Moisture content (MC) is the weight of water contained in the wood compared to the wood’s oven-dry weight. A change in the size of a piece of lumber is related to the amount of water it absorbs or loses. For moisture contents from 0 to about 28 percent, the moisture is held within the walls of the wood cells. At about 28 percent MC the cell walls reach their capacity or fibre saturation point (FSP) and any additional water must be held in the cell cavities.
Figure 2.3 Moisture Content and Shrinkage of Lumber
and manufactured Wood Products
Moisture Control in Dimension Lumber
Lumber stamped S-Grn (surfaced green) is lumber which had a moisture content exceeding 19 percent (unseasoned) at time of manufacture.
The designation S-Dry on the grade stamp stands for surfaced dry and means that the lumber was surfaced at a maximum moisture content of 19 percent (seasoned) or less. The grade stamp will not indicate whether seasoning resulted from air drying or kiln drying. Some mills apply a voluntary stamp indicating that the lumber was kiln dried but service performance is the same for kiln dried lumber as for lumber dried by seasoning.
Regardless of whether S-Grn or S-Dry at the time of manufacture, careless storage can lead to absorption of water which reverses the seasoning process and therefore increases the possibility that dimensional change will take place when the lumber has been placed into service which of course is not desirable.
The table below gives shrinkage coefficients for Canadian Softwoods
Careful storage and handling of S-Dry lumber will ensure that it remains in seasoned condition when put into service. Careful storage of S-Grn lumber will allow further drying after service, thereby minimizing dimensional change which might occur after going into service.
S-Dry lumber is up to 15 percent more expensive than S-Grn lumber owing to packaging and drying costs.