Cupping and crowning are two unfortunate results
of excessive moisture in maple flooring.
All moisture content increases
cause wood products to expand. Due
to the cellular structure of flat-sawn maple flooring,
expansion takes place primarily across the width
of each strip. When flooring strips
in a maple system take on enough moisture to expand
and eliminate all available horizontal expansion
space, individual boards will expand
upward at the edges causing the surface condition
commonly known as cupping. Cupping is caused by a moisture
of the maple — moisture contents in each strip of
flooring are higher at the bottom than on the surface.
cupping is severe enough, a condition known as "compression set" can
occur. Compression set is caused by severe expansion
pressure from excessive moisture causing individual
boards to crush each other.
Individual cells on the edges of each maple strip are
permanently deformed or crushed, leaving excessive
cracks and ragged edges when
the material returns to its normal moisture content.
is the opposite of cupping. The center of each flooring
strip is higher than its edges. Moisture imbalance
is sometimes the cause of crowning if excessive moisture is introduced
on the top of the floor due to roof leaks, spills
or improper maintenance
However, crowning is more commonly caused by sanding
a cupped floor before the moisture content in the
maple returns to a uniform and
normal condition top to bottom.
Sanding while the
flooring is still cupped will result in the loss
of flooring material on the edges of each board.
Once all excess moisture works its way out of the flooring
maple will return
to a flat condition - except where the original edges
of the strips were sanded off, leaving voids at the
edges of each flooring row.
Some slight cupping and/or
crowning may occur naturally and is acceptable. The "bark" side of
a maple log will shrink/swell more than the center
of a maple log, and this minor expansion/contraction variation is
more noticeable in areas of the country that experience
significant seasonal moisture content changes and on
floors containing wider
face-width maple strips.
MFMA and all its member manufacturers
have published specifications which prescribe optimum
temperature and humidity ranges to ensure
proper flooring performance and reduce the likelihood
that cupping or crowning will ever develop on a maple
MFMA recommends maintaining indoor temperatures
between 55 and 75 degrees and indoor relative humidities
between 35 percent and 50 percent year round.
If the flooring materials are properly acclimated,
a 15 percent fluctuation in indoor relative humidity
will not adversely affect the maple. Excessive
shrinkage and/or expansion may occur with indoor
relative humidity variations in excess of 15 percent.
you have additional questions, please contact MFMA's Technical
Director at 847-480-9138.
Rev. February 2005
© Copyright 2005