Building Foundation for the Deck
Decks and overheads require a foundation for each load-bearing post. The type and dimensions of the foundation you’ll need depends on the height of the structure, the types of materials you plan to use, how deep the ground freezes, and the load-bearing capacity of the soil.
Because they support wood posts or beams, which deteriorate with ground contact, post supports typically extend above grade. They are usually cylinders or blocks, at least 8 inches across, with straight or beveled sides.
Footings are typically 18 inches square and at least 6 inches thick. You should dig the holes for them to a depth prescribed by local building codes—anywhere from 1 to 6 feet or more, depending on the local depth of the frost and the stability of the soils in your area.
Footings and piers
When it comes to supporting the posts for your deck or overhead, you have several options.
- PRECAST PIERS: A precast pier generally looks like a pyramid with its top cut off. You can purchase precast piers at your local materials supplier in sizes that are appropriate to your installation. The better piers have post anchors already embedded and are ready-made for setting in a poured footing while the concrete is still wet.
- POURED-IN-PLACE PIERS: These are piers that you make yourself, molded in a form and poured at the same time you pour the footings.
- You can buy prefabricated forms (usually they are large cylinders that you place in the footing hole) or make your own configurations with wooden forms. Poured-in-place piers offer decorative options that precast varieties do not.
- POST ANCHORS: Some post anchors are designed to be set in the footings or piers while the concrete is still wet; others can be added later by drilling through the concrete, but these tend to be less stable.
Mixing concrete for postholes is usually easier with premixed bags; you just add water and provide the muscle power to mix it. Using premix is slightly more costly than mixing your own concrete from dry materials, but the expense is more than offset by the convenience.
Holes for 4×4 posts (3 feet deep) require only a little more than 2 cubic feet each—two 60-pound sacks per posthole. You can fill about three postholes with four 90-pound sacks of premix. Make sure to buy extra. Concrete isn’t expensive, and you won’t want to come up short in the middle of pouring.