Checks and splits in lumber and timbers, especially timbers, are often misunderstood when assessing the condition of a structure. I'm going to outline the basics here. There are two means by which checks and splits can form in wood elements: during seasoning, or drying, and during manufacture. This blog is concerned with checks and splits which are the result of seasoning after installation. If after reading this blog you have any specific questions, please do not hesitate to contact Wood Science Consulting.
The development of checks and splits after installation occurs after the element has dried in place. Quite often in older buildings the elements were installed green (i.e. "wet"), especially timbers. Due to their size, it's not practical for timbers to be kiln dried. Some are air dried for a period of time prior to installation, but for the most part they are installed green, and therefore, are allowed to dry in-situ. The same applies to a lot of dimension lumber.
During the seasoning process, stresses develop in wood as a result of differential shrinkage which can often lead to checking, splitting and even warping. Separation of the wood fibers results in checking and splitting. Due to the innate characteristics of wood, it shrinks and swells differently. This is best explained in the image below. As a general rule of thumb wood shrinks (swells) approximately twice as much in the tangential direction of the annual rings as compared to the radial direction. Additionally, during the initial drying process the outside of an element inevitably dries quicker than the interior, which also causes differential stresses to develop within an element. The combined effect of these drying stresses in a wood element often, and sometimes inevitably, results in the formation of a check or a split. Since the weakest strength property in wood is tension perpendicular to the grain (similar to the manner in which wood is split using an ax), drying stresses can result in a check or split that form in a radial direction across the annual rings. However, while these seasoning characteristics may initially appear as problematic, they likely are not. It is important to remember that as as wood dries, it becomes stronger. Furthermore, the development of these seasoning characteristics is, quite often, normal. Most importantly, both are accounted for in the derivation of design values for lumber and timbers and are also accounted for in the applicable grade rules.
A check is separation in wood fibers across the annual rings of a piece of wood and a split is a separation of wood fibers across the annual rings but through a piece wood. A third type of fiber separation, known as a shake, occurs along annual rings and is generally a naturally occurring phenomenon in standing tress, not the result of seasoning. There are several types of checks and splits which are defined and handled in the grade rules for dimension lumber and timbers.
For the in-situ evaluation of dimension lumber and timbers normal checks and splits can often be interpreted as problematic by some design professionals with respect to allowable design values. However, in most cases they are not. In the image below, the ends of both timbers are exhibiting various sizes of normal checks that developed as the timbers dried. If these timbers were being examined in a structure they would look similar to each of the adjacent images.
There are instances, however, where a check or split may reveal an important issue or a problem. For example, in the image below, the timber has a relatively large split. This particular timber is actually demonstrating a much more important structural issue, severe slope of grain. This particular timber is considered non-structural by current grade rules, and has a partial failure.
In the image below, a unique situation was encountered with a large timber. Three sides of the timber were visible from two adjacent rooms. Large splits were a concern as shown on the left side of the image. In the adjacent room the separations appeared to be normal checks and much smaller in size. A Resistograph was used to drill and examine the timber at several locations through the cross section and along the length. Based on those results the relative density profiles demonstrated sound wood, except that each profile also detected what appeared to be either a large shake or split, and one large check. Since this timber was in full compression, neither was considered to be a structural design issue.
In summary, checks and splits are often not an issue with in-situ lumber and timbers. Normal checks and splits are often encountered when assessing a structure, but they are accounted for in the derivation of design values and they are each handled in the grade rules. Checks and splits are generally only an issue when they reveal other characteristics such as low grade material or even failures.