Building Proper Concrete Deck Footings
Here are five ways to build deck footings that will last a lifetime.
Surely you have experienced or seen the work and effort that goes
into digging and preparing a proper foundation for any light outdoor
structure like a deck or a shed.
Well did you know that even with all that work it can all be for
not if you don't follow a few very simple standards or instructions
during the construction?
If you dig too shallow and not wide enough the pier can be pushed up with frost heave.
If your footing width is wider than the thickness it can crack and break apart.
Don't let this happen to your deck.
Check out these illustrations and follow these tips that are based on the building code recommendations and you should be fine:
A Straight Concrete Pier
This is a great dependable style of footing that uses a standard concrete form without any flared or bell shaped bottom.
If you are planning to dig and you have soil with a high
bearing capacity this style of concrete pier will do the trick for you.
Notice the post anchor on top is secured to the concrete with a J bolt
dropped into the cement and the post is set up off the concrete surface.
"X" is the desired width of the bottom of the pier and it is the area
that will bear all of the weight imposed on it from the tributary area
of the structure above it.
So whatever load value is imposed on that surface area, make sure it
is less than the maximum load that the soil can bear, or your deck
footings will sink, and they will not support the weight.
To see larger images, just click on each picture.
A Straight Concrete Pier With A Wider Flat Footing
Just add a wider base and you substantially reduce the imposed load on the soil below.
Simple enough concept if you think about it.
By using the same diameter of pier as shown in the first diagram you
can easily increase the surface area below it by digging the hole wider
and pouring a footing with a larger surface area.
This will spread or dissipate the weight from the above tributary
loading area over a larger area of soil and reduces the per square foot
load on the soil.
It's a bit like walking in the snow with snow shoes or high heels. What footwear would float you up higher on the surface? :)
Key Points to Remember
"Y" is the distance the deck footings extend beyond the cylindrical
pier. The thickness of the footing must always be greater than that
If it is less than "Y" and you over load this pier, the concrete is
likely to break along the bottom surface "X". The break line would be
like a 45-degree angle flaring outward from the wall of the pier.
A Straight Concrete Pier With A Wider Flat Footing
Going one step further, use a pre formed bell shaped footing to adapt to a variety of form dimension diameters.
It takes a lot of time and effort to lay down a square 2x4 footing frame in order to make the footing seen above.
But instead you can use a plastic bell-shaped form like this one.
There are a couple types available on the market and they are usually
adaptable to sonotube forms of differing diameters.
A good size for a common backyard deck is a 10" or 12" diameter for the sonotube and a 20" base diameter.
Of course, you can get a 24" base size and it will spread weight over an even greater surface area.
The one down side to this is the extra digging involved. For example, if you live in a place like New Hampshire where the frost level might be
48" or more, you will have a lot of back breaking digging to do.
You could hire an auger for about $50 a hole which will speed things up, or you can check out here a new concrete form system that mimics this style and shape and will save you time, money and labor. I think it is a brilliant idea.
Or Use A Bell Shape Form From Top To Bottom
This footing is not moving anywhere!
As you can see here, you can take the concept of a bell shape and go
literally hog wild. Here, the cone shape begins at the surface or just
below and flares outward at 30 degrees or less off vertical.
The flat part at the bottom must be at least 6" thick. This is a common rule for footing so don't forget that.
You will notice that the support post is also embedded deeply into
the concrete and extends no closer than say 4" from the bottom of the
With these style of deck footings there is a lot more work to do. You will
have to dig a wide hole and straight down to match the diameter of the
base. Also, anytime you permanently set a wood post into concrete you risk rot and decay.
So, any post used in this manner should be treated for ground contact because overtime it will be subject to a lot of moisture.
Save time and money on concrete with this style.
For a quick and dirty concrete foundation, this method can do the trick.
You can get away with usually a 15"x15" base and then at least 6"
thick so you don't crack the deck footing. That is assuming you are
using a 4x4 which is 3.5" square.
You will have to wait for the cement to cure and then back fill over top.
Since the post will be embedded in the concrete and in the soil, using a
post treated with preservative for ground contact is a must.
This method has the main benefit of saving a lot time mixing cement.
Five Deck Footings Styles To Support Your Deck
If using concrete, frost heave protection for a ledger connected deck is needed.
The ways of installing concrete deck footings above are best
practices. Remember to always be sure that the underside of any footing is
indeed below the frost line.
Forgetting to do that one simple thing can render all you hard work, time and money.
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