How To Pour Frost-Proof Deck Footings

The fact that water expands when it turns to ice makes life possible on our planet. However, it also creates headaches for homeowners in the form of busted plumbing pipes, ice dams and damaged gutters. It can even cause deck footings to heave, resulting in leaning deck posts and unstable decks.

The climate along California’s Central Coast, where I live, is mild enough for builders to ignore this issue. But that’s not the case in more northern regions. Ruben Saltzman, CEO of home inspection service Structure Tech, has identified frost heave as one of the most common problems with decks where he works in Minnesota.

“Sandy, well-drained soils aren’t particularly susceptible to frost heave,” says Saltzman. “But soils with higher clay content are.”

You only need a few basic concrete skills to pour deck footings, but you do have to ask a few important questions— the depth of the frost line chief among them. In Minnesota, the frost may go as deep as 42 to 60 inches, but a warmer state like Missouri may have a frost depth of only 12 inches. In most of California, it’s five inches, which is considered insignificant. Any footing you pour will be deeper than that.

Because the frost line varies with region, it is important to check with your local building department to ensure your deck footings are deep enough to withstand the winter freeze.

What Causes Frost Heave?

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The fact that water expands when it freezes is the underlying cause of frost heave. When water freezes, it expands about 9 percent—think of how ice cubes are domed above the original water level in an ice cube tray. Ice exerts a pressure of about 50,000 lbs. per square inch when it freezes, which is enough force to lift even a large building. A puny little deck on inadequate footings doesn’t stand a chance.

Water doesn’t freeze uniformly in the soil. Instead, it creates thin layers of frost called ice lenses, and when these form under a footing or latch onto the sides of wood or concrete, they actually push the footing upward. That leaves a space under the footing that can get filled in by the surrounding soil, which prevents the footing from returning to its original depth when the soil warms. This is less likely to occur in sandy, well-draining soil than in soil with a high clay content because sandy soil gives water a chance to drain off before it freezes.

How to Prevent Frost Heave

As Saltzman points out, there are three ways to prevent frost heave:

Dig the base of the footing below the frost line

The bottom of the footing should be several inches below the frost line, so if you’re setting posts in a cold place like Minnesota, you’ll need to dig to a depth of four to six feet, depending on the region.

Flare the bottoms of the footings

If you create a bell shape at the bottom, it will push against the soil above it to resist heaving. However, Saltzman cautions that ice latching onto a concrete footing and exerting upward pressure above the bell can actually fracture the footing, so for deep footings in clay-rich soil prone to freezing, he advises reinforcing concrete footings with re-bar.

Make the edges of the footing smooth

You can bury wooden posts, but since there’s no way to give wood a smooth surface that will resist ice lenses and won’t degrade underground, you’re better off sticking to concrete footings in cold climates, especially in dense soil. The conventional way to make the edges of concrete smooth is to pour it into waxed cardboard tubes known as sonotubes.

• Pro tip: Helical piers offer an alternative to concrete footings for frost-proof deck support. They are galvanized steel posts with a spiral on one end that screws into the ground and a bracket on the other to hold a post. Helical piers resist heaving and fracturing, but they don’t work in rocky soil.

How to Pour Frost-Proof Footings

Here’s a simple three-step procedure for pouring a footing that will withstand harsh winter conditions:

1. Dig a hole

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Verify the frost depth for your area. Dig the hole for the footing four to six inches deeper than the frost depth using a post-hole digger. Flare the bottom of the hole by scraping the side with a shovel and using the post-hole digger to remove the extra material.

2. Order pre-mixed concrete

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Forget about mixing the concrete yourself. You’ll be shocked at how many bags of concrete mix you’d have to mix by hand to fill a form tube. Figure concrete volume by multiplying the height of the post and column in feet by the radius of the sonotube in feet squared times 3.14. Divide the total by 27 to get the cubic yardage you need to order. Add extra for the wide footing bases.

3. Pour the footing

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Hold the sonotube about 12 inches up from the bottom of the footing and secure it by nailing the sides of the tube to the center of a tic-tac-toe grid of 2x4s at the top of the hole. If you’re using re-bar reinforcement, insert it into the tube now. Dump concrete through the tube into the bottom of the hole. After the wide part at the bottom of the hole has filled, the tube will fill, too. Set metal post-holding brackets in the concrete when the tube has filled, and the concrete is fresh.

About the Expert

• Ruben Saltzman is the owner and CEO of Structure Tech home inspection service. He teaches home inspection seminars nationwide.