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 constructions?

If you dig to 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 tips based on the building code recommendations and you will be fine.

A straight column

A column pier and flat base

Add a bell shaped base

Full length bell shape

Simple post on footing

A Straight Concrete Pier

A straight cylindrical column is the simplest type of footing but not always the best depending on your soil type.

This is a great dependable style of footing using a standard concrete form without any flared or bell shaped bottom.

If you are planning to dig and you have soil with a fairly high bearing capacity, this style of concrete pier will do the trick for you.

Notice the post anchor on top secured to the concrete with a J bolt dropped into the cement and the post set up off the concrete suface.

"X" is the desired width of the bottom of the pier and 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 the the soild can bear or your deck footings will sink and will not support the weight.

To see larger images, just click on each picture.


A Straight Concrete Pier With A Wider Flat Footing

By adding a wider and larger surface area footing under the pier, the load is spread out over a greater area reducing stress on the soil below

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 spreads or dissipates the weight from the above tributary loading area over a larger area of soil and reduces the per square foot load on the soil.

Its a bit like walking in the snow with snow shoes or high heels. What 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 distance.

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 outwards from the wall of the pier.


A Straight Concrete Pier With A Wider Flat Footing

Go one step further and use a bell shaped form rather than a flat footing.  This has the benefit of being quicker to install and adaptable to different diameter forms.

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 say 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 or more and that will speed things up.

Simple enough concept if you think about it.

There is a new concrete form system that mimics this style and shape. I think it is a brilliant idea. You can check it out here.


Or Use A Bell Shape Form From Top To Bottom

Sink the post deep into the cement with a large full length cone or bell shaped form.

This footing is not moving anywhere!

As you see here, you can take the concept of a bell shape and go literally hog wild. Here, the cone shape begins and the surface or just below and flares outward at 30 degrees or less off vertical.

The flat part at the bottom has to 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 footing.

These style of deck footings are a lot more work to do. You will have to dig a wide hole and straight down to match the diamter of the base. Also anytime to 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. This post will be subject to a lot of moisture over time.


Simplest Concrete Footing Method

Sink the post into the ground supported on top of the concrete footing

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 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 concrete, frost heave protection for a ledger connected deck is needed, you have the knowledge.

The ways of installing concrete deck footings above are best practises and so just 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 moot.

Some Other Great Reading...

See A Unique Bell Shaped Concrete Form

Back to Top of Deck Footings

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