Decks California - builder - contractor - design - construction

Deck Footings and Piers

Designing decks for our location

We live in the beautiful wine country of Northern California. When we design decks for our area these are the building practices we follow. Always refer to your local building codes or a structural engineer.

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Serving North Marin & West Sonoma Counties in the San Francisco North Bay area of Northern California

Footing sizes in inches - diameter or square x thickness - 1,500 psf bearing
Joist Span ft	Beam Span ft
	6	8	10	12	14	16	18
10	15 13 x6	17 15 x6	19 17 x7	21 19 x8	22 20 x9	24 22 x9	25 23 x10
14	17 15 x6	20 18 x8	22 20 x9	24 22 x10	26 24 x11	28 26 x12	30 28 x12
18	20 18 x7	23 21 x9	25 23 x10	28 26 x11	30 28 x12	32 30 x13	34 32 x14

Footings recommended by the 2010 deck prescriptive

The above table for footings is from the deck prescriptive based on the 2009 IRC being adopted by most building depts. but they can vary so check your jurisdiction.

The loading conditions

The deck 40 psf live and 10 psf dead. The soil bearing 1,500 psf. The concrete 2,500 psi compressive. The footing bottoms are located a minimum of 12 inches below undisturbed ground at native soil or below the frostline whichever is the deepest. The ground slope not greater than 2 horizontal for every 1 vertical.
Footings adjacent to the top of a retaining wall must be set-back from the top of the wall a distance equal to the height of the wall, or be placed below grade an amount equal to the height of the retaining wall.

Example: If the deck has a beam span of 6 ft. or less and a joist span less or equal to 10 ft they recommend a footing either 15" in diameter x 6" thick or a square footing 13"x13"x 6" thick

Footings closer than 5'-0

Footings closer than 5'-0" to the existing exterior house wall must bear at the same elevation as the existing wall footing. Most jurisdictions in California change this to within 3'-0"

Decks building codes - Footings - IRC & IBC

IRC R403 & IBC 1805 Footings
Footings shall be supported on undisturbed natural soils or engineered fill. Concrete footing shall be designed and constructed in accordance with the provisions of Section R403
IRC R403.1.1 & IBC 1805.4.1 Minimum size
The size of footings supporting piers and columns shall be based on the tributary load and allowable soil pressure in accordance with Table R401.4.1 (above)
The minimum width of footings shall be 12 inches, spread footings shall be at least 6 inches in thickness
IRC R403.1.4 & IBC 1805.4.1 Minimum depth
All exterior footings shall be placed at least 12 inches below the undisturbed ground surface.
IRC R403.1.5 Slope
The top surface of footings shall be level. The bottom surface of footings shall not have a slope exceeding one unit vertical in 10 units horizontal (10% slope).
IRC R404.1.2.3.1 Compressive strength
The minimum specified compressive strength of concrete, shall be not less than 2,500 psi

TABLE R401.4.1 - PRESUMPTIVE LOAD-BEARING VALUES OF FOUNDATION MATERIALS
CLASS OF MATERIAL	LOAD-BEARING PRESSURE LB/sq ft
Crystalline bedrock	12,000
Sedimentary and foliated rock	4,000
Sandy gravel and/or gravel	3,000
Sand, silty sand, clayey sand, silty gravel and clayey gravel	2,000
Clay, sandy clay, silty clay, clayey silt, silt and sandy silt	1,500

Decks - How we design our footings

Decks - Footings - Soil bearing

Decks over 30": We would calculate the tributary loads (including the spa load if any from the manufacturer) and size the footing for the soil type. We prefer to use a footing to stabilize the deck from settlement and use a pier with rebar to get the post connection above grade.

Decks under 30": We typically use a 8" dia x 8" thick footing with 4x4 P.T. uncut end embed in concrete to the footing at 24" deep due to the height restriction for the framing. Spas would be set on a concrete pad with the same specifications as a garage slab.

Sandy gravel, gravel: Gravel and sand will settle in water almost immediately. It's usually pretty easy to recognize. Moistened it can't be rolled into a ball. 3000 LB/sq.ft.

It’s hard to identify soils precisely in the field, sometimes an engineering soil report will be required, here are some quick on-site tests we use.

Sand, silty sand: Finer silt particles will take fifteen minutes to an hour to settle. Granular, gritty texture. Moistened it can't be rolled into a ball or readily fragments. 2000 LB/sq.ft.

Clay, silty clay, silt: Clay particles will remain suspended in water for a day or longer. When moistened it has a sticky texture and if it can be rolled as thin as 1 in. without breaking apart, it probably has a substantial percentage of clay. 1500 LB/sq.ft.

When in doubt: Common sense just tells you to use 1500 LB/sq.ft. but most building dept. officials know their territories so give them a call or check on-line. Especially when building on a slope here in California some areas are known for their slides.

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