Homeowners also think that wood can support both furnitures and parties. The reality is your deck is as solid as dirt beneath it. If footing is too small, in five years you’ll have a wobbly platform.
Plan the foundation first, this cannot be corrected after concrete hardens; you don’t want to cut boards until you know how they are going to sits. There is two main factors that determine the size of a footing: The amount of soil it will support (soil strength), and the size of the area being supported by each post.
How to Build a Strong Deck Base
Heavy clay will hold weight different than sandy soil, or even loose silt. A weak base require a greater footprint so you can spread out its load. It’s a matter of balance; the more weight your footing needs to support, the larger it has to be to resist sinking in ground.
Typically twelve inches is a standard size for most backyard projects because the majority of soil types falls somewhere in the mid-range in terms of capacity. Twelve inches will work for good soil conditions and moderate loads. But if you’re working with a long span between post, or if you’ve got clay as your base, you’ll probably want to bump up the size.
Width is only part of the story; so is depth, and that depends on climate. To determine how deep to dig, you have to know the frost line: the point beneath surface where earth freezes during winter months. When water freezes, it expand… Pushing outward, upward, and trying to push up your deck. That upward force will twist your whole deck out of square if it lifts one end while not affecting other.
In colder climates, it may require two feet (or even more) of digging; in milder ones, a mere foot may be sufficient. But take no chances. Don’t skip this step. It would of cause structural problems later, even if it saves you an hour of work today.
The last part of the puzzle is post placement and concrete choice. Don’t skimp on concrete; inexpensive varieties doesn’t have enough compressive strength to stand up over time. A 6×6 post resists buckling much better than a 4×4, particularly in a tall deck. It is stronger and easily withstand sideways pressure.
Never place the wood right against the concrete. Any moisture that wicks up into timber will rot the wood from inside out in short order. Use a steel base plate, positioned atop the cured footers. This establishes an air space which protect the wood from getting wet and makes any necessary repairs down the road easier.
The mistake many make is that they think all posts are created equal. Often corner posts takes less load than mid-frame ones but it’s usually better to simplify construction by making each footing as large as you need. Doing so provides a safety factor while eliminating guesswork. Tributary area translates into diameter (see chart), but the point is to be consistent.
What you don’t see matters. A lot of building a deck is what you don’t see. All that goes into structure depends on what’s underneath the surface. Measure the soil. Look up frost depth. Pour the concrete for the footings. Take the time and do it right, because nothing makes a better deck than a good foundation.
After all, you want to be able to relax on your deck in the summer instead of worrying if it will stand.
