Concrete Pier Size Calculator

Building a deck is one thing (you need some lumber and some nails). Until you get out there and break ground, then the real work begins.

Sure you can buy the best composite decking and make sure your joists is spaced perfectly, but if your piers aren’t sitting on solid ground below frost line, that entire structure will heave up, crack, or even tilt over in a harsh winter. Building a concrete pier has nothing to do with digging a hole and guessing how big it need to be.

Why You Need to Calculate Your Deck Piers

There’s a balance between the dead weight of the structure. There are also live loads like people and snow. Finally, there is invisible force of wind trying to pry your roof frame apart as it move. The calculator does all that tricky math and geometry for you while you figure out if your design would really work under these site conditions.

Let’s start with the most important part: What lies beneath your feet? Within just one backyard, soil bearing capacity can range from several thousand pounds per square foot (e.g., compacted gravel) to half that amount (loose fill or soft clay). Before you dig, you need to know.

And if you strike poor footing, don’t automatically assume “dig deeper.” Occasionally, it makes sense to expand the base width. Enter the bell footing. Flare the lower portion of the pier. This increases the surface area pushing down on earth, but it also increase the volume of concrete required.

Spread the load. When the ground cannot support concentrated weight, this is a smart approach. The tool is helpful because it tell you how much extra concrete you need for the expansion. Hauling 50% more mix than necessary cost more and produces waste.

In addition, there’s one non-negotiable limit in most places: Frost depth. When water freezes, it expands and tries to push up against whatever is surrounding it. That includes a foundation. Your pier might be two inches taller than the frost line where you live, but as that pier rises with expanding water, it’ll lift itself away from solid frozen earth around it. And all that uneven movement create massive amounts of stress between the wood posts bolted to concrete and the concrete itself.

Always pour a pier so its base extends past maximum historic frost depth for your zip code. That could mean over four feet down in the chillier areas. It’s worth it, though, because the cost of trying to fix a heaved foundation would of been much greater then simply pouring an extra few inches of concrete now.

And then there is the steel. Concrete can be super-strong in compression but very weak when bent or pulled apart. This is why you see rebar cages all over piers that supports heavy decks, carports, or even roofs. If the soil shifts a bit, the vertical bars pulls on the concrete and help prevent the concrete from cracking and splitting.

Cover also makes a difference. To keep the steel protected from corrosion and moisture, you want some concrete around the steel. Ideally, there should be several inches. If the rebar gets wet, it will rust out in a decade or so. So most builders will run number four bars evenly around the perimeter with a couple of inches of concrete covering them. No, this isn’t about making it look industrial. This is about keeping the structure from falling apart when side forces hit it during storms.

Uplift is another factor you need to consider. The wind not only blows downward onto your roof but it also tries to pull upward on the edges and corners. In doing so, it wants to literally lift your whole frame right off the supports. That’s a negative pressure at the base of your posts. And if your piers are not set in the ground properly or don’t weigh enough they will pop out.

The calculator takes all these things into account when recommending how much depth or volume is necessary for proper anchoring. It makes sure there’s enough concrete weight to counter the upward force.

Last, keep in mind that these are all planning estimates. This gets you close enough to order materials and start sketching out your plan. Depending on where you live, there may be very specific code requirements around things like anchor bolts, post bases and how inspections must occur. Sometimes what makes sense on paper can need tweaking depending upon the rules of your local building code. These rules include how they enforce setbacks and how strictly they require you to build something strong.

This gets you in the ballpark as a starting place. Just make sure you check with your town, city, or municipality ahead of time for their rules and regulations before you break any ground. The foundation is only as good as what is built on top of it.