Introduction
If you removed the mason's lines to dig the postholes, carefully replace them. Make sure the postholes are deep and wide enough for the footings required for your deck. Remember, the base of the footings must be below the frostline. If the site gets lots of moisture, place gravel in the holes to aid drainage. Cut fiber-form tubes with a handsaw, taking care to make square cuts. These forms should be long enough so they are at least 2 inches above grade and about 6 inches above the bottom of the hole.
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1. Build the forms
Set a fiber-form tube into each hole and fasten 2x4 braces to the sides with decking screws. Center the tube under the mason's line and check for plumb. Drive short cleats into the ground and attach them to the braces. Cut a piece of rebar to length and place it in each tube. Fill each tube with a foot of concrete. After a few minutes, lift the rebar 4 inches off the bottom.
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2. Pour the footings
Fill the remainder of the tube with concrete. Before the concrete sets, insert an anchor bolt into the center of the footing. Leave the threaded end protruding upward with the shank perpendicular to the footing surface. Smooth the surface of the concrete. After the concrete is set, the post anchor can be positioned on the bolt.
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- Whether you mix concrete yourself or have ready-mix concrete delivered by truck, you need to know in advance how much you will need. Most ready-mix companies require a minimum order, which you may be able to meet only if you are building a large deck requiring many postholes.
- To make your own concrete, you can buy premixed bags weighing up to 90 pounds each. These generally make from 1/3 to 2/3 cubic foot per bag, but check the label to determine how much mixed concrete can be made from each bag.
- To calculate how much concrete you need, determine how much is required for each hole, then multiply that amount by the number of holes. Calculate the total volume of each hole using this formula: 3.14 times the squared radius (one-half the diameter) times the height (total depth of the posthole). A 1-foot diameter hole, 3.5 feet deep, would require 3.14 square feet of concrete. (3.14 x .25 x 3.5 = 2.75) If you set the posts in the concrete footing, subtract the space that the post will fill in each hole to determine accurately the amount of concrete needed.
