Before digging footing holes, call 811, your local utility locator. It will find and mark utility lines, but dig carefully; this service doesn’t typically locate sprinkler lines, and here it missed a telecom cable conduit, too.
You’re always disturbing the soil at the bottom of the hole as you dig and it’s hard to scrape it out, especially from deep holes. One trick for removing the loosened soil is to vacuum it out.
Alternatively, loose soil can be compacted with a tamper, a framing offcut, or a section of a 4x4 or 6x6 post.
You can form piers using metal duct, which costs 30% to 40% more (about $4 more per footing) than cardboard tubes, but is more durable in wet conditions. To reduce costs, the author recycles his HVAC sub’s offcuts, as well as old ductwork removed from remodeling jobs.
To increase a pier’s bearing area without adding a lot to the total volume—and weight—of concrete needed to build it, many deck builders add a wide-base form to their standard cardboard-tube footing forms.
Garbage-bag footing bases are useful in wet conditions and when your footings are wider than concrete tubes but smaller than prefabricated wide-base forms. The author digs a 12-inch-diameter hole about two-thirds of the footing depth, then widens the last third of the hole to the calculated size. After duct-taping a heavy-duty garbage bag to a footing tube cut 6 inches longer than two-thirds of the depth, he pokes rebar horizontally through the form 6 inches from the top to suspend the form in the hole. Finally, he pours concrete in the bottom third—the bag—then backfills the space between the hole and the form before filling the form to the top with concrete.
A pier should extend a few inches above grade so that the hardware used to connect the PT post to the pier won’t get covered with soil.
Even though PT posts are treated for ground-contact, end grain will wick up moisture and is particularly vulnerable to rot.
Most post bases have an elevated plate that prevents the bottom of the 4x4 or 6x6 post from coming in direct contact with the concrete.