Digging Footings

This project covers digging the footings for a deck using a clamshell digger.

Intro

If building codes allow shallow footings and your deck requires only a few of them, you're in luck. You can probably dig the footings with a clamshell digger. In soft soil it's easy and inexpensive.

If you have a lot of deep footings to dig, however, rent a power auger. Rent a one-person machine if you have a way to get it home. Otherwise go for a two-person auger. In either case make sure the auger has a bit the right diameter for your footings. It should come with extensions so you can start close to the ground and dig to the proper depth.

Working a power auger can be a lot like working out at a gym. You and your assistant should plant your feet firmly and keep a tight grip on the machine. And when you lift the bit out of the hole, bend your legs and lift with them, not your back.

Don't try to dig through rocks. Remove the auger and try to pry the rock with a pry bar. If you hit bedrock, your code may allow you to pour concrete directly on top.

Prestart Checklist

Time
Figure about one hour for each 42-inch footing with a flared bottom

Tools
Clamshell digger or power auger, round-nose shovel, small sledge, handsaw, pry bar, 2-foot level

Skills
Digging or using a power auger, leveling

Prep
Remove sod, lay out site

Materials
Footing tube forms, mason's line

Step 1

If you haven't done so already, mark the center of the footing by spray-painting an X on the landscape spike you have driven in your layout. Use upside-down paint (available at your home center). Untie the mason's lines to get them out of your way, and remove the spike.

Step 2

To start the hole, spread your feet about a foot on either side of the X and with the handles of the clamshell digger parallel to each other, drive the blades into the soil. Put your shoulders into the work; your arms will tire quickly. Let the weight of the digger do as much of the work as possible.

Step 3

Pull the handles apart to capture soil between the blades, then lift the digger straight up. Move the digger to the side and push the handles together to release the soil. Knock clumps from the blades with a 2x2. At the proper depth, work the digger at an angle to flare the bottom of the hole.

Step 4

Clean out any loose soil from the bottom of the hole and pour in a layer of gravel. Tamp the gravel with a tamping bar or a 2x4 to the depth required by codes. The gravel allows water to drain away from the bottom of the footing and helps reduce frost heave.

Step 5

Mark your tube form so it will be long enough to reach the point where the flare begins in the bottom of the hole -- plus about 2 inches so you can attach it to braces. Cut the form with a handsaw, keeping the saw perpendicular to the form.

Installing the tube form: Step 1

Before setting the tube in place, fasten it to 2x braces with short screws. Set the tube in the hole and drive stakes at both ends of the braces. Fasten one end of each brace to a stake and set a 2-foot level on the form. Raise or lower the brace to level the tube, then screw the braces to the stakes.

Installing the tube form: Step 2

Backfill around the outside of the tube, tamping the soil lightly with a 2x2 as you go. If the hole you've dug is more than 3 inches wide on all sides of the tube, tamping the soil may fill the flared bottom of the hole. In this case pour the footing first, then backfill.


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