Had some red oak logs (cut green 1 year ago) rough milled into 5/4 boards in December 2002, stickered outside under metal roof section (open front, sides, and back) in SE Minnesota.
What could I expect the moisture content to be now? Would like to use them for fairly non-critical, non-joined, trim work…a bit of shrinkage would be acceptable.
I know there’s a few Minnesotans that post here that air dry hardwoods. Would appreciate you chiming in…before I start unstacking and re-stacking. Should I let it sit another year?
Replies
Little drying generally occurs during the colder winter months. And, if it was milled this past December, it really didn't get drying until then. For 1" lumber, at least 6-8 months of optimum drying weather would normally be required after the boards were milled and stacked.
That said, you might want to invest in a moisture meter. Be sure to mesasure the moisture at a number of different places and rip some boards so you can get an accurate internal reading.
Thanks, Howie...that's kind of what I thought, but was hoping someone in MN was just now pleasantly surprised at how dry thier similarly treated lumber was. I have access to a meter, but don't even want to unstack without a pretty good chance at 20%. On top on the 5/4 stuff are the 4X5 center-cuts, and my back isn't what it used to be. 8<(
Contrary to assumed belief, wood does dry during the winter. It all relates to the triple point of water.
From the USDA Forest Service publication "Air Drying of Lumber", Table 4 it is stated that the approximate time to air dry green 1 inch lumber to 20% MC is between 70 and 200 days. As your wood is 25% thicker, you should estimate that it will take 50% longer than the times listed in the Table. Seasonal variation is part of this difference in time but other factors such as air velocity through the pile due to orientation, site micro-climate and stickering will affect drying times as well.
You can expect (in MN, after the wood reaches its lowest air dry MC) the following seasonal equilibrium moisture contents as follows:
Stanley Niemiec - Wood Technologist
I would have a tough time believing that wood in Minnesota would dry at the same rate in the winter as it does in the summer. I could be wrong, but wood we cut in the fall in northern NY state, had sawn and stacked outdoors, had little or no change in moisture content when measured after a week of above freezing temperatures in the spring.
That was the reason for my statement that wood dries very little when stack outdoors in below freezing conditions.
I could be wrong but it's not really relevent if a moisture meter is used and actual determination of the wood moisture content indicates it is ready for use.
The rate at which wood dries is obviously a function of temperature but it also is a function of the Relative Humidity of the air and the velocity of the air. That material can dry at a temperature below the freezing point of water is a well known and applied technology called freeze drying.
And when I refer to the triple point of water, what I am stating is that water in a solid state can become a vapor (transmutate) without liquifying first. If you apply energy, in the form of heat as when you boil water, the rate of vaporization increases and part of the technology of kiln drying is to heat the wood. However, in establishing a kiln schedule the most important aspect is to control the relative humidity of the chamber. This establishes a gradient and the moisture content of the wood will equalibrate at some Equilibrium Moisture Content with the moisture in the air.
Mills saw lumber from logs year round with seasonal advantages and disadvantages. Sawing frozen logs is done regularly and generally, the saw tooth configuation is changed to facilitate this. Feed rates are lower and more energy (horsepower) is required but with regard to drying, the likelihood of surface molds and bacteria staining is greatly reduced. Additionally because the rate of drying has decreased due to temperature (as you indicate) the wood is less likely to develop surface checking (and therein honeycombing), collapse and the end splitting is generally lessened as well. Insect degration is also greatly diminished.
But the reality is that the wood does dry. This is confirmed with the following citation from The Air Drying of Lumber publication I referred to in an earlier post:
Even in Minnesota, day time Winter temperatures SOMETIMES rise above the freezing point and stacks will absorb solar energy (if and when the sun does shine) especially if dark shade cloth is used. And once the temperature of the wood does rise above zero (C), then capillary action will occur. While capillary action (of the water) does not equate to drying, it does tend to equalize the moisture content within the wood and therein continues to provide a greater surface area for vaporiztion to occur.
Moisture meters are a handy tool but I find them generally a somewhat inaccurate way to determine moisture content. In many cases they are misused because most individuals do not adjust for both species and temperature as is required. The other reason that moisture meters are a poor choice is that they cannot measure the moisture content of wood if it is above its Fiber Saturation Point (FSP). This hypotheical point exists when all the free water in the wood (ie the liquid in the cellular openings) has been removed and the only water remaining in the wood is that which is chemically bonded to the ligno-cellulosic mattrix. The FSP is generally about 25%MC.
Thus when you stickered the wet wood in the fall and measured it with your meter, the meter probably registered the scale maximum (say 25% MC) even though it could have been actually at 80% MC. If you measure the wood again in the spring, you might get the same meter reading (25% MC) but the wood at that point in time might have actually attained that 25% MC level. The problem is not with the wood or whether or not there was a loss of MC but instead with the inaccuracy of the tool.
The only way to correctly determine moisture contents for wood above the FSP is to dry a sample in a forced air oven to a bone dry condition (0% MC) and then compare the difference between the initial wet weight of the sample and its final (dry) weight. The difference in weight is the weight of the lost (evaporated) water. If you divide this number by the weight of the dry wood and multiply that value by 100, you will obtain a correct oven dry MC.
Stanley Niemiec -- Wood Technologist
"...one-inch northern red oak os often air dried to an average moisture content of about 20% in southern Wisconsin in about 60 days when yarded in June or July. If piled the late fall and winter, the stock must stay on sticks for 120 days or more."
I take that to mean that I very well could be at 20% by now, given that in SE MN we've had a relatively dry winter and spring. Only now is the dew point getting much above 60.
Well, I might take the top few off, and rip some edges. I'll let you know how it comes out.
Please remember that I indicated that 5/4 lumber takes 150% longer to dry than 4/4 lumber. The rule of thumb is that as lumber doubles in thickness, drying times increase fourfold.
Secondly, a 20% MC is generally unsuitable for interior wood work. The dimensional changes associated with changing MC (ie shrinkage and swelling) occur not because the free water in the cell walls is removed but instead result with losses/gains in the bound water. Thus most dimensional changes occur and are calculated in the MC range between the FSP and 0%. Radial directional shrinkage in Northern Red oak is 4.0%, tangential shrinkage is 8.6%. To use this wood for interior applications, you will need to kiln dry the material to an appropriate target MC of approximately 8%.
The best way to determine the MC of wood is to follow the oven drying procedures I previously mentioned. Another drawback of moisture meters (the probe type especially) is that they measure the electrical resistance between the two probes and only to the depth of penetration of those probes. As such, with thicker lumber, the surface MC reading may not necessarily be the average moisture content of the wood. If you resaw a sample into three pieces of equal thickness, you might find that the outer jacket boards may have a considerably lower MC than that of the core board. This is due to the fact that wood dries from the outside in.
I should additionally add that in oven drying samples, you continue to dry them (weighing them periodically) until there is no change in weight. Laboratory sample drying ovens typically operate in the range of 200 to 220 degrees F, have a fan to circulate the air, and are vented. Basically, when the board stops lossing weight, there is no more water within it to evaporate and you have attained a bone-dry state (0% MC).
And when you are choosing samples for MC testing, insure that you select your samples so that they are at least 18" from the end of the board to avoid an erroneous value due to end grain effect (end grain drying rates being 10 to 15 times greater than flat grain).
It's funny how people often tend to focus on temperature instead of humidity when thinking about how weather affects wood drying. Following that logic, ChapStick sales should be at a low in the wintertime!
The only time it approaches "dry" in the Puget Sound between October and May is when it's below freezing outside. Most especially when it's been snowing.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Howie,
Neimiec is right, my actual experiance here in Minnesota verifies that..
I have air dried over 30,000 bd.ft. of wood here in Minnesota.. You can do it in a month if you bring it inside and turn on a dehumidifier. Or stack it inside for a month during the winter..
On the other hand I had some 5/4 ash that was still at 10% moisture after a year outside under a tarp.. (some had reached 8% , but not all) I don't use Red Oak but my white oak 4/4 will typically dry in one spring/summer/fall outside.
I've gotten Black walnut to trim levels in a couple of months outside under a tarp but getting full sunlite.. I get rather severe end checking evan with the ends well sealed but accept it in order to finish the project..
Curious about the ash, although I've never dried ash boards, I've put up and used alot of ash firewood...and it seems to dry much faster and better than oak in branch and split forms. Not to say I don't believe you...but there are two or three different species of ash growing around here. Maybe they have different drying rates depending on species.
My need for the oak at this time is for mainly decorative trim, so I'm willing to put up with quite a bit of movement.
Whereabouts on MN do you live?
Oh I see, Minnetonka...rain enough for you? We're going to get it today.
Edited 6/25/2003 8:37:10 AM ET by johnnyd
This forum post is now archived. Commenting has been disabled