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"The flooring
store said to leave one quarter inch gaps for
hardwood floor expansion at the walls...now
the boards are lifting off the floor." Ill
informed advice like this often leads to
floor problems. Many flooring sales people
unfamiliar with hardwood flooring assume
expansion is the same as laminate flooring.
In reality that 1/4" recommendation can
sometimes cause problems for laminate (that
plastic looking stuff) also.
What Is An Expansion Gap? Why Do We Need
It?
Expansion gaps are spaces left around the
perimeter of rooms, against fixed objects
such as columns,
thresholds,
hearths, baseboard, and other
stationary items built or secured into the
framing structure of the home. Even though
hardwood flooring used is no longer living
and breathing it still reacts to moisture
changes in the environment. The cells in the
hardwood will take on or absorb moisture
when the relative humidity is high, or when
exposed to water. Expansion takes place, and
the hardwood grows, for lack of a
clearer term, across the grain (width) of
the plank (see below, not all will react
this way). Conversely when air moisture
levels decrease, moisture content
evaporates, shrinking the hardwood.
When we say spaces, they are defined as
installing the hardwood flooring up to and
away from the fixed objects. For example;
"we left 3/4" expansion at the baseboard."
Solid Hardwood Floors
Solid hardwood floors by far, will expand
and contract more than engineered products.
How much will depend on the layout involved,
seasonal change in relative humidity, specie
selection, and type of hardwood cut. For
smaller areas such as 10' x 12' rooms,
leaving 3/8" expansion would be
satisfactory. However getting into larger
layouts over a few thousand square feet it
would be wise to stay within the age old
guideline of at least 3/4" on the perimeter.
Solid hardwood flooring will expand more in
the direction of the tongue and across the
face of the product. In
large layouts it is suggested to start your
installation near the center to offset the
expansion properties. The use of slip
tongues or splines would be used to reverse
the direction; having the flooring installed
with the tongue facing out both ways from
the center.
Our example to the right shows the use of
the slip tongue highlighted in red. Slip
tongues fit into the grooves of the plank by
way of gluing with a yellow carpenters type
or Titebond. They are generally shipped in
four or six foot lengths but many
experienced installers have been able to get
by making them from material on the job run
through a table saw. If you plan on doing
such, double check if the spline fits snug into
the grooves.
Splines also come into play when reversing
the direction of the installation. In some
cases you may have most of your installation
going forward (tongue facing out) through
the house. Other areas may fall behind
you, requiring a direction change with
the tongue facing out. Floors should not be
fastened into the groove. Another area where
splines are often used would be starting or
running an installation off stair
nosing. All stair nosing is grooved.
Glue down floors do not fall into this
category as most are of the engineered
variety. Slip tongues can be used but are
not mandatory. Installations begin with the
groove facing out, enabling a cleaner way of
installing; glue is not scooped into the
groove. When planning the layout, determine
which starting area will work best,
minimizing areas that require reversing
direction, also called back filling.
When back filling be extra careful with
scooping glue.
Engineered Hardwoods
Engineered hardwood flooring does not
require the expansion area solid products
need due to the way they are constructed in
cross ply layers similar to plywood.
Expansion and contraction is limited as the
ply layers act to restrict the movement in
the overall product itself. The
exception to the rule is a floating floor.
Floating floors once installed
basically become one large sheet of
flooring. The entire unit will expand and
contract as a whole.
Now Wait A Minute...That Does Not Make Sense
Granted, it does sound confusing. Let's
clarify this. When three inch wide
engineered hardwoods are installed for
example, by way of stapling they become one
unit alone. The next piece installed
adjacent becomes another. They have their
own built in but barely noticeable expansion
if you look at it close enough in the manner
each piece is manufactured. On the other
hand, most floating floor products are
milled with such precision there is no
expansion. They become connected.
What Kind Of Log Cut Makes A Difference
Another wrench thrown into the equation lies
in how much each type of hardwood specie
will expand and
contract without any reference to the
moisture level changes. Much can depend on
the way it is cut from the tree. Most solid
hardwoods sold today are considered flat or
plain sawn, where the expansion takes place
across the grain when installed.
Our illustration to the right shows more
common cuts used for solid hardwood
flooring. In older homes it is not uncommon
to see quarter sawn or rift sawn flooring.
These products are still available but the
cost increases substantially.
What Do They Look Like?
Quarter sawn hardwoods expand vertically
because of the method in which they are
milled from the log. Rift sawn is a form of
plain and quartered as the sawing takes
place at a 30 degree angle to the growth
rings. Comparing the differences with
expansion and contraction, quartered will be
the most stable (nill movement
horizontally), rift will take on both
horizontal and vertical expansion
properties, while plain sawn will be the
most susceptible to movement across the
grain once installed.
 Other
types of hardwoods that have limited
expansion and contraction properties are
sources taken from the heartwood portion of
the tree. Annual growth rings are tighter.
What happens in this situation is the
tighter the grain the less susceptible to
movement the floor becomes.
Straight grain
hardwoods are ideal for those seeking
unusually wide width hardwood. The manner in
which some are cut minimizes adverse effects
that are common with wider plank floors.
Stability -
Oak Expands Less Than Hickory
Okay, this one gets harder to explain. The
Phd wood experts call it the dimensional
change coefficient. Yea,
that kind of stuff was not on my list of
favorite subjects in high school either.
What they're trying to say is each hardwood
species will react different when moisture
is present or not present. In the chart
shown to the right the higher the number the
more susceptible each hardwood species will
be in relation to movement. These figures
should be used when relating to flat or
plain sawn hardwoods.
Keep in mind,
many factors can affect the expansion and
contraction of hardwood floors. Maintaining
a stable level of relative humidity in your
residence will minimize movement. Installing
floors in warmer more humid conditions
combined with dry winter heating months can
prove troublesome, unless some precautions
are taken.
*Although some
tropical woods such as Australian cypress,
Brazilian cherry, merbau and wenge appear in
this chart or have excellent moisture
stability compared to domestic oak, actual
installations of many of these woods have
demonstrated significant movement in use. To
avoid problems later, extra care should be
taken to inform potential users of these
tendencies prior to purchase.
Source:
Stability ratings taken from Wood Handbook:
Wood as an Engineering Material (Agriculture
Handbook 72, Forest Products Laboratory,
Forest Service, US Department of
Agriculture; revised 1987)
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