What Size Screws to Use for Decking Boards, Frame, Railings?

When I was building my deck a few summers ago, I was amazed at how many different fastening options were available. It would’ve been handy to fasten everything together with just one size of screw, but when I did my research, I realized that would’ve been a really big mistake. So I needed to find out what size screws to use for decking.

You need a different type of screw for each part of your deck. An 8 gauge, 2.5” coated deck screw is most commonly used when fastening deck boards to joists. For deck framing, structural wood screws such as Simpson SDS 1.5” screws work with joist and stringer hangers, as well as post/beam brackets. Railing posts require ½” diameter galvanized or stainless steel through bolts, while railings will need 3.5” coated deck screws for balusters and rails.

Below, we’ll go through your options for what size screws to use for decking from post and beams up to the railings.

What Size Screws for Decking

Understanding Decking Screw Sizes

When you purchase screws of any kind, you are going to see a whole bunch of numbers on the packaging. These numbers are important, and if you don’t know what you are looking for, you’re going to end up with the wrong screws.

Also, understand that screws and nails are different and have different numbers. Don’t assume that 16d nails are the same as ¼” screws because they aren’t. Screws are a whole different animal than nails, and when used properly are more effective.

To save you some trips back and forth to your nearest Home Depot, let’s break down what those numbers mean.

Gauge

The gauge is the diameter of the screw. The higher the gauge, the larger the screw. For deck screws, you are likely looking at either 8 or 10 gauge screws. To be specific, 8 gauge roughly equals a diameter of 4 mm.

If using structural wood screws, then you’ll likely be using ¼” diameter screws. These are also known as 14 gauge, but are usually labeled as ¼”. Any screw with a diameter greater than ¼” is listed in its actual diameter, not gauge.

Length

This is pretty self-explanatory. After the gauge or diameter, the next number listed on a package of screws is the length.

A common rule of thumb is that a screw needs to go at least halfway into the wood beneath the piece you are attaching. But when attaching deck boards into joists, a 2.5” deck screw is usually long enough since over half the screw is buried in the joist.

Using screws to affix brackets such as hurricane ties and joist hangers, you may need shorter screws that will not protrude through the ledger boards or joists. In this case, ¼” SDS structural screws of either 1.5” or 2” are good.

Thread Size

Many deck screws will not have the thread size listed, as the term “deck” usually indicates a coarser thread. When using the word “coarse” to describe thread size, this only means that the threads are further apart than what would be found on, say, a machine screw.

Screws that are wood, deck, or multi-purpose and rated for treated-wood use are going to have a similar thread size – which is fine.

Types of Deck Screw

There are tons of different types of deck screws out there and it can be overwhelming when standing in the fastener aisle at Home Depot. Let’s have a look at the different types of deck screws.

Regular Deck Screws

A “regular” deck screw is a steel screw coated with zinc or another weatherproof coating. Usually, they are green or brown and come in a variety of lengths and gauges. These are the most commonly used deck screw, the cheapest, and are effective. Do not use in shear force applications.

Structural screws

Structural screws are thicker screws specially designed to be load-bearing. That means these screws can be used in place of nails when attaching joists to ledger boards, beams to joists, and for railing posts.

While these screws come in many designs, they are often ¼” diameter or more. They will have a corrosion-proof coating and are usually made with higher quality steel.

Hidden Decking Screws

Hidden deck screws are just that – hidden. These are brand-specific screws that work in tandem with a jig or bracket. Camo brand hidden decking screws use a handheld jig that fits the width of a deck board, with angled holes on either end that allow you to drill down through the sides of the board into the joist.

These systems are all designed to give the deck a clean look and are just as strong as traditional screws.

Stainless Decking Screws

Stainless steel decking screws are ideal in any exterior application because they are extremely rust-resistant. However, the cost is higher than a regular wood decking screw making them less common.

When installing cedar decking, stainless screws are a must. Coated screws may leave a residue or stain over time as the coating bleeds into the wood. Stainless screws, which are not coated, will not discolor cedar.

Composite Fasteners

Screws used with composite decking typically have a much smaller head and tighter threads. The smaller head reduces composite tear-out. In some cases, these screws have a reverse-thread just under the screw head. A reverse thread ensures the screw sits cleanly at level with the composite deck board.

Composite screws, with their narrow thread, also have the benefit of not needing any pre-drilling. This is a huge time saver. All composite fasteners come with a weatherproof coating and a star or torx head.

Lag bolts

Lag bolts are a tried and tested method to affix two pieces of lumber in shear force applications. These bolts typically have a hexagonal head and are at least ¼” in diameter. However, unlike a typical bolt, a lag bolt has a pointed end like a screw.

Lag bolts are ideal for posts or ledger boards, where the construction of the deck requires the bolt to be buried in the lumber.

What Size Screws to Use for Decking

Now that we know the difference between the various screws available for your next deck project, let’s investigate the ideal size of screw to use for every application.

Deck Boards

The prime focus of your deck, you want your deck boards to be secure and to look good. For most of us, working within a strict budget, coated deck screws are the most viable option. Standard green or brown coated screws are the cheapest. A 3” length deck screws are preferable to 2.5” as they hold better.

A #8 diameter deck screw is ideal, as any higher would split the wood. Remember, if you are using cedar, then you should opt to use a hidden fastening system as these coated screws will stain your cedar.

If you prefer a cleaner look for your deck boards, then the CAMO fasteners are ideal. Unlike other hidden systems that use brackets between the deck boards, the CAMO system relies on a handheld jig that fits over the width of a deck board. On either end, the jig has two angled holes that ensure the screw is fastened diagonally, out of sight.

CAMO deck screws come in several lengths, with the most common being 2 ⅜”. These screws are also coated but are narrower at #7 diameter. Each pack of screws comes with a bit, which is star-shaped. As well, the screws have a reverse-thread near the head. This pulls any wood that has torn out during the screw back in, reducing any tearout.

Finally, the CAMO jig is adjustable so if you’ve milled your deck boards that are a true 6” or any other width, the jig will shift to fit. Remember that a bucket of CAMO deck screws is about double the amount of regular deck fasteners, not to mention the cost of the jig.

Another advantage of CAMO installation tool is that it provides automatic decking spacing.

Post to Beam

When affixing a post to a beam, making use of a galvanized post cap/beam holder is ideal. First, it puts the weight of the beam entirely atop of the post, and it allows you to sink a number of structural screws through the galvanized post cap.

Simpson Strong-Tie galvanized post caps should be used with Simpson SD #10 diameter, 2 ½” long galvanized screws. They have a higher load rating than 16d nails, commonly used in bracket applications. The SD screws are hex-head, and a driver comes with the screws. Finally, these screws are designed to fit within the holes of the brackets, as they are both Simpson brand.

Joists to Beam

Using hurricane ties to attach joists to a beam is ideal. Again, I recommend using Simpson SD #10 diameter screws. However, hurricane ties require screwing into the thickness of the joist. Therefore, 1 ½” length SD screws should be used for this connection.

There are several different styles of hurricane tie, but this type of screw should suffice as long as they are Simpson brand. While the cost is roughly four or five times the cost of 16d galvanized nails, these screws are much stronger. As well, driving screws into brackets can be faster than nailing.

Joist to Ledger Board

Affixing a joist to the ledger board requires a joist hanger. Use of a #9 diameter, 2 ½” long Simpson SD galvanized screws work best with the joist holders. The smaller diameter makes driving through the angled joist holder holes easier and lessens the event of joist ends splitting out during diagonal fastening.

When installing joist hangers, space is sometimes at a premium depending on your deck installation. Swinging a hammer is not always possible due to space restrictions, which makes driving screws all the more desirable.

Railings

Installing railing posts depends on your local building code requirements. For most, ½” diameter galvanized, lag bolts driven through joists into the railing post works fine. These bolts have an aggressive thread, a pointed tip just like a regular screw and a hexagonal head. A 6” long bolt is sufficient to use with a 4×4 railing post.

Some building codes require a tension tie to be used in conjunction with a carriage bolt. A tension tie is a galvanized bracket that is screwed into the deck joist behind the railing post. The tie has an eye that is flush to the railing post. A hole is drilled through the rim joist, railing post, and joist on either side so that the carriage bolt goes through all three pieces of lumber and the tension tie.

The use of a carriage bolt with a Simpson tension tie is ideal. It is much stronger than a lag bolt simply because the railing post is fastened on both sides, whereas a lag bolt only attaches the post to one joist. Tension ties are connected to the joist using Simpson SD #10 screws that are 1 ½” long.

Stairs

Treads are non-load bearing. Therefore standard deck screws can be used. If using standard, pressure treated deck boards as treads, then #8 3” coated deck screws is the best option. For cedar, stainless deck screws of the same diameter and length are best to avoid discoloration.

Wood stringers are held up by stringer brackets, which are screwed into . Galvanized Simpson stringer brackets are designed to take Simpson SD #10 screws. Since these brackets screw into the rim joist on your deck, use of 1 ½” long SD screws are fine.

Metal stringers require galvanized lag bolts instead of SD screws and brackets. These lag bolts must be ½” diameter and long enough to penetrate nearly the entirety of framing behind the stringer itself.

Using Screws for Different Deck Materials

The important thing to remember when choosing a screw is to be consistent. If you are buying joist hangers and post caps that are galvanized, then you’ll also want to use galvanized screws. Mixing and matching can result in corrosion and structural failure.

Pressure Treated

Zinc-coated screws are the cheapest, and one of the most reliable, screws to use for pressure treated wood. However, the copper commonly found within today’s pressure treated wood will eventually eat away at the zinc.

If you have the money, splurge for stainless steel screws in pressure treated wood. They’ll last much longer because they aren’t coated. Stainless steel uses nickel and chromium mixed with steel to make an ultra corrosion-proof product. While the cost can be up to ten times the amount, it will still be peanuts compared to what you spend on your decking material.

Cedar

Cedar requires a hidden decking system or stainless steel deck screws. Coated deck screws will, over time, bleed into the cedar and tarnish the natural look of it. Since stainless screws are not coated, then there is no risk of discoloration of the wood around the screw.

Composite

Composite deck screws need material-specific screws. Simpson Strong-Tie makes an excellent Deck-Drive DCU composite screw. I recommend their #10 3” composite screw. The head of the screw is star-shaped and comes with a driver.

The small head of these screws reduces any mushrooming in the material while the thick diameter provides more than enough fastening strength. A small head also improves the look of the surface of the deck.

Conclusion

I hope you have enjoyed this article on how to find which screws work best for your deck. As decking products proliferate, it is important to understand that not every fastener works with any bracket or lumber. Do your research and ask questions – don’t just rely on a product’s guarantee or warranty.

As always, please feel free to comment below if you liked what you read or if you have a comment or question to share.

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