Poured concrete piers are commonly used, alone or topped with wood posts, to support porches and decks. Properly sized they do an equally good job of supporting an entire structure. The pier foundation is an easy one to build and considerably less expensive than most other kinds. A series of piers is installed around the perimeter of the house and another set within the perimeter to support girders, which in turn bear part of the weight of the structure. Additional piers, usually of smaller size, can be used for porches, decks, carports, roof post supports, and similar elements.
One of the big advantages of pier foundations is that the piers can be of various lengths and suited to the specific topography of the building site. Though all are buried to approximately the same depth below grade, the above-grade portions can be set to whatever length, within reason, that's necessary to compensate for variations in the topography and still pro vide a level platform for the house. Piers also require considerably less site preparation and cause far less site disturbance than most other foundation types. Only a series of relatively small holes need be dug, instead of a huge excavation, and spoil dirt disposal presents little problem.
To build a pier foundation, first lay out the foundation lines and locate the points where the pier holes must be dug. Poured concrete piers are generally round, though they can be formed square as well. Round ones are usually about 12 inches in diameter, and other form sizes are also available. Set the pier centers so that the outer edge of the sill or floor frame will be even with or set back a bit from the outer most edges of the piers. Interior piers should be centered under appropriate support points along the girders.
Dig a series of holes down to frost line or slightly below but at least 2 feet deep, and the same size as the required pier footing (which an be either square or round). Rough-level the bottoms of these holes and pour in concrete to a depth of 4 inches or more, depending upon the foundation design. Short lengths of rein forcing rod crossed and wired together at the crossing points in a # pattern may also be set in place, about at mid-depth of the footing. Level and smooth the top of the footing with a chunk of wood or a small hand float, then bed a pair of right-angled (and bent cold, not with a torch) reinforcing bars in the concrete. Place the elbows of the bars at about mid-depth of the concrete pad, with the stubs pointing straight up (these can be wired to the rebar, if used, and set at the same time). This arrangement forms a footing or punch pad for the pier, and a pair of anchors to bond the pad and pier securely together (Fig. 4-23). Allow the pads to cure for 3 or 4 days before pouring the piers.
Fig. 4-23. Typical concrete pier pad with an angled reinforcing bar.
Note that in some cases the footings are not needed, if the piers themselves are of sufficient size and number that their total bottom area provides enough bearing surface to support the weight of the house. In that case, the forms can dug just slightly larger than the pier forms, the forms can be set directly in the holes, and the piers poured immediately.
To pour the piers, first set a series of level lines over the pier locations so you can tell where the pier tops should be located, and so you can determine the location of their center lines and the total length, or height, of each form. Cut sections of the specially made card board pier form tubes—which are available at most lumberyards—to the correct lengths and stand them in place in the holes. Or, make up your own forms from plywood, well braced. Locate the forms accurately, using your level lines. If you are using tube forms, shovel clean dirt (no rocks bigger than fist-size) in around the tubes to lock them in place, checking their positions as you do so. Tamp this fill down firmly, but be careful not to knock the form askew, or you might have to start all over again. Fill around the tube up to grade level. Wood forms must be securely braced in place in the holes with stakes, braces, and rocks, all of which must be removable.
Pour the forms full of concrete, periodically shoving a stick or old broom handle up and down in the mix, like churning butter, in order to consolidate the mix and work the air bubbles out. After you have poured each form to about the 2-foot level, stick a pair of precut pieces of No. 4 reinforcing bars down into the fresh concrete until they bottom. Top up the forms (Fig. 4-24), then level the tops and insert anchor bolts or pins deep into the mix and plumb. Cover the top of each form with a plastic bag secured with string or mechanic’s wire.
Fig. 4-24. A heavy cardboard pier form topped up with freshly poured
concrete.
Allow the piers to cure for at least a week before you load any weight onto them. If possible, wait 2 weeks, because by that time the concrete will have nearly reached maximum strength. The cardboard form can be peeled away after 5 or 6 days, but is better left for a couple of weeks. The below-grade portion can be left on; it will do no harm and will eventually rot away. Wood forms and braces can be dismantled and stripped away, and the back-filling finished after about a week.