If you're building a deck in Minnesota or Maine, you've probably heard the term "thermally treated wood decking" thrown around in lumberyards. It's the newer cousin of pressure-treated pine and cedar that's been showing up on job sites for the last decade. I've installed it, I've watched it age through two winters, and I've seen homeowners love it and hate it. Here's what I'd tell a neighbor before they spend the money.
What Is Thermally Treated Wood Decking?
Before we get into costs, let's be clear about what this stuff actually is. This material starts as regular softwood — usually spruce, pine, or ash — and gets cooked in a kiln at 400-plus degrees with steam and no oxygen. That heat changes the cellular structure so the wood can't absorb water the way it used to. Bugs don't want it, rot is slow to start, and the boards come out a rich chocolate color that fades to silver over time. It's not a chemical treatment. No copper, no borate, nothing you have to worry about with kids or pets.
I first ran into this material on a lakeside project in Wayzata. The homeowner had pricey composite on the front porch but wanted real wood in the backyard because he liked the look. We used a thermo-ash product from a regional mill. After three years, the boards looked close to new. The same deck's joists — regular pressure-treated lumber — had cupped and thrown a few fasteners. The difference was night and day.
What Does Thermally Treated Wood Decking Cost?
Now let's talk dollars, because that's what most people ask me first. This decking runs about $9 to $14 per square foot for materials, depending on species and supplier. That's noticeably more than cedar at $4 to $6, but less than premium composite at $12 to $18. Installed, you're looking at $18 to $25 per square foot with fasteners, flashing, and the substructure. A typical 16x20 deck comes in around $6,500 to $8,000 installed. That's in the same zip code as a high-end composite job, but you're getting a natural product that doesn't have plastic's feel problem.
How It Handles Snow, Ice, and Summer Sun
Start by thinking about snow and ice. It handles freeze-thaw cycles well because the boards are stable. They move less than cedar and far less than standard PT lumber. That means fewer popped nails and fewer warped boards come spring. That said, it's not hardwood. It dents if you drop an engine block or move a fire pit across it. And it burns — the boards will char like any other wood if you have a grill mishap.
Another thing nobody tells you: those dark boards get HOT in direct sun. In July, on a south-facing deck, they can be uncomfortable to walk on barefoot. Lighter woods don't soak up heat the same way. If your deck is in shade for part of the day, it's fine. If it's cooked all afternoon, consider a lighter species or go with composite and lose the debate.
Maintenance: Do You Need to Seal It?
Here's where I've seen homeowners go wrong: they treat it like cedar and wait two years before putting any finish on. You don't need to seal thermally treated wood decking to keep it from rotting, but you do need a UV-blocking oil if you want to hold that brown color. Otherwise it weathers to that silver-gray in about one season. Some people love the gray. Others call me saying they paid extra for "mahogany" and got driftwood. Set expectations on day one.
Common Questions From Homeowners
Does it need to be stained? No, but if you want color, use a semitransparent oil-based stain rated for hardwood. Can you screw into it? Yes, and it actually holds fasteners well, but you have to predrill near the ends to avoid splits — same as cedar. Will it last as long as composite? Depends on the deck you compare it to. A good thermally modified board on a proper substructure will easily go 25 years. That beats cheap PT and beats most budget composite boards, though it doesn't beat the 30-50 year warranties on the premium stuff.
Pricing varies a lot by region. In the Midwest, thermo-ash and thermo-pine come in at $1.20 to $1.80 per linear foot for a 5/4x6 board. Out West, you'll pay more for shipping. If you're handy, you can save the $4 to $6 per square foot installation markup and DIY it in a long weekend. But I'll say this: subframe prep matters more than the decking. Level, solid, flashed joists — get that right and the decking will do its job quietly for decades.
Should You Choose Thermally Treated Wood Decking?
If you want the warmth of real wood, hate the plastic feel of composite, and can handle a little maintenance, yes. If you want zero upkeep and live in a climate with brutal sun, go composite. And if you're on a tight budget, cedar is still a solid choice — just plan to replace a few boards in ten years.
There's a reason I keep a bucket full of failed deck samples in my garage. Most failures aren't the material's fault. They're from skipped flashing, bad flashing details, or waiting too long to seal. This material is one of the few I've installed that consistently performs better than homeowners expect. That doesn't happen often in this trade.
If you're shopping for a new deck and want a second pair of eyes on the estimate, print out the material specs and send them over. I'm not selling lumber. I'm just saving you the trip up the ladder.
Stay warm. Stay dry. — Dean
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