Deep Dive: Structure of a Deck - Foundation

Concrete piers for a deck foundation

The Foundation of a structure is the part of it which supports the rest of the structure and anchors it to the earth. With decks, we usually do this through concrete piers dug into and poured in the earth.

Any time a structure is placed on the earth, it imposes a downward force due to gravity, compressing the soil. Most of us think of the ground as pretty solid, but with large compressive forces like a deck or house, long time spans (years and decades), and with weathering and changes in moisture content, freezing, etc, soil can become a lot less predictable.

Soil type plays a role in how soil expands and contracts with moisture changes, how it reacts to freezing, etc, but all that is a subject for another blog post.

The main 2 factors we’re concerned with when building a deck are:

1) Distributing the weight of the deck and potential loads (people, furniture, snow, etc) over a large enough surface area of soil given the strength, or Bearing Capacity, of the soil.

Bearing Capacity of Soil - the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. In simpler terms: how much weight the soil can hold over a given surface area without shifting significantly.

2) Preventing Frost Heave - Placing foundation supports deep enough that they won’t be affected by freeze/thaw cycles, i.e. below the Frost Line.

Distributing the Weight of the Deck comes down to taking the square footage of the deck, multiplying it by the load requirements (60 lbs/sq ft, or psf, for decks in our area) and then dividing that load into the tributary loads (explained below) carried by each deck footing (usually a cylindrical or rectangular block of concrete embedded deep in the soil; used to anchor a deck to and distribute loads into the earth).

Footing - for decks, these are commonly pier footings (we like to use bell pier footings) usually a cylindrical or rectangular block of concrete embedded deep in the soil used to anchor a deck and spread loads into the earth. In houses, this often takes the form of a strip footing, a continuous rectangular footing that spreads the load of a structure over a large area of soil.

Tributary Load - the area load of the structure that is supported by a particular structural member, such as a beam, post, or footing. Calculating the exact load per footing can get complicated, but here is a photo that shows the general idea.

For this 10ft x 10ft deck, we have one half of the deck supported by a continuous ledger at the house and the other half supported by a beam with 3-evenly spaced footings. Notice that the tributary area for each footing is different; the center footing actually supports twice as much load as the outer footings because it supports half the load from each side. The center footing will therefore need to be made wider to distribute the larger load over a larger surface area of soil.

For this deck, with a 60 psf load, the center footing would be designed for 1500lb (25 sq ft x 60 psf), and each outer footing would need to support 750lb (12.5 sq ft x 60 psf)

Soil Bearing Capacity can vary quite a bit. Certain soils like sedimentary rock or compacted gravel can withstand upwards of 4000 psf or more, whereas other, softer soils such as clay and silt may only be able to handle 1500 psf. Because a soil compaction test is expensive and a bit overkill for most decks, we tend to design our footings assuming the lower end of soil bearing capacity (1500psf), that way our decks are resting on a solid foundation.*

*Side note: It’s very important to have foundations rest on undisturbed native soil, or soil that has been resting there, compacting for hundreds of years. This is because uncompacted, or poorly compacted soil, can compact by a lot over time when a heavy weight like a deck or house is built upon it, causing excessive settling and structural issues. Usually, we don’t need to dig deeper than 18” in our area for undisturbed native soil, but within a few feet of a house’s foundation, poorly compacted fill dirt can require deeper footings to reach solid earth.

Using the 1500 psf Soil Bearing Capacity above, we can calculate the surface area needed for each footing.

Center Footing: 1500lb Load / 1500 psf Soil Capacity = 1 sq ft

Outer Footings: 750 lb Load / 1500 psf Soil Capacity = 0.5 sq ft

To find the width x length of Square footings, take the square root of the surface area, then convert to inches.

Outer Footing

√0.5 sqft=0.707ft*12"/ft = ~8.5"

-or for Round Footings -

Center Footing

1sq ft= π*r^2 → 1 sq ft/3.14=r^2 → 0.318=r^2 → √0.318=r=0.56ft

0.56 ftx12 = 6.8” radius x 2 = ~14” diameter footing

Preventing Frost Heave

When the ground freezes, it expands, and then it contracts again when it thaws. This phenomenon, often called frost heave, can easily lift entire buildings, and can wreak havoc on structures over time if the foundation is not installed deep enough. In areas that experience freezing weather, your house’s foundation is installed deep below what’s called the frost line (the depth of soil beneath the surface expected to freeze in a given area) so that your foundation doesn’t experience this repeated up/down movement.

It’s important to make sure your deck avoids the same issue, as your deck lifting and dropping over many freeze/thaw cycles, much like bending a paper clip too many times, can cause the connection to your home to fail over time, and cause a deck collapse.

The depth of the frost line depends on your area, in our area, we install footings at least 18” below the soil surface. Check out this Washington State Frost Line Depth Map

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