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;
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
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.
Michael Poster operates Michael Poster
Woodworking in Montrose, Pa.