The dream begins with a sketch of the ideal deck. What will it look like? How about the view? The evening breeze? Then you open the blueprint software and the math doesn’t add up. That gap between your deck posts looks like a tricky algebra equation. Don’t worry: you’re not alone if you freak out when confronted with how much space those posts can covers.
Most homeowners view their post spacing as purely cosmetic. But it’s a structural negotiation. It must be rigid enough to stop the floor from bouncing. It must also support all the furnitures, people, and snow above it. It’s a simple give-and-take. You give away span for material; you pay a price for knowing just how much.
How to Plan Your Deck Structure
Once you can see the path of these loads, the visual breakdown makes perfect sense. Downward is the path of gravity. The path goes from the decking through the joists into the beam, through the post and then into the footing. Any weakness or undersizing anywhere along that line take its toll on the entire system. As the chart shows, this hierarchy explains why your beam span depend entirely off the spacing between your posts. And remember, it’s not magic, it’s physics.
You cannot have posts ten feet apart if you don’t have a thick-enough beam to span that distance without sagging. And if you reduce the size of the beam? You had better close the distance between those posts. The relationship is rigid, you’d best respect it.
The first wrong turn is that most builders choose post size as their starting point. Because 4×4’s look good and seem dainty. However, 4×4’s are extremely prone to buckling on a tall post. Buckling means a sudden sideways failure. It hasn’t even started to crush yet. This is a weak point in your design that will ruin everything.
Jump right to 6×6 if you can for any residential deck. It offers better bucking resistance, costs only a little bit more and provides a much beefier look under heavy beam. Those two extra inches in width will never make you regret that decision.
But there’s more: Species matters too. Most common are Southern Pine and Douglas Fir. But they don’t act alike. A Southern Pine is stronger, allowing for longer spans between supports. That translates into fewer footings and fewer posts which saves you money. And Douglas Fir is a little softer, but also great for structure.
On paper, this doesn’t make that much difference. The spec sheet will look similar, but it makes a world of differance in your pocketbook. When you’re making the most of open space, not only does the size of the wood matter (the dimensions), but so does its strength (the species). By the board foot, you’re paying for strength, so think carefuly.
Hardware is hero of the build. The hardware is the unsung hero of the build. The best timber in the world won’t hold up if it’s nailed with the wrong nail. Post bases lift the wood off the concrete, keep it from rotting and add years to life of the deck. Post caps secure the beam against the post. This keeps the thing intact even when the wind pushes up trying to rip everything apart.
Joist hangers clean transfer the load and don’t depend on toe-nails that is suspect when put under great force. Those little pieces of metal connect the logic together. They’re the glue. If not for them, then you just got yourself a pile of wood that waits for a storm to come along and wreck it.
The last piece of the equation is the footings. Footing sizes depends on tributary areas (how much deck area each post bears). A center post bears more weight than a corner post, so a smaller footing is enough for a corner post. Soil also makes a difference: Firm soil will hold up smaller footings and soft soil needs bigger footings. Below-freezing weather may cause the soil to heave; therefore footings need to be below the frost line.
This is the only portion of the project that you can’t retrofit if done wrong. If your footing doesn’t work, the deck will sink and could of costs big money to correct.
These factors play off one another in planning a deck. I want long spans for an airy feeling but strong posts and beams to achieve them. The chart defines the limits; the decision is yours. Beginning at the desired span, select the required beam to bear the load; then select the post to bear the beam; finally, determine the footing to bear the post.
This is a chain of dependencies. Honor the load path, and your deck lasts decades. Ignore it, and you’re constructing a liability. The view is worth the work, but only if the structure can sustain it.
