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Moisture Content in Timbers - What You Need to Know
Moisture content in timbers (not framing lumber), is one of the least understood specifications seen in many plans. 19% Moisture Content: o This is a spec for framing lumber, and is based on design values that depend on the MC to improve strength.  2” to 4” thick lumber can, and is, dried to a consistent 19% throughout the piece. o Timbers above this size are very difficult to dry beyond the first couple of inches on each side, and in fact the 19% moisture content can only be achieved by drying the timber in a Radio Frequency Kiln (wood dried this way is referred to as RFKD).  This is like a huge microwave with hydraulic hold-downs that help to keep the wood from twisting as it dries in the RFK.  There are only 2 of these facilities in North America - one in Canada, and one in the Pacific Northwest.  The kilns are expensive (about $2M for one the size of a shipping container), so consequently the process may add as much as 200-300% (or more) to the cost of a job (including freight costs). Surface Drying o Surface Drying gets the MC to a 19% level in the outside 2” layer but the interior of the timber is still “green”, and this moisture will come out over time as the piece continues to dry, which can still lead to instability in the timber down the road. Air Drying. o Timbers sometimes are touted as “Air Dried”.  This is (in best cases), where the timbers are placed on stacking sticks and left in an open warehouse for a period of months.  The outcome is similar to Surface Drying, but with a little more inconsistency. Why does my Architect or Engineer specify 19% MC? o This is the industry standard for “Framing Lumer”, and the benchmark in all grading rules and design value tables - it’s simply assumed that 19% can be easily achieved for timbers as well. What happens in my project over time, and how can I be assured of the most stable timber product? o Once the AC is turned on, your timbers will really start to dry and at this point you encounter the possibility of checking (cracks), twisting, or  other instability. OK, I get it but what can I do to make sure I have trouble-free timbers? The answer is in this specification
Sellwood Bridge reclaimed antique timber 40' long