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Concrete deck foundation details determine whether a deck settles, moves with frost, or safely transfers its loads to the ground. There is no single nationwide footing size or depth: the correct design depends on tributary deck area, beam and joist spans, verified soil bearing, local frost penetration, and the deck’s relationship to an existing house foundation. For example, Houston’s residential table starts at a 15-inch round (or 13-inch square) footing that is 6 inches thick for a 6-foot beam span with joists up to 10 feet, but reaches a 34-inch round (32-inch square) footing 14 inches thick for an 18-foot beam span and joists up to 18 feet. Those values assume 1,500 psf allowable soil bearing and 2,500-psi concrete; they are not universal minimums.
Use your jurisdiction’s current permit detail first. Then verify soil and frost conditions, calculate the load carried by each post, select a footing area that can support that load, and use a listed post-to-concrete connector. The examples below show how to make those decisions without treating a local table as a substitute for project-specific design.
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A footing spreads a post’s load into the soil. Its required bearing area is therefore tied to the load delivered by the post and the soil’s allowable bearing pressure, not simply to the post’s nominal dimensions. A post supporting a large tributary area, a long beam span, or heavy features such as stairs and a roof can require a much larger footing than a post on a small, low platform.
Start by identifying the deck’s beam spans, joist spans, post spacing, total supported area, and design loads required by the local code. Divide the deck into tributary areas so each post is assigned the portion of the deck it supports. Compare the resulting post reaction with the allowable soil bearing capacity confirmed for the site. If the soil may support less than 1,500 psf, the Houston residential deck guide says to investigate the soil rather than rely on its prescriptive table.
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Undisturbed, solid ground is important. Fill, organic soil, loose sand, expansive clay, or wet zones can have different capacities and settlement behavior. A printed table states assumptions; it does not prove that those assumptions exist beneath your yard. Have the building department or a qualified designer identify when a soil report, special inspection, or engineered footing is required.
How deep should a deck footing be?
Depth is set by the deeper of the local frost requirement and any minimum depth measured below undisturbed ground. Because frost penetration varies by location and soil drainage, a number that is compliant in one jurisdiction can be inadequate a few miles away.
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- Fairfax County example: the typical drawing shows a 24-inch minimum depth and separately references frost depth. The drawing and its span tables are Fairfax-specific; the PDF is version 2021.0, dated January 5, 2024.
- Ontario: the construction note explains that frost penetration is established from local experience and that required depth changes with soil type and drainage. For clay or soil that cannot be clearly classified, it describes at least 1.2 m and not less than frost depth for certain foundations without heated space. Confirm the current municipal interpretation before excavating.
Measure depth from the finished grade or the reference elevation specified by the local detail, not from a temporary pile of soil. The bottom must be level, free of loose material, and on the bearing stratum identified by the detail or designer. If excavation exposes soft pockets, groundwater, roots, or undocumented fill, stop and obtain direction; simply pouring more concrete does not correct an unsuitable bearing layer.
How wide and thick does a footing need to be?
Width (or diameter) provides bearing area; thickness provides a structurally adequate pad beneath the post or pier. The governing table may select both dimensions from beam span, joist span, post spacing, and soil assumptions. Do not copy a diameter from a different span line or mix a local table with another jurisdiction’s depth rule.
| Published example | Span and assumptions | Footing dimension |
|---|---|---|
| Houston guide | 6-ft beam span; joists up to 10 ft; 1,500 psf soil and 2,500-psi concrete assumptions | 15-in round or 13-in square; 6-in thick |
| Houston guide | 18-ft beam span; joists up to 18 ft; same stated assumptions | 34-in round or 32-in square; 14-in thick |
| Fairfax typical details | Dimensions selected through Fairfax deck-span tables | Square sides shown from 12 to 26 in; round diameters 14 to 29 in; thicknesses 6 to 10 in |
The Houston values illustrate how dramatically footing dimensions can change as the supported spans grow. They are not minimums for every deck. Fairfax’s ranges likewise belong to Fairfax’s details and should not be transplanted to another state, province, soil, or code edition.
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Ontario’s construction note mentions a minimum footing area of 0.40 m² for columns spaced 3 m on center supporting one floor, but cautions that this table does not straightforwardly establish deck-column sizing. Use tributary-area and allowable-bearing calculations when the listed limits are not met; do not turn 0.40 m² into a general deck-footing prescription.
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Excavation near an existing foundation can remove the soil that supports the house. Both the Houston and Fairfax details require a footing located within 5 feet of the existing exterior wall to bear at the same elevation as the house foundation. This requirement is especially important where a basement, crawlspace, stepped footing, or sloping site is present.
Locate the house footing before digging, and show its elevation on the permit drawing when possible. Do not assume the visible basement wall or slab marks the bottom of the foundation. If the new excavation would pass below or alongside the existing footing, obtain the building official’s or designer’s direction for sequencing, shoring, underpinning, or relocation.
Which concrete deck footing arrangement should you use?
Two common details appear in the cited guides. One uses a manufactured metal post base anchored to a concrete footing, with the wood post centered over the concrete. The other uses a narrower concrete pier or stem rising from a wider load-bearing pad. The stem is a form and load-transfer element; it does not eliminate the need for the wider footing selected for the soil and post load.
Choose the configuration shown in the approved local detail or project design. A galvanized post base and cast-in-place anchor must have the capacity, dimensions, embedment, edge distance, and corrosion resistance required for the post and exposure. Follow the connector manufacturer’s installation instructions and coordinate footing thickness with the anchor hardware. Do not substitute an unlisted strap, cut-down bolt, or interior-rated connector.
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Center the post on the footing unless the engineered detail explicitly permits an offset. Eccentric placement reduces the effective bearing area and can introduce bending that the simple prescriptive table does not cover.
How do you size a footing before applying for a permit?
Use this sequence to turn the deck geometry into a footing decision:
- Identify the governing rules. Record the project address, code edition, local amendments, frost-depth reference, permit handout, and whether the deck is attached, elevated, roofed, or more than one storey.
- Map the support layout. Draw beams, joists, posts, stairs, and any roof or hot-tub loads. Mark each post’s tributary area and the distance from every footing to the house foundation.
- Establish soil conditions. Use the local allowable bearing value only when the site meets its assumptions. Ask whether fill, expansive clay, high water, poor drainage, or rock changes the required detail.
- Calculate or select bearing area. Compare each post reaction with the verified allowable soil pressure. Select the round diameter or square side from the applicable prescriptive table, or have a designer size it when the table limits are exceeded.
- Set depth and elevation. Apply the local frost and minimum-depth rule, then check the five-foot house-foundation rule and any slope or stepped-footing requirements.
- Detail the connection. Specify the post base, anchor type, embedment, galvanizing or other corrosion protection, and post-centering requirement. Ensure the connector is compatible with the concrete strength and post dimensions.
- Show inspection points. The permit drawing should identify footing size, depth, reinforcement if designed, soil-bearing assumption, and the point at which the excavation will be inspected before concrete placement.
If any input is unknown, the correct next step is clarification—not choosing the largest-looking number from an unrelated table.
What should happen during excavation and concrete placement?
Excavate to the approved dimensions without undermining adjacent footings. Keep the bottom free of loose soil and standing water, and have the bearing surface verified before concrete is placed where the local guide requires field verification. If the excavation is deeper than planned because of soft soil, do not backfill with random dirt; ask whether the design calls for compacted structural fill, a larger footing, a pier, or an engineered revision.
Set forms or a concrete form tube plumb and at the elevation shown on the detail. A cylindrical tube forms the pier stem; the load-bearing pad below it must still have the specified diameter, square dimension, and thickness. Place the cast-in anchor or connector so its location and embedment match the approved hardware instructions. Keep the post base centered and protect galvanized components from damage during placement.
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Use concrete that meets the specified compressive strength. The Houston table assumes 2,500-psi concrete, but a project detail may require a different mix. Do not add water at the site to make placement easier, and do not cover a footing before required inspections are complete. Cure and protect the concrete according to the mix supplier and local requirements before loading the post.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What does Ontario’s small-deck frost exception allow?
Ontario has a narrow exception that cannot be generalized to other provinces, states, or municipalities. The construction note reproduces Sentence 9.12.2.2.(7) of the 2024 Ontario Building Code (O. Reg. 163/24):
“The foundation depths required by Sentence (1) do not apply to foundations for decks and other accessible exterior platforms (a) of not more than 1 storey, (b) not more than 55 m2 in area, (c) where the distance from finished ground to the underside of the floor joists is not more than 600 mm, (d) not supporting a roof, and (e) not attached to another structure, unless it can be demonstrated that differential movement will not adversely affect the performance of that structure.”
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Every listed condition must be met. A deck that is attached to a house, carries a roof, exceeds the area or height limit, or has a different movement relationship does not automatically qualify. Confirm the current municipal interpretation and permit requirements for the actual site.
The same Ontario note reproduces Sentence 9.17.4.3.(1): “Wood columns shall be separated from concrete in contact with the ground by 0.05 mm polyethylene film or Type S roll roofing.” Treat that separation as a code detail where it applies, not as a universal substitute for a listed post base.
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What are the most common footing mistakes?
- Using post size as footing size: a 6×6 post does not tell you the bearing area needed below it.
- Applying one frost depth everywhere: frost penetration is local and affected by soil and drainage.
- Ignoring weak or filled soil: a prescriptive table cannot certify an unknown bearing layer.
- Mixing jurisdictions: Houston dimensions, Fairfax depth, and Ontario exceptions are not interchangeable.
- Digging beside the house without matching elevations: excavation can destabilize the existing foundation.
- Confusing a form tube with a footing: the tube forms a pier stem; the pad below carries the bearing load.
- Improvising connectors: use a manufactured, corrosion-resistant base and anchor with documented capacity and correct installation.
- Pouring before inspection: cover-ups can require costly exposure and may fail permit approval.
What materials belong on the footing plan?
List materials by performance rather than brand: concrete meeting the specified strength; a form tube where the approved detail uses a cylindrical pier; a galvanized or otherwise code-compliant post base; a compatible cast-in anchor; and any separation membrane, reinforcement, or drainage materials shown by the local detail. Product dimensions and capacities must match the design. The cited guides illustrate categories, not universal product selections.
How can you document the final decision?
Keep the approved footing table or engineered calculation with the permit set. Annotate the drawing with footing diameter or square size, thickness, depth below reference grade, concrete strength, soil-bearing assumption, post location, connector model or capacity, and the distance to the house foundation. Photograph or record the bearing surface if the local inspector permits it, and document any field change before concrete is placed.
The safest conclusion is straightforward: determine load and soil first, then apply the local frost and foundation-clearance rules. A footing that is deeper or wider than necessary may waste concrete, but one selected without verified assumptions can settle, heave, or compromise the house.
Frequently Asked Questions
Can I use the Houston footing table for a deck in another state?
Not without confirming that your jurisdiction accepts the same prescriptive basis and that your soil, spans, frost conditions, and concrete assumptions match. The Houston table is local guidance built around 1,500 psf soil bearing and 2,500-psi concrete; another building department may require a different table or an engineered design.
What should I do if excavation finds loose fill or groundwater?
Stop at the affected footing and notify the building official or project designer. The approved solution may involve removing and replacing unsuitable material, redesigning the footing, improving drainage, or using an engineered pier; simply pouring deeper concrete is not automatically acceptable.
Does a sonotube or form tube count as the footing?
A tube forms the narrow concrete pier or stem. The wider pad, diameter, square footing, or other bearing element below it still has to be sized for the post load and soil capacity shown in the approved detail.
How do I verify the frost depth for my address?
Ask the local building department for its current frost-depth reference and deck-footing handout, and confirm whether amendments or soil classifications change the requirement. Do not rely on a value from a neighboring city or an old permit drawing.
