Whether you’re using wood for a small scale woodworking project or as a heavy duty construction material – the moisture content of your timber can have a profound affect on its performance.
Before, during and after it’s in place – your timber will continue to absorb or lose moisture until it reaches the same level as the surrounding environment. This process, also referred to as equilibration, can cause a range of issues – most commonly involving the wood shrinking unequally or becoming damaged if the process occurs too quickly.
The type of timber you’re using can also play a part in this process, with hardwoods typically being more difficult to dry due to their decreased permeability. This is one of the reasons why softwoods continue to dominate the commercial wood supply around the world.
Obviously, this is far from ideal and in the worst case scenarios can lead to costly delays on construction projects. To avoid this, there’s several ways that the moisture content of your wood can be dramatically decreased before it sees any use.
If drying takes place shortly after felling, it’ll also help protect the timber against decay, fungal infections and infestations from certain types of insect. There’s also a range of beneficial side effects to the drying process, including (but not limited to):
· Decreased weight and subsequently lower handling costs
· Increased strength when compared to fresh timber
· Better insulation properties
· Enhanced suitability for machine work, finishes, glues and paints.
By drying the wood ahead of time, you’ll ensure that any major structural changes (like shrinkage) will take place in the drying process – rather than in-situ. There’s two main techniques for carrying out this process, namely:
Air Drying: Air drying simply involves storing your sawn timber in a clean, dry, temperate and shady area, stacking it on raised foundations and separating each level. This allows nature to take its course and gradually, the wood’s moisture content will become level with that of the surrounding area.
How quickly this process occurs will depend on several factors, including the climate of the surrounding area and to what extent the stack is exposed to air. The effectiveness of the technique can also be improved by coating the planks with substances like oil or wrapping them in a material that still allows some moisture movement.
Kilning: Kilns are basically a type of oven created within a thermally insulated chamber. They’ve been in use for thousands of years and are used to tackle a wide range of tasks including cooking, creating cement and smelting ores.
While air drying might do the trick for small-scale jobs and woodworking projects – the overwhelming majority of the world’s commercial timber is dried in industrial kilns. The kilning process is superior to air drying in several ways, including:
· More effective moisture removal
· Substantially shorter drying times
· Greater control over degrading
· Obliterating fungi and insect infestations
· No seasonal or climactic variations in drying time
Strength Class: D50
Weight per m3 (12% moisture content): 740 kg
Modulus Elasticity (12% moisture content): 13400 N/mm2
Durability: Very Durable
OAK / English or European
– Weight – Variable according to growth conditions. 670 – 720 kg/M3 when Kiln Dried
– Durability – rated as durable meaning that it is suitable for external uses.
– Bending – Oak is good for bending, the timber must be defect free.
Strength Class: D60
Weight per m3 (12% moisture content): 950 – 1100 kg
Modulus Elasticity (12% moisture content): 18500 N/mm2
moisture content – 14% – 18% (+/- 2%)
The timber dries well and fairly rapidly, with only a slight tendency to distortion and splitting
Wood Movement — You Can’t Stop It
Wood is hygroscopic, which means its moisture content will fluctuate based on the relative humidity (RH) of the surrounding air. As humidity increases, the moisture content increases, and the wood expands, and as the humidity decreases, moisture content decreases, and the wood shrinks. This relationship is referred to as Equilibrium Moisture Content (EMC), and can be accurately predicted.
Understanding Equilibrium Moisture Content
The moisture content of wood is tied directly to the relative humidity of the surrounding air. The higher the relative humidity, the higher the MC of the wood. Period. If you’re installing wood that’s recently been transported, or installed on a job, it might take a little while for the material to reach its equilibrium moisture content (EMC) with the air—in other words, for the wood to accommodate to the humidity level for the climate around the wood: the wood may take on more moisture or it may dry out. For example, if wood at 10% MC is exposed to 25% RH, the wood will dry to 5% MC (and shrink as it dries).
How Wood Moves
If the moisture content of the wood you install is too high, excessive shrinkage may occur, along with the risk of problems of unacceptable gaps and cracks in the wood itself. When the moisture content is too low, the wood may expand, and may buckle, bow, and distort surrounding material.
Width of material
The wider the board, the more movement will occur (the term “board” technically refers to wood 1 1/2 in. thick or less, but for this article its use will refer to wood typically used by finish carpenters). It’s a direct proportion: an 8-in. board will move twice as much as a 4-in. board, and a 12-in. board will move 3 times the amount as a 4-in. board. And it’s important to keep in mind that a glued-up panel behaves basically as one wide piece of lumber.
Grain orientation matters
Boards are characterized as being either “flat sawn” or “quarter sawn.” Quarter sawn lumber (also referred to as “rift sawn” or “vertical grain”) shrinks and expands roughly half as much as flat sawn. Most over-the-counter finish material is flat sawn, and you should assume flat sawn values unless you’re sure your material is quarter sawn. Quarter sawn lumber has annular rings that are oriented between 45 and 90 degrees to the board’s face. Flat sawn grain orientation falls between 0 and 45 degrees to the board’s face.
Applied finish does not stop movement
While it’s true a high quality finish will slow the rate of moisture exchange, it will not stop it. Material finished on all surfaces will expand or contract at a slower rate than raw wood, but make no mistake—finished wood will eventually acclimate to EMC levels.
Excessive Moisture Content in delivered material
Optimum moisture content for interior millwork is 6-8%. In the real world, your material may arrive around 9-10%. For installations in unheated areas, the preferred readings are in the 12-14% range, assuming an area is protected from the weather. In most cases, you can deal with material that’s a couple of points high, but keep in mind that the wider the stock, the greater the movement. Ideally, the moisture content of wood should not change more than 2% when put into use.
Typical on-site humidity
At certain points during construction, such as when pouring concrete, plastering or drywalling, tremendous amounts of moisture are often added to the air, causing humidity spikes as high as 80-85% RH. If you are storing finish material on-site during these periods, be sure to keep them wrapped in a vapor impermeable material (like plastic) with as few gaps as possible. Wood stored in this manner will not pick up any appreciable moisture.
Wood is stable at below freezing temperatures.
The moisture in wood is chemically bound in the walls of the wood cells and cannot freeze, and expansion and contraction continues at below freezing temperatures. Wood does acclimate more slowly at lower temperatures.
Wood will expand on warmer days and contract on colder days.
For all practical purposes, thermal expansion and contraction of wood is not an issue. That said, warmer temperatures speed the exchange of moisture within the wood. Moisture exchange will happen more rapidly at warmer temperatures, but there is no thermal movement of wood worth measuring.It doesn’t matter if lumber is kiln-dried.
Kiln-dried hardwood lumber typically leaves the kiln near 6% MC (softwoods at 10-12%). But all kiln-dried material will acclimate to the surrounding EMC levels. The significant advantages of kiln-dried material is that it is typically heated to at least 130 degrees in the kiln, which will stop any insect activity, and also “set” the sap in resinous softwoods (sap in resinous air dried material can bleed from the board after it’s installed, especially when interior temperatures rise in the summer).They don’t make wood like they used to.
It’s true that most of the old growth timber is gone, but properly dried vertical grain material has highly desirable movement characteristics. If you’re seeking material that will move the least, choose one of the more stable species, and specify vertical grain