Can You Use Composite Deck Screws on Pressure Treated Wood?

Image of composite deck screw but Can You Use Composite Deck Screws on Pressure Treated Wood?For nearly 70 years, pressure treatment of wood has been used as a preservation method to increase its durability, protect it from destruction by fungi and insects, and even act as a fire retardant in some cases. Companies make pressure-treated wood by putting the lumber in a pressure chamber, removing the air, and flooding the chamber with the treatment chemicals under pressure, thus driving the chemicals into the lumbers cell structure. While this method has undoubtedly proved efficient in preserving wood, there are some limitations on the type of fasteners you can use with pressure-treated lumber. So, the vital question is, Can you use composite deck screws on pressure treated wood?

Yes, you can. While most manufacturers make composite deck screws to work well with composite materials, you can also use them with pressure-treated wood as long as they meet the specifications found in the ASTM codes. This restriction is because, depending on the number of chemical preservatives retained in the wood, non-recommended screws may get corroded.

Composite deck screws are durable, strong, and provide a higher quality finish. Apart from working to secure your boards, composite deck screws also play a role in the final aesthetic of any wood projects you undertake; thus should be selected with this in mind.

Can You Use Drywall Screws in Pressure Treated Wood?

Image of a dry wall screwNo, we do not recommend using drywall screws in pressure-treated wood.  Different ASTM codes determine the specifications on the coatings for various fasteners to use in ACQ treated lumber. According to the 2006 residential codes, fasteners used in ACQ pressure-preservative treated wood should be of either stainless steel, copper, silicon bronze, or zinc-coated galvanized steel. The weight of the coating is also supposed to follow the ASTM A153.

Though you can use drywall screws in temporary constructions and areas where safety is not a factor to be considered, they are not suitable for building projects. This restriction is because drywall screws are usually brittle, and instead of bending, they snap clean off. This feature can cause frustration, especially if the shaft is already embedded in the wood, as you cannot remove it. Also, the threads in drywall screws go nearly to the head. When fastening two boards, the screw threads at the top may anchor in the top board and keep any panels that you have not tightly clamped apart.

Do I Need to Pre-drill Deck Screws?

Yes, we do recommend that you pre-drill deck screws. Pre-drilling refers to drilling a hole in the board whose diameter is smaller than that of the screw. This pre-drilled hole is known as a pilot hole. Pilot holes essentially serve three primary purposes. First, they lighten the job of the larger drill and guide it to the correct location. Second, you can use them to allow you to insert another hole-making tool. Thirdly, and perhaps most importantly, pilot holes provide clearance for self-threading screws in boards to avoid damaging them. Note that pilot holes are even more crucial when screwing at the board ends.

Deck screws cut their threads when driven into wood. If you drive in a screw without first pre-drilling, the screw can act as a wedge. The screw thus creates outwards pressure that may cause the wood to split or crack and weaken it over time. Screwing without a pilot hole can also cause a breakage of the screw by binding the core.

So, how do you correctly install pilot holes into your deck boards?

  • Use the right-sized drill bit. The pilot hole that you drill should be slightly smaller or approximately the same size as the minor diameter of the screw. The aim is to drill a hole whose diameter is equal to that of the screw’s main cylinder but have the screw threads cut the wood to create a solid connection. If you cannot eyeball it, you can use charts that show the recommended drill for different screw sizes.
  • The pilot hole depth should be the same length as your screw. Most experienced woodworkers can tell when they reach this depth and stop drilling, but if you cannot, use a drill stop to get the appropriate depth or mark the length of the screw on the drill bit.
  • If you are drilling at board ends, drive the pilot hole at least 25 mm from the boards’ end.

How Many Screws Should I Put in Decking?

When fastening screws in decking, the general standard is to use 350 screws for every 100 square feet of decking. This number is when you are using the standard 6-inch-wide boards with actual width of 5 ½ inches and 16-inch spaced joists. If you are using hidden clips, you can divide the number by half to use 175 screws per 100 square feet. This number is lower since you will need only one per hoist.

Below are the steps to calculating how many screws you will need for your deck size.

Image of an outdoor deck. So, Will an Outdoor Rug Damage a Wood Deck?Step 1: Measure the area of your deck.

You need to know how big your deck is to find the number of screws required. Measure the deck dimensions and determine the square footage.

Step 2: Determine the joist spacing and the number of deck boards.

The joists are what keep the boards attached to your deck. After finding the joist spacing, you can then measure the deck boards’ sizes and determine the number of deck boards that are needed.

Step 3: Calculate the number of screws needed.

Determine the number of screws that you will need at the joist intersections. We recommend at least two screws per intersection, though you can add some more depending on the design you want. With the number of screws per joist, the number of joists, and the number of deck boards, you can get the number of screws needed by multiplying them. 

To save you a lot of time and effort, and if mathematics is not your forte, you can look up an online deck screw calculator that will quickly and efficiently find the number for you.

What Size Screws Should Be Used to Install Wood Decking?

When installing the deck boards, an #8 diameter deck screw is the best option. A screw with any higher diameter would cause the wood to split. You can use screws that are 3-inch long as they are preferable to shorter (2.5-inch) screws since they provide a better hold. In attaching the joists to the beam, a #10 diameter screw with a 1 ½ inch length is the best option. 

Should I leave a Gap Between Deck Boards?

Yes, you should. The gap in deck boards refers to the small edge-to-edge spaces between the deck boards. Decking without leaving the right-sized gap can cause the deck boards to buckle out of place, expand or cup. The deck boards will then need replacement or reinstallation, a process that is both time-consuming and costly.

So, why is it so crucial that you leave gaps between your deck boards?

  1. To account for the expansion and contraction of the wood

The changes in wood volume are primarily driven by moisture, with wood fibers capable of retaining it. When you expose the deck boards to moisture, they readily absorb it and swell. Conversely, in drier conditions, the wood will dry out and shrink. Since you install the deck boards outdoors, you continually expose them to changing weather conditions, and their volume will change, respectively.

Installing your deck boards without accounting for this and leaving no gaps or too small gaps will cause the wood to buckle off the deck, displace fasteners and crack when they expand.

  1. For proper drainage

Leaving gaps between your decking board will create an effective drainage channel for any water on the deck. This feature will reduce or even eliminate the chances of water pooling on your deck. Good drainage is vital in preventing moisture damage on the boards and mildew growth and rot.

  1. To allow good airflow

The right-sized gaps between your decking boards will also help in allowing airflow through the deck. The gaps will let the evaporating air escape, which will work towards keeping your deck boards rot-free.

  1. Cleaning

As the deck is outdoors, it will inevitably require a fair bit of cleaning from time to time. You may find that you have a fair amount of debris raining down on your outdoor deck. Deck boards with proper spacing will allow some trash to fall through the gaps to the ground, making your cleaning job more manageable.

Though leaving gaps between the deck boards undoubtedly has its advantages, some hazards come with leaving too large a gap. More significant gaps can diminish the beauty of the deck and make it seem unsightly. They can also be a hazard for people and pets, causing accidents. Another problem with larger spaces is too much airflow which can make the deck more susceptible to fire.

With this in mind, what gap size should you leave between deck boards?

The size of the gap you leave between deck boards will vary depending on various factors such as the humidity of the area, the season of installation, and the wood’s moisture level. Typically, the acceptable spacing is between ¼ inch and 1/8 inch.

If you are installing wood with high moisture content or pressure-treated lumber, you will often buy it wet, so you should install the deck boards butted against each other. When the wood dries out over time, it will shrink and create the necessary gaps.

You should also consider carefully the type of wood that you plan to use. Different wood types absorb moisture differently and expand and contract differently, factors that you should plan for.

When installing your deck boards during hotter months, you should leave a gap that anticipates the boards’ expansion in the wetter, more humid periods. The final deck gap for kiln-dried wood with a low moisture content should be around ¼ inch, while for air-dried decking, the gap should be less than ¼ inch as the boards will likely shrink over time, creating more significant gaps.

Below are some expert tips and tricks to help you choose the proper deck spacing.

  • Acclimate the wood – most of the wood you purchase has previously been stored in a climate-controlled area. After purchase, let the wood acclimate and adjust to the local climate for 1 to 2 weeks in a dry place outdoors. This acclimatization will work to limit any changes it may undergo after installation. 

If you plan to acclimate your wood, make it remains covered and that it does not sit directly on the ground to keep it from soaking up any groundwater or rain.

  • After acclimating the wood, you can finish the boards with a penetrative oil finish.
  • Perform a dry run to see the final look of the boards before fastening them in place.
  • Install the first deck board at least 1/8 inch from the sliding. This space will allow for expansion and contraction in that direction and facilitate drainage.
  • You can easily estimate a 1/8-inch gap using an 8d nail as a spacer. Other DIY methods you can use to space the boards include a plywood strip of the required thickness and a speed square. Alternatively, you can also invest in deck board spacing tools.

Final Thoughts

The combination of composite deck screws with pressure-treated wood can rarely go wrong. Not only do you get a durable, high-quality fastener, but one that also improves the overall aesthetic. All this while using preserved wood that you are sure will last you for a good long while without quickly succumbing to rot, insects, or even fire. So…

Can You Use Composite Deck Screws on Pressure Treated Wood?

You definitely can. As long as your deck screws meet the ASTM code specifications, this is a match that will yield great results. While you should keep in mind the final desired look, you should consider many other factors before settling on the fastener type. Such factors include the environmental conditions and climate of the area, the material (how much chemical treatment the wood has retained), and the type of wood.

We appreciate you taking your time to go through this article, and we hope that it has been informative and answered any questions you had. You are welcome to leave any questions, comments, and suggestions in the comment section below.

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Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.