3 Best Framing Options for Trex® Composite Decking

When people think of decking, they often think about what goes on top of the deck. But rarely do they consider the most vital component – framing and substructure. As in, what goes below the deck.

Trex Decking can be installed on one of three options green pressure-treated lumber, kdat lumber or steel framing. Here is a summary of the three framing options for Trex Decking, as well as the Pros and Cons for each.

Option #1 – Trex® Decking Installed on Green/Wet Treated Decking:

The standard application for framing and substructure is traditional wet/green pressure treated wood. The wood is pumped full of chemicals and preservatives that prevent rot and decay for lasting performance. Green treated decking is the least expensive and most readily available option, but is often heavy, filled with moisture, and as it dries leads to warps, cups, twists & bows in wood. That means your beautiful new Trex Deck may be waving, or popping at no fault to the product, but to the substructure it rests on. And for deckbuilders? You can expect more customer complaints with wet treated decking, as it minimizes the stability of Trex Decking. Also, since it’s still “wet”, the homeowner is often surprised that they cannot paint or stain immediately. Green/Wet treated wood is the least expensive option, but also offers the least amount of stability and support for composite decking.


• Most inexpensive option.
• Prevents Rot/Decay.
• Widely available.


• Heavy, filled with moisture.
• Tendency to warp, twist after cutting.
• Can lead to decking instability and deck complaints.
• Cannot seal, paint or stain for up to 6-12 months.

Option #2 (BEST)– Trex® Decking Installed on KDAT Pressure-Treated Wood

According to the Southern Forest Products Association, “KDAT is highly recommended. Re-drying the treated material will return each piece to a workable moisture content, generally to 19% or less. The advantages of KDAT flooring material included enhanced dimensional stability, plus reduced tendencies to warp, twist and cup.” (2014 Southern Pine Decks And Porches Magazine)

Yet, despite its above recommendation, deck builders usually choose to use standard wet/green treated wood, an inferior substructure for composite decking.

Kiln Dried After Treatment lumber has gone through the normal pressure-treating process like traditional wet/green treated wood. But, with one huge difference – the moisture is removed from the wood (down to 19%) or less for a drier, lighter and much more stable substructure for Trex Decking. Only a modest price difference, yet rarely used by deck builders, KDAT wood will add lasting performance, while minimizing cups, warps, twists normally associated with wet treated lumber. Plus, homeowners are elated to discover that you can paint, seal or stain KDAT framing immediately. It is the superior choice for customers who want an affordable, more stable option for their composite decking.


• Lighter, easier to handle.
• Prevents Rot/Decay.
• Recommended nationally as more stable substructure/framing material.
• Fewer deck complaints.
• Minimized warps, cups, twists.
• Can be painted, sealed or stained immediately with no wait.


• Slightly more expensive than green/wet lumber.
• Not as widely available as wet/green lumber.

Option #3 – Trex® Decking Installed on Steel Frames

(©Trex Company, Trex Elevations, trex.com)

It’s hard to deny the allure of an all-steel deck frame. Unlike wood, it won’t shrink, warp or twist. And, it definitely isn’t susceptible to rot and decay. However, the biggest drawback of steel frames for Trex Decking is that it’s by far the most expensive option. Metal Frame substructure has become more common as a substructure for composite decking. However, it comes with a hefty price tag that consumers aren’t usually willing to pay. Similar to KDAT, the benefit of metal frames is rot resistance, and added stability, but at a cost that adds anywhere from 50%-80% more to your deck project.


• Rot and decay are no issue.
• Provides added decking stability.
• Does not need to be painted or stained.


• Very expensive, adding 50% or more to entire deck project.
• Intricate assembly for deck projects.
• Harder to replace pieces
• Rust, corrosion issues

KDAT Pressure Treated Wood Framing with Trex® Decking Clear Winner

For customers looking for the best performance and enhanced stability without adding over-the-top costs to their deck project, KDAT wood is the clear winner. Not only will you minimize moisture problems like rot, decay, warp, twists and cups, but you’ll add a solid, stable substructure to your Trex Decking that’ll stand the test of time.

Northern Crossarm Company is a leading KDAT treated wood supplier located in Chippewa Falls, Wisconsin. We specialize in KDAT lumber and are the largest KDAT treater in the state. To learn more about Northern Crossarm’s KDAT, or find your closest KDAT dealer, please call (715) 723-4100 or visit us online at https://crossarm.com.

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