Before you dig postholes, check with your local building department for specific code requirements for the type, depth, and strength of deck posts and footings. These codes are based on local climate and terrain. Footings must be stable in soft soil, withstand frost heave, and provide a base to keep posts or beams above decay-causing moisture.
The depth for post footings should be below the frostline (the depth to which frost permeates the soil) to prevent movement caused by freezing and thawing. This depth varies with local climate. You may be required to place a precast pier pad on gravel below the frostline. A posthole normally is between 24 and 42 inches deep, depending upon the soil type, the depth to the frostline, and the height of the post. If the soil at the base of the hole seems loose, compact it with a tamper. Don’t try to dig postholes with a shovel; at the least you should borrow, rent, or buy a posthole digger. If your design requires a large number of holes, rent a power auger or hire a contractor to handle the job. If you rent a power auger, be sure to get thorough instructions on its use.
Tools: Garden spade, clamshell posthole digger, digging bar, sheet of plywood or plastic.
1. Use a posthole digger. If needed, use a garden spade to remove sod. With the handles squeezed together, jam the posthole digger into the ground. Spread the handles apart and lift the dirt out. If you hit large rocks, rent or buy a digging bar (basically an elongated crowbar). Roots can be cut with a tree saw. Do your best to keep the hole as plumb as possible and the bottom level. Flare the bottom of the hole to widen the footing base. Holes should be about 12 inches wide for a typical 4x4 post.
2. Keep dirt away from hole. If you have never dug a posthole, you may be surprised at how much dirt is removed. Don’t make the mistake of piling dirt too close to the hole. Set it several feet away, preferably on a sheet of plywood or plastic to make your clean-up job simple.
Caution! Take Your Time - Digging postholes is demanding labor. If you need to dig holes that are 3 to 4 feet deep, do not be surprised if you spend up to an hour on some holes. You may spend a good bit of this time coaxing out rocks and cutting through roots. This work can be tough on the arms and shoulders and even tougher on the back. Even if you are in relatively good physical shape, it makes sense to take your time and take frequent breaks. As hard as the digging is, don't cheat on the required depth. Rest assured that once this task is finished the rest of the job will seem easy.
Preparing the Foundation
If you removed the mason’s lines I 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.
Tools: Mixing tub, mason’s hoe, shovel, water hose.
Estimating Concrete Needs
■ 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 2.75 square feet of concrete. (3.14 x ,5(2) 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.
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.
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.
Prepare Stair Landing - If you plan to rest stairs on a concrete pad, you may be able to save time and energy by preparing the pad now so you can complete your concrete work at one time. The trick is determining exactly where the pad will be placed, which is difficult to gauge when the deck hasn’t been built yet. To position the pad accurately you have to trust your plans and know where you want the stairs located.