Building outdoor decks can be a profitable business for builders and remodelers. But you have to get the details right to avoid failures from weak structural connections, weathering and corrosion.
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Deck Foundations
Most
decks can be supported by wood posts resting on concrete footings, piers, or a
combination of the two.
Size. Assuming a minimum soil bearing capacity of
1,500 psf, 8-inch-diameter concrete piers bearing on square footings measuring
2 feet on a side and 9 to 11 inches thick are adequate for most single-story
decks where beams are spaced 14 feet apart or less and joist spans are 14 feet
or less (see Footing Size table below).
Beam
Span
Joist
Span
Round
(diameter)
Square
Footing
Thickness
8'
<10'
20"
18"x18"
8"
<14'
24"
21"x21"
10"
<18'
27"
24"x24"
11"
10'
<10'
23"
20"x20"
9"
<14'
27"
24"x24"
11"
<18'
31"
27"x27"
13"
12'
<10'
25"
22"x22"
10"
<14'
30"
26"x26"
13"
<18'
34"
30"x30"
15"
14'
<10'
27"
24"x24"
11"
<14'
32"
29"x29"
14"
<18'
37"
33"x33"
16"
16"
<10'
29"
26"x26"
12"
<14'
35"
31"x31"
15"
<18'
40"
35"x35"
18"
Depth. Keep deck footings at
least 5 feet from a house foundation. Deck footings closer than 5 feet should
be set at the same depth as the house footing. Otherwise, place footings below
the frostline or at least 12 inches below grade.
Reinforcement. To prevent cracking of
the footing due to point-load forces, add steel reinforcing. Make sure an
unreinforced footing is not too wide. It should extend at least 2 inches beyond
the pier but no more than the thickness of the footing. In all cases, center
piers or posts on footings.
Engineered alternatives. For problem soils, steep
slopes, or sites with difficult access, or where environmental or other
conditions preclude excavation, several engineered alternatives to conventional
concrete footings and piers are available:
Hydraulically driven helical piles
(photo above) are typically
installed by subcontractors who use torque readings to achieve the required
bearing capacity in a variety of soils. (For more information, see Working with
Helical Piles)
Precast pin footings (photo, above) bear on steel pipes driven using
a hammer drill through sleeves in the piers to a depth determined by the
frostline. (For more information on pin
footings, see Slideshow: On Site with Diamond Pier)
Call 811 before you dig. Whether you plan to dig
holes for footings or use an engineered pier system, take the precaution of
calling ahead to determine the existence and location of any underground
services in the project area. In most of the U.S., simply call 811 at
least three days before digging (in Canada, each province has a separate phone
number for information).
