Deck projects need strong foundation

August 26, 2011

Q: My upcoming deck project requires concrete support piers. I’ve never dealt with them before and have many questions. What are my options? How do I make sure the piers are in the correct locations? What type of concrete pier foundation would you install? Can you share some best practices?

A: You have options galore, and what you end up doing will probably be a function of the tools and cash at your disposal.

Piers are structural elements that transfer loads to the earth. They can be made from many materials, but commonly you’ll see them made from concrete, brick, concrete block, stone and sometimes steel. At the end of the day, what is important is the bearing capacity of the soil the pier rests on. You don’t want a pier to sink once the final load is applied to it.

The biggest challenges you face are determining whether your soil is strong enough, and then figuring out how deep you have to place the bottom of the pier. Unless you live in a tropical or subtropical climate, you’ll need to think about frost heave. The bottoms of the piers need to be below the frost line in your area.

You can discover the frost depth by contacting your local building department. You’ll be going there anyway to get a permit for this project. The building inspector will probably want to check the soil to ensure it is strong enough. This means that before you proceed to work on the project, you have to dig the hole for the pier to the required depth and then have the inspector come out and look at it.

As for your options, you can use any number of plastic or dense cardboard forms or tubes that will allow you to pour a concrete pier. You can also buy precast concrete deck piers and have them delivered to your home. The trouble with precast piers is that you need a backhoe to move them and lift them into place. Nonetheless, they are competitively priced when you consider how much it will cost you to buy the forms and possibly bring in a truck with all the concrete you’ll need for the piers.

You can also use a concrete block to build your own piers, but this requires that you pour a concrete footing at the bottom of the hole for the block to rest on. You then have to fill the cores of the concrete block with steel rods and a concrete mix to make them solid. You need to be somewhat skilled to lay a concrete block level and plumb.

Here is a trick for making sure you place the piers in the right place: If the deck you’re building is a simple square or rectangle, and it’s not too far off the ground, it might be best to construct the actual outline of the deck. Then use simple wood posts to temporarily support this box in the air. Once you ensure it is the right size and is level and square, install temporary diagonal braces at the corners to keep the box square. You can then drop plumb bobs or use a laser tool to find the exact position on the ground where your deck posts will be. If you do this carefully, the deck posts will be centered on the exposed pier.

I’ve used the traditional round, dense cardboard tube forms with great success. There are new plastic forms that also work well, and I especially like the ones that have a base shaped like an upside-down funnel; this shape creates its own footer when you pour the wet concrete into the form.

I feel it’s important to make sure that the bottom of the pier is wider than the top. This shape helps prevent frost heave from lifting the pier. If you use the traditional round tube that has the same shape top and bottom, it’s possible for the frozen ground to grab onto the pier at the top and lift it from the ground as the frozen soil moves up. This is not common, but it can happen.

To prevent lift, I would pour a concrete footer that is at least a foot wider in diameter than the pier. I would place bent concrete steel rods in this footer that extend up into the tube. This way, once the pier is poured, it is connected to the large disc of concrete that makes up the footer. If the frost tries to lift the pier, the footer provides lots of resistance.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site, www.
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