustrations below).

A thickened edge slab footing has deeper (thicker) concrete at the edge(s). That concrete is used to support columns/posts for gravity and uplift forces.  there are also code minimum footing requirements for patio slabs in both the Florida and International Building Codes.

larger thickened edge footings are used for construction of slabs on grade with integrated foundations.

There are other types of foundations such as stem wall, structural foundations on piles, and others which can be found online and are not ‘at-grade’.  They are outside the scope of this description of at-grade footings. It’s important though to consider integrating different footing types since they will have different settlement characteristics over time.

Posts can be surface mounted or embedded into the foundations to achieve different results. an embedded post typically takes more uplift and overturning forces and resists bending/listing which is useful for design of lateral and uplift forces.

Important to note that in some areas footings like this are required to be below the frostline to prevent heave.  Generally however, footings for patio type use are exempt.  check with your local code and building official for clarification of needs in your area before installing any foundations and always consult a licensed professional engineer.

SEE ALSO: FROST PROTECTION FOR CONCRETE FOOTINGS ON GRADE – HEAVE

Here are some patio related illustrations to further describe the above

Isolated footings for signs (unconstrained)

Isolated Footings (constrained)

Ribbon Footing for Sign