Plywood is made up of varying layers or plies of wood, usually with a hardwood or veneer top surface. these layers help give the material its strength, and ensure that it remains stable enough to be used for many different purposes, from creating a roof deck to building cabinets.
But like all wood, each layer of the plywood can absorb moisture, whether from direct contact or through humidity in the air. and when the wood absorbs the moisture, it can expand, then shrink again once it dries.
This type of expansion and contraction may have little effect on some areas beyond the need for an expansion joint to absorb some of the movement, but in the case of specialty items like cabinets or furniture where tight tolerances are required, too much expansion and contraction over time can lead to warping of the finished piece.
In addition, traditional plywood is often made using an adhesive made from urea-formaldehyde, which bonds the different plies together. formaldehyde is sensitive to moisture and can degrade over time if subjected to enough water or high humidity levels. this can cause the plies to delaminate or to begin to pull apart from one another – not something that you want to happen to the products you build.
By using moisture resistant plywood, you hinder the expansion and contraction process and you prevent this degradation from occurring between the layers. there is no need to account for an expansion joint and no worry about warping over time. in addition, it means that the plywood can now be used in areas where higher levels of moisture and humidity are found, such as the bathroom or below grade, where traditional types of plywood cannot be used.
