Medullary checking occurs perpendicular to the direction of the annular rings in the wood. As wood dries out and seasons, it will shrink. If the wood shrinks too quickly or too drastically, it will split and open up into what are called "checks". They are very visible on large timber and posts, as these timbers will shrink considerably and quickly, resulting in the development of checks.
While checks can be unsightly on fence posts, they present more of a hazard on boats. First, these checks will allow water to pass through the wood and cause leaks to form. Second, any fasteners that are in the check will become loose!
To reduce the risk of checking, some easy steps can be followed:
- Don't use boxed in heart wood.
- Don't use green wood.
The heart of a tree will almost always check in a characteristic plus sign. The checking will usually run all the way out to the sides of the timber, resulting in a piece of wood that wants to be quartered. If you select a timber that does not have the heart boxed in the center, the risk of checking is greatly reduced. If the timber has a heart, try to orient your cuts so that the heart will be cut out of the finished product. This will remove the heart and the risk of severe checking as the wood dries out.
The other method to avoid checking is to properly season your wood. If you use wet, green wood, it will dry out too fast and shrink. As it shrinks rapidly, it will check and ruin your finished product. To reduce this issue, you want to season the wood slowly. Seasoning is simply the act of drying the wood in a controlled manner. The rule of thumb is one inch per year. If your lumber is 1 inch think, it will take a year to dry out properly. If your lumber is 2 inches thick, it will take 2 years to dry out properly.
This is a rule of thumb. The truth is the wood is dried by the relative moisture content of the room it is stored in. Letting the wood dry slowly will prevent checking by controlling the rate at which it shrinks. If your storage area is rather humid, the wood will dry slower. Likewise, if the room is bone dry, the wood will dry out very quickly.
You may be wondering what difference does it make if the wood seasons in a room or on a boat? It dries just the same, right? Wrong. The reason seasoning reduces checking is the wood dries evenly. If you take a green piece of wood and install it on the boat as a frame, the three sides that face the inside of the hull will dry out as they are exposed to a lot of air, but the side facing the planking will not dry. Three of the four sides will dry out quickly and shrink while the fourth side will remain wet and not shrink as much. The irregular shrinkage will lead to the development of checks. If the wood were allowed to dry out entirely in a room where all four sides are exposed to the air and can shrink together in a controlled manner, the incidence of checking is greatly reduced.
If you do all of this perfectly, and use wood with no heart that has been well seasoned, the risk of developing checks in the future is greatly reduced but not eliminated.
To mitigate the risks of fasteners coming loose because the wood they are in checks, make sure that the screws are oriented properly to the grain. If you insert the screw with the grain, so that the screw runs parallel to the rings, the screw will have very little holding power and runs the risk of causing the wood to split along the grain. If you insert the screw perpendicular to the rings, it will have the most holding power, but will also be set in line with the medullary rays. Medullary rays run perpendicular to the rings as do the medullary checks. While the screw may hold well at first, it may lose holding as checks develop in the future.
The alternative is to set the screws at forty five degrees to the annual rings of the wood. This will place the screws through the rings, giving you the holding power of driving the screw through the rings while not lining up perfectly with the medullary checks that may develop. Now the screws are oriented at an angle to any checks that may develop while still offering an acceptable amount of holding power.
As you may have imagined, this is why quarter sawn lumber is so popular. You can place the screws into any of the faces and the screws will always be oriented diagonally to the annual rings of the wood. This is why quarter sawn lumber is preferred when being used on a boat.