When you start a weekend building project, you usually starts with the pretty stuff. You might choose nice composite boards or decking with minimal upkeep. You picture all of those fun summer parties in your future.
Next thing, you’re up to the frame. Now things get complicated. And here’s what matters most: Does the structure support the load? That’s what a span chart answers for you. And it’ll save you from a bouncing (or worse) structure.
The Strong Parts of Your Deck
There is two factors that determine how widely you can space your posts. One is size of the beam; the other is the span, or distance… Between the beam and the house ledger (see chart). Where those intersect, that’s where you check.
Let’s say you have a 10-foot joist span, and your beam is doubled two-by-ten Southern Pine. Then according to the chart, you can space your posts up to eight feet apart. Change that to a Douglas Fir, and you drop down to seven feet apart. That doesn’t seem like much…until you realize that every additional foot of separation equals one fewer post. And fewer posts means fewer hole is dug and fewer bags of concrete are set.
The post: Many builders don’t give this one its due. For a low-level deck close to the ground, a four-by-four will do fine. But as the post lengthens, it begin to behave like a narrow column, which can be prone to buckling under pressure. The deck builder’s guide advises that, in cases of bigger tributary areas and taller decks, six-by-six posts is better. Tributary area refers to section of the deck that loads weight on a given post. Bigger tributary areas mean more load on a given post. Without considering the width-to-height ratio, you risk having a wobbly structure. People might not want to stand so close to the edge.
The foundations (footings) are critical to the whole process. These should of been dug at least to frost depth (which will vary from area to area). In colder zones, it might be almost four feet deep. The diameter of footings is based off the load and the soil type. A heavier load need a larger footing, as does softer soil which cannot support weight as well. Guessing the footing’s diameter can cause the footing or foundation to shift one side more than another. This can create cracks, structural stress, and potential tilting over time.
“The biggest thing that will fail is usually the connection back to the house. The water comes in at the siding/ledger board interface. You can’t bolt your ledger to the sheathing or you’ll trap water right where it starts to rot. A lot of builders do nails but not drainage. An inch standoff (air gap) separates the house from the ledger board. This allows water to drain out, rather than get soaked up into the wood. With proper flashing plus this gap, it’ll last as long as the house.”
The right lumber isn’t everything; you’ve got to have good hardware, too. Use joist hangers with the proper amount and type of nails. It’s tempting to cut corners here, but it’s dangerous. There’s a reason why building codes spell out the quantity and size of nails used in this application, testing has shown these connections won’t pull apart even when faced with heavy loads or uplift from wind.
Permits serve a specific purpose. In most locations, if you have a deck over 30″ above grade, you need a permit. Why? This process ensures that your ledger is attached properly, your footings are dug deep enough, and so on. It is all done so you and your neighbor can use it safely.
Don’t forget, it’s going to last a while, you don’t want something that looks great temporarily but falls apart later. Know the loads and know the load paths. Start at the bottom. Allow the structure to hold up the rest of itself. When the weather comes, your deck won’t budge.
