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Wood Deck Kiln Drying Process

A kiln is essentially a low-temperature oven used to dry wood used for decking. The lumber is placed in a chamber where the temperature, humidity, and airflow is monitored and controlled for optimum results while the wood dries. Kiln drying is used where air-drying is not feasible, for example in tropical regions where humidity is high, and has some advantages over air-dried woods.

Benefits of kiln drying include:

  • Reduced shipping cost, since more water is extracted from the wood, leaving it lighter.
  • Controlled drying results in fewer defects like cracking and checking.
  • More accurate cutting. Wood contracts as it dries. Cutting wood to size before it is fully cured can result in inaccurate measurements and additional expense, since you’ll have to replace miss-cuts. Purchasing kiln-dried deck wood ensures that the wood is fully cured and ready for installation upon delivery.
  • Kiln drying helps the wood resist decay and reduces sap staining.
  • No chemicals are used in the process.
  • As the wood dries, insects and other parasites die, and mold is much less likely to develop.
  • Individual boards are less likely to contract, leaving a more uniform batch.
  • Wood fully dried to a specific moisture content in a kiln is harder, stiffer, and stronger than green wood.
  • Oils or finishes can be applied immediately after the deck is installed. No need for additional acclimatization time.

Kiln drying is a standard industry practice that changes the dimensional stability of the wood, making it more stable and leaving the wood with a specific, desirable moisture content. Too much moisture, and wood used for decking or other outdoor installations can warp, shrink, or crack when exposed to humidity. Kiln dried wood has lower moisture content and, as a result, fewer changes due to humidity.

Air-dried lumber continues to shrink as it cures, tropical hardwoods included. Dimensional stability can change drastically as the moisture content changes, leaving unacceptable gaps in a finished deck. Acclimating the wood on a jobsite helps, but cannot fully compensate for extreme changes in humidity, including air humidity, rain, sleet, and snow. Following installation, extremely dense hardwoods like ipe, will not absorb enough moisture to cause expansion warping.

When installing a deck, it’s important to know the moisture content of the wood in order to determine how much gap the floor requires for expansion. This is especially important for pre-grooved planking with fixed expansion gaps. Air-dried wood tends to shrink more than the 1/8” allowed and can result in unsightly gaps. Kiln dried wood decking comes with a stable moisture content and will not shrink or expand. The tongue and groove construction is smooth and stable, without gaps, although areas exposed to full sun may have slight shrinkage.

(5) Comments

  1. In your article you mentioned that kiln drying is a standard industry practice that changes the dimensional stability of the wood, making it more stable and leaving the wood with a specific, desirable moisture content. Are there certain projects that require kiln dried wood? I can imagine that being in control of how the wood dries can be crucial for certain projects. Using a kiln to dry could be very valuable for a project.

    • BuildDirect Product Expert Team
      BuildDirect Product Expert Team - Reply

      Hi Derek,

      Thanks for reaching out to us! Kiln is typically used in construction as it’s more stable. There aren’t really any projects that have to include kiln dried wood – it’s more seen as a preference for building projects. Please let us know if you have any other questions!

  2. We pressure washed a 11 month old kiln dried deck and after drying some grainy white fibers
    were left on top. Before using its first coat of semi-transparent oil stain we thought we would use
    an orbital sander with 120 fine to remove the fibers and vaccume up any dust.

    Are these small white fibers of concern?


  3. Hi Liz,

    Typically, the Hardwood decking is Kiln Dried. The majority of the Hardwood is being imported into the US. Kiln drying the wood ensures it does not have pests and pass customs inspection. Also, it lowers the overall weight, which lowers the transportation costs. The Pine and Cedar is typically used outside where you want the wood to have a natural moisture level. If the wood is going indoors generally it should be kiln dried. If the wood is going outside like the decking, best to have it green or air dried so that it does not swell and expand with the first bit of rain. Do answer your question…the pine and cedar is typically used outside where a low moisture level is not required….where as the Hardwoods are typically being kiln dried for import and to keep the weight down.

  4. What type of wood is typically available as “Kiln-dried” and used for decks? Do they only use hardwood? or do they use softer woods, like pine or cedar?

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