Pier foundations can be as simple as a sonotube foundation for a porch, deck or small addition, or as complex as an engineered pier-and-grade-beam foundation for expansive soils.
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Simple Pier Foundations
Simple pier-and-grade-beam foundations can support decks, porches, pole barns, and small home additions, providing good insulation and ventilation (below).
Figure: Simple Pier Foundations
For sizing pier diameters for small foundations, the rule of thumb is “1 inch per foot of span.” Thus, a deck that spans 8 ft. will stand comfortably on 8-in.-diameter piers, while a deck that spans 10 ft. requires 10-in.-diameter piers. For spans longer than 12 ft., add a second row of piers and a second girder at the center of the joist span.
Footings for shallow piers (less than 6 ft. deep) will help prevent the pier from settling. The pier footing should be as thick as the pier’s diameter, with sides that measure twice that much. So an 8-in. pier, for example, should rest on a footing that’s 8 in. thick and 16 in. square, while a 12-in. pier should rest on a footing that’s 12 in. thick and 24 in. square.
Simple Pier Foundation Details
Use strap ties to anchor piers to a triple pressure-treated 2-by girder. The ties wrap around the center stick in the built-up girder and lap over the top. (The girder looks better if the ties aren’t exposed on its face.)
Install a poly vapor barrier over the ground beneath the joists, and run it up and staple it to the inside of the girder. This helps prevent water vapor from building up in the shallow space beneath the joists.
Additional moisture control can be added by cantilevering joists 2 in. beyond the rim of the girder, and installing a 2-in.-wide standard metal soffit vent on the underside of the overhanging joists.
Finished grade should slope well away from a pier foundation — about a slope of 4 in. in 10 ft.