Your average wooden deck is going to give you between 10 and 15 years of reliable service. This doesn’t mean that you have to tear the whole thing down after a decade and start fresh, though.
No, it usually means you have to give it a close inspection, repair and replace parts as necessary, and maybe sister a deck post or joist (or two). Yes – you can definitely sister a deck post or joist that needs a little help holding things up…in most cases, anyway!
Read on to find out more about how to safely and reliably sister your deck posts to squeeze even more life from your deck going forward.
Answer To The Question Can I Sister a Deck Post?
Sistering a board is a valid way to make sure that your deck stays strong and stable, even if it isn’t the most ideal fix for the long term.
The idea with a sister is pretty simple and straightforward:
All you’re really doing is running a second post (or joist) right alongside the original, tying the two together and allowing the new “sister” to carry a chunk of the load that the damaged original was carrying on its own.
This is a super popular move for making sure that deck joists are in good condition, but it can be used on posts as well – with one major caveat.
You need to have solid bearing (even if it isn’t full bearing) on the pier that you’re running down to.
Full bearing is the ideal, but the odds of being able to tuck two full size posts on the same pier designed for one are pretty slim. There’s not always a lot of room to stick a second 6×6 alongside another and still have full bearing.
If you’re not able to get full bearing on the new sister beam, though, you might be able to get a little creative with a “split sister”.
The idea here is to cut that new post down the middle and sort of sandwich the original between. Now you’re adding another 6×6 into the mix but split across two opposing sides of the original. The odds of getting better bearing increase with the reduced footprint available.
Some deck builders use engineered wood products (like high quality, structural plywood – OSB and the like) in these sistering situations, too.
You won’t be adding quite as much strength or stability with these cut down kind of sisters, but you may be able to shore things up long enough to secure the materials needed for a bigger tear down or repair and replace project!
Avoid Scabbing When You Can
A sistered post is always going to run the full length of the original that it is giving support to. Or at least close to full support, anyway – maybe a half inch or a full inch short in some circumstances.
New material brought in to add support that doesn’t run the full length of the original isn’t called a sister, but instead is called a scab.
Scabs definitely have their place in construction. There’s a reason that they are installed, after all!
But when you’re talking about sistering up something as important (from a structural standpoint) as a deck post you don’t want to play any games.
Avoid the scabs and go for a full on sister at that point.
The Value of a Sistered Joist
The real value behind a fully sister joist is shoring things up a little bit and buying you some extra time – often a couple of years, even – before you need to address the root cause issues that led to the original parts becoming compromised in the first place.
With deck posts especially you aren’t just going to be able to pop one out, have the rest of the deck fully supported on the other existing posts (without anything bad happening, anyway), and then pop a new one right back in.
Most of the time you are at minimum going to have to pull up some deck boards, access the entirety of that deck post and its multiple connection points, support those points individually when you remove the old deck post, and then tie in a new one that is hopefully a little bit beefier.
There’s a lot involved with that kind of project.
Sistering, on the other hand, can usually be done in 30 minutes to an hour or so if you’ve got the material and tools on hand.
It’s a bit of a no-brainer!
Nail Pattern and Glue Really Matter
You almost want to go a little bit overkill on your sister component, especially when it comes to your nail pattern and the amount of glue you’re using to tie this new sister into the original post.
Failing to secure these boards together is asking for some real structural issues, usually sooner rather than later.
If you don’t secure your sister to the original component correctly you’re not only going to be crippling your ability to carry the extra load that the sister was intended for, but you’re going to be adding even more weight onto that already compromised component.
That’s a surefire recipe for disaster!
Lazily attaching a sister with a couple of nails isn’t going to cut the mustard, either.
This doesn’t mean you should fill the whole sister with ring shanks or giant nails, either. But you are going to want to use a quality construction adhesive designed with exterior use in mind and you’re going to want to consider lag screwing or leg bolting the sister post into the old one.
Get beefy with the fasteners without compromising the utility of the fix itself.
Consult the Code
As with any other project involves you tinkering with the structural components of your deck you want to consult the code and see what they say about using sister joists or sister posts.
This info will always point you in the right direction (and will usually give you a good idea of what the bare minimum is versus a more overkill solution that you’ll want to trend towards).
Closing Thoughts
At the end of the day, as long as you remember that a sister deck post project is always a temporary fix – even if it’s going to buy you a year or more worth of time – you shouldn’t have much to worry about.
Be smart about how you go through the sistering process and you’ll be able to shore that deck up until you can do a total tear out and replace of the parts that have become compromised.
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