Q. In my area of South Carolina, changes in humidity often cause miter joints to open up over time. I plan to use biscuits to keep my miters tight. Would I be better off using epoxy instead of yellow glue when gluing up the biscuits? Should I check all my lumber with a moisture meter?
A. Cabinetmaker Michael Poster responds: To many carpenters, "moving" miter joints can be a vexing problem. The customer complains about unsightly gaps in trim joints that fit perfectly when they were first installed. While it’s true that open miters (and other defects) can be caused by changes in humidity, other factors also affect joint movement.
As a general rule, the moisture content of trim should be 8%. This figure increases to 11% in damp coastal areas and drops to 6% in arid climates. Variations in desired moisture content of 1% to 2% are acceptable. You should definitely use a moisture meter. I use a no-frills version with a range of 6% to 30%, made by Delmhorst Instrument Co. (51 Indian Lane East, Towaco, NJ 07082; 877-335-6467).
Wood movement is proportionate to board width. Trim wider than 4 inches is more likely to cause problems. Whenever possible, build up wide casings from narrower profiles.
Some wood species are more prone to movement (beech and maple, for example), while woods like cherry, red oak, and white pine are more stable. Quarter-sawn lumber of any species is more stable than flat-sawn, but is often expensive or difficult to find.
Biscuits are an excellent way to ensure that a miter joint "stays put," but they should not be used to compensate for improperly dried lumber. When gluing biscuits, epoxy offers no advantage over yellow (aliphatic resin) glue. The moisture in the yellow glue causes the compressed biscuit to swell, closing the gap between the slot and biscuit, and strengthens the joint.
