What Causes Tapered Ends on Kiln Dried Wood?

Extra Moisture

• Large, green timbers have fibers that are filled with moisture. While a kiln is a powerful drying device, it can sometimes dry the wood too quickly. This leads to a situation where the ends of the timber dry quickly and the outer fibers lose their moisture, but the core of the wood retains most of its water. As a result, cells shrink along the outsides of the wood where moisture can escape most easily, leading to a slight tapering toward the ends of the boards.

Poor Cuts

• Sometimes tapering is not the fault of the kiln-dry process at all, but a result of poor cutting practices. Timber that is not cut precisely can taper off center, narrowing as the board is cut from end to end. These flaws in the cut may be more noticeable after the kiln dries the wood out. Of course, some boards are cut purposely to taper for a special construction project, in which case the change in slope should be expected.

Species

• The species of wood can also affect the likelihood of tapering. Species that produce many knots may be more subject to tapering issues when kiln-dried. Grain also matters. Some species, like oak, have grains that can actually pull at the ends of the board as the board dries. This results in a slight tapering curve at the ends of either side once the board is finished drying, even if it was straight and level as green timber.

Similar Issues

• While tapering problems can occur in a kiln, they are not the only issue that may arise from poor cuts or uneven drying. Warping and cracking are problems that can occur instead of tapering. They are different ways for wood to react to the same underlying issues, and typically point back to either bad wood or wood that was cut improperly and is showing the effects. This wood cannot usually be used in any serious construction project.