Building a beautiful, functional, and (most importantly) safe deck involves a lot more than intelligent design.
The right materials and the right fasteners play a huge role in the safety and stability of your deck. Lag screws in your deck posts are a major piece of the puzzle, securely locking posts and joists together in a way that other fasteners never could.
Does this mean you need to be using lag screws all over your deck? Not exactly.
But below we run through (almost) everything you need to know about why lag screws should be utilized in all of your deck posts.
Can You Use Lag Screws for Deck Posts?
The beautiful thing about lag screws – and why they are recommended to be used in deck posts – is that they can help add a lot of strength and a lot of security compared to other fasteners.
You see, lag screws (sometimes called lag bolts) are designed and engineered to support significantly heavier loads than your every day screw or nail ever could.
This is hugely important when you’re talking about the kinds of loads that deck posts are carrying. We are talking about multiple hundreds of pounds here – and sometimes even more than that – all the time, 24/7.
Using a flimsy fastener in these applications would be a surefire recipe for disaster.
Yes, there are definitely parts of your deck where lag screws would be a little overkill. Fastening the deck boards themselves to the deck joists is a perfect example.
But when you’re talking about 4 x 4, 6 x 6, or 8 x 8 deck posts (or even chunkier pieces of pressure-treated material) you’re going to want a really beefy, heavy duty fastener to get the job done right – one that you can rely on for years and years to come.
Lag screws fit the bill perfectly.
Lag Screw 101
What are lag screws, though?
Well, these overbuilt fasteners rank up there with some of the strongest, most durable, and most reliable fasteners available on the market today.
Not only are these fasteners massive in size (many of them are at least an inch long and at least a quarter of an inch thick, and they get bigger from there) but they are specifically engineered to not only offer a tight fastened between materials but to carry heavy loads, too.
The “front-end” of a lag screw looks a lot like a gigantic drywall screw or metal screw. It has threads that chew into material, biting deep and locking in an extra firm hold.
That’s hugely important when you are tying multiple gigantic pieces of pressure-treated wood together on your deck.
The “back half” of these screws look like a traditional bolt – complete with the traditional bolt hexagon head. The hexagon will head allows these lag screws to be driven deeply into the material that you’re working with, and the bolt body adds that extra stiffness and strength required to support big loads.
Lags are so beefy that they require impact drivers (power impact drivers) to be pushed deeper into the material as well. You’re going to want to make sure that the batteries on your impact driver are fully charged and that you have a firm grip.
These things bite big time!
Pick the Right Deck Fasteners
Of course, certain deck post installations will require you to be a little discerning in the type of fastener that you choose to use – picking either a through bolt or a lag screw.
Through Bolts
Through bolts are a “classic” way to tie deck posts to other components, but the thing here is that you have to first predrill the through hole between the materials that you are joining together.
That might not be a big deal if you’re able to tack these materials up to one another before you start drilling out the hole. If you have to fight this freehand, though, you can quickly see how much of a nightmare through holes can become.
On top of that, you need to have access to both sides of the materials that you are combining with through bolts. You’ll need to have a tool on the head of the bolt itself and a tool on the nut to get it torqued in and tightened down.
Through bolts used to get a lot more use in the past than they do today. Lag screws have displaced them in a lot of previously common applications, especially when it comes to deck posts.
At the same time, if you have to notch deck posts together to get a longer span (for example) nothing beats a couple of through bolts and a lot of construction adhesive.
Lag Screws
There are a lot of benefits to using lag screws, not the least of which is that you can drill and drive them all in one shot – and only need to have access to one face of the materials that you are fastening together.
Not the best thing at handling shear load when you have notched posts together (through bolts are the way to go, like we mentioned a moment ago), lag screws and lag bolts can handle the rest of the deck post fastening heavy lifting for you, though.
The odds are pretty good that if you have to tie joists into posts or posts into ledger boards you’re going to be using lags to get the job done.
Go Galvinized All the Way
When selecting the ledger screws for your project you not only have to make sure that you’re getting the right length and the right diameter for your specific application, but you also need to be sure that the lags you’ve chosen are properly treated.
Steel, stainless steel, steel and zinc, and hot dip galvanized options are pretty common.
In most applications (when we are talking about decks and pressure-treated lumber) you’re going to want to go with a galvanized finish.
Galvanized lag screws are relatively affordable, incredibly durable, and aren’t going to be reactive with the pressure-treated materials that you have for deck posts. They last longer, they hold up to exterior “use and abuse”, and they’ve been proven to be effective for decades.
Sure, stainless steel and special treatment options might offer a slight performance enhancement. But you’ll end up ponying up a lot more money to get your hands on those specialty lag screws without seeing a dramatic bump in the results they produce.
Closing Thoughts
Putting your deck together requires a lot more than having blueprints, the right materials, and proper cuts.
You need to think about the fasteners you’re using, the responsibility those fasteners are going to be charged with, and choosing the right options that are more than up to meet the task.
When it comes to fastening deck posts (or things to deck posts) you can’t go wrong with lag screws.
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