Are you getting ready to build a deck but aren’t sure of how – or even if – your deck posts need to be centered on the piers sunk into the ground?
While code enforcement is going to tell you that you need your post centered on footings (according to the 2021 IRC, anyway), those footings are buried underneath the ground – and it’s almost impossible to confirm. Builders (pro and amateur alike) are recommended to center deck post on the piers to stay safe.
In the rest of this detailed guide we run through (almost) everything you need to know about centering those deck posts, why it’s so important, and whether or not you can get away with being “sort of” off-center.
Let’s jump right in!
Do Deck Posts Need to Be Centered on Piers?
The International Residential Code (IRC) – basically the bible of building codes not only in the US, but around the world – was set up with one singular purpose:
To establish minimum billing requirements to keep people safe – and to keep project standing upright!
The 2021 IRC (the most up-to-date as of this writing) clearly states that posts that support deck framing should be centered squarely on all footings.
As we highlighted a moment ago, though, the footings for your deck are going to be buried underneath the surface of the ground. Deep, deep beneath the surface in fact – down below the frost line.
That might be 4 feet underneath the surface in some localities. It might be 6 feet. It might even be deeper than that, the betting on just how chilly things get in the dead of winter.
At the end of the day, though, those footings are always (ALWAYS) going to be buried underneath feet of dirt.
Trying to center your posts perfectly on footings just won’t “cut the mustard”. You can guess – and get pretty close – but you’ll never know for sure.
This is why professional deck builders and amateurs alike are recommended to instead center the posts of their deck structure on the piers that come from the footer underground up out of the surface and provide the actual weight bearing component necessary to support your deck construction.
Full Bearing is the Ultimate Goal
You absolutely, positively want to make sure that your deck posts (upon which your joists are going to be run) are as centered on these piers as humanly possible.
The idea is to get what’s called “full bearing” in the construction trade.
Full bearing means that every square inch of your deck post is sitting on top of the surface of your piers, without any unsupported components that could cause the structure to weaken.
In a perfect world, lighting up your posts to be perfectly centered on your deck piers would be effortless.
You’d simply run your tape, snap your lines, and set your piers without even thinking about anything else – squaring them up right dead center on the piers so that they are getting 100% full bearing.
They also be evenly distributing all the load that they are carrying perfectly across that foundational component, too.
But we don’t live in a perfect world.
No, sometimes after we set our piers they settle a little bit and come “out of whack”.
Sometimes our deck framing plan doesn’t perfectly line up with the way that our piers were laid in the ground. At least not so that every single post is going to hit that pier dead center every time.
In the real world, though, that’s (generally) acceptable so long as you are still getting full bearing – or close to it.
Obviously, you don’t want half of your deck post to be hanging off of your pier completely (giving you 50% or less bearing instead of full bearing). But your post a sliver out of center isn’t going to be the end of the world.
Shoot for full bearing and you’ll be good to go!
What Kind of Soil Are You Working With?
The amount of “wiggle room” you might have to play with when it comes to centering your deck posts on your piers is pretty significantly impacted by the kind of soil that your piers – and footing – have been poured in.
Footings and piers that have been poured into strong, stable soil with fantastic drainage aren’t going to shift or move that much. This is doubly true if these footings and piers are going to be poured into soil that isn’t going to have to deal with a lot of frost, freezing, or heaving, either.
At the same time, if these footings and piers are going to be poured into soil that is a little looser, a sandy, or has drainage problems – and is subjected to a longer frost, a lot of freezing, and the potential for significant heaving every year – those components are going to be moving around a little more.
Throw in the weight of your deck and the force being exerted on those foundational components (the footings and the piers) and you have the potential for things to shift and wiggle on you a little bit.
Maybe even a lot!
This is why there is so much value in testing your soil and making sure that the footings and piers that you establish for your deck posts to be put on are built correctly.
Everything (and we mean everything) starts with the foundation.
If the foundation is a little wonky, if the piers don’t have full bearing on the footing, or if the posts don’t have full bearing on the piers you are going to have to deal with a whole bunch of headache and hassle not only building your deck but enjoying it in the years to come.
Don’t be surprised if poorly supported deck components start to drift or shift on you over time. Don’t be surprised if your deck starts to buckle and heave, either.
These kinds of decks become safety hazards in a hurry, too.
Square things away with your foundation – and center up those posts on each individual pier – and you’ll have a whole lot less to worry about!
Conclusion:
We hope these blog posts have given you a better understanding of why it is so important to install your deck piers and footings correctly.
Poorly installed footings and piers are the root cause of many deck-related problems down the road – from posts that drift and shift, to decks that start to buckle and heave.
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