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Cracks are the most common cause of complaints on wood floors.
It is normal for the interior of homes to become dry during heating
seasons, for obvious reasons. Under this circumstance wood floors
also dry out and shrink slightly. Properly made and properly
installed wood floors should be expected to have normal
cracks, separations between the edges of boards, in dry months
is most areas of North America. Depending on the width of the
board (or parquet members) used, the size of the room and the
severity and duration of low outside temperatures (and hence
the intensity of heating), the term normal cracks have
various interpretations.
NORMAL CRACKS - -
Generally, cracks are normal in strips 2-1/4" wide or less,
if they close up during the season associated with higher humidities,
or the non-heating season.
Normal cracks may vary in width from "hairline"
cracks, thickness of stationary, to more significant cracks,
up to or greater than the thickness of a quarter. The larger
cracks are expected in those geographical areas associated with
an extended dry heating season with warm humid summers with abbreviated
air conditioning i.e. the Great Lakes or New England area.
To further complicate cracks, plank or strip floors sometimes
"panelize" due to movement of underfloor construction.
Also, if the finish cements individual boards into panels, normal
winter seasonal shrinkage may be concentrated into only a few
cracks. Other joints between these lager cracks generally remain
tight together. In this event some cracks may be considerably
wider than the thickness of a quarter.
Plank floors, because of widths involved, and some parquet
floors, can shrink individually up to 2 1-2 times as much as
2-1/4" strip floors. Seasonal cracks found in those floors
therefore can be much larger than in strip, and still be normal.
Again, if the floor expands so that the cracks disappear during
the humid non-heating season they should be considered normal.
Note: Normal cracks between ¾" thick parquet
units, installed in mastic, can remain near perimeter walls if
cork expansion joint filler is omitted (See NOFMA "Installing Hardwood
Flooring).
ASSESSING CAUSES FOR CRACKS JOB-RELATED
CAUSES
Large cracks in wood floors, which do not close up in the
summer months, can have either job-related or manufacturing defects
as the cause. Job site inspections should be designed to determine
which is the case.
When the complaint is cracks between boards, the moisture
content of flooring will normally be significant lower than when
the flooring was installed. The subfloor and joists will also
contain less moisture than when the flooring was installed. A
very moist environment and associated expansion about the time
the flooring is installed may be causes of cracks. As this excess
moisture is lost, shrinkage of the flooring and underfloor materials
results.
Energy-conscious home buyers have, in recent years, demanded
building practices which sometimes increases the moisture within
a structure during the building process. Vapor retarders,
ostensibly made to prevent warm or cool air loss, may seal in
the new home's moisture. Literally hundreds of gallons of water
used in concrete, masonry, thin-set tile mortar, plaster, dry
wall joint compound, latex paint, and many other building components
evaporate into the home's interior. This moisture is not only
absorbed by a wood flooring system but other wood materials in
the home. The moisture may take far too long leaving the home
thanks to vapor retarders. This moisture will often cause wood
flooring to expand before or soon after installation. When this
happens, the strips, planks, or parquet units close on one another.
The flooring will move or reposition itself, and if the pressure
is sufficient, cupping or buckling may result or the flooring
strips may crush against one another.
Then, usually when the dry (or heating) season arrives, the
total moisture environment changes, and the flooring and underfloor
structure will dry out. If the earlier moisture absorption was
great enough, the drying season will produce "abnormal"
cracks. From this point the cracks will probably never close
completely in humid months if seasonal environmental changes
are normal.
Identify abnormal vs. normal cracks by determining the
proper fastening schedule and making proper observations such
as moisture content and measurements of flooring and underfloor
materials.
For Example====
Flooring moisture content=== 6% to 9% (average of readings)
Measurement characteristics=== Using the 2-1/4" strip
as our example, it is difficult for a set of twenty 2-1/4"
strips to span only 45" due to slight variations. The actual
measurement may be 45-1/8" slightly more or less, even when
every strip measures 2-1/4" when installed.
If the flooring was either too moist when installed and was,
therefore, slightly oversized, or went through a moisture related
expansion after installation, the 20 strip span will be well
over 45-1/8". Now the individual boards, under normal occupied
conditions, will measure very close to 2/14" at 6 to 9%
average moisture content. The shrinkage back to the normal conditions
will show up in cracks. If individual boards have gone through
an extensive moisture introduced expansion, their edges may have
been crushed so their actual width is perhaps slightly less,
on average, than the original manufactured width.
Plank flooring will have all the characteristics described
for strip flooring except that under identical circumstances,
plank will exhibit more movement per board, hence larger cracks.
Wide planks are also more likely to be cupped-a slight amount
being normal.
Nailing Nail every 10" to 12" with a MINIMUM of
2 nails per board within (1" to 3") of the ends.
Inadequate nail spacing can also contribute to cracks. An
inadequately nailed floor has more opportunity to move under
pressure. Since flooring strips can more easily move to new locations,
during the high moisture conditions, cracks are the result.
Squeaky floors are another indication of floor movement after
installation. Sufficient side movement will loosen nails slightly,
resulting in squeaks when foot traffic puts pressure on the floors.
ASSESSING CAUSES FOR CRACKS MATERIAL-RELATED
CAUSES
Another cause of abnormal cracks can be improperly manufactured
wood components. This usually occurs because lumber is not adequately
dried before the flooring is milled.
NOFMA member flooring mills are operated to produce a
product precisely milled to the intended width (i.e. 2-1/4")
as the product exits the flooring machine. This is the case regardless
of the moisture content of the wood being processed.
If the moisture content of the wood is too high when flooring
is milled-generally in the range of 12% and above-the flooring
will later shrink to the range normal for its environment, usually
6% to 9%. In this situation, 2-1/4" strips will shrink more
than 1/32" in width leaving cracks between boards. These
strips will be less than 2 ¼" even during the humid
season.
MEASUREMENT CHARACTERISTICS.
The 45" span used in our earlier example will likely be
very near that exact measure-perhaps a bit over (1/16" to
1/8") or maybe even less than 45". But the key is the
face width of the boards within the set, some of which will be
below the normal 2-1/4" face width, as indicated previously.
The difference will be cracks between boards. As in the fist
assessment these principles apply to plank flooring also, except
that difference in width will be proportionately larger in relation
to the width of the planks used.
OTHER CAUSES FOR CRACKS IN STRIP
AND PLANK FLOORS
There are several other reasons for cracks in floors, and these
have little relationship to job-site moisture problems. Some
are:
.. Foundation settlement. When outside walls settle-or the
center supports under the house's center beams move-the area
pf the floor actually stretches, causing cracks over joints in
plywood subfloors. This can be detected in foundation walls or
by checking the levelness of the floors.
.. Over-drying around forced air heating ducts and vents.
If cracks are associated with areas above heating plants, heating
supply vents (particularly closed vents) etc., check NOFMA
"Installing Hardwood Flooring" for correct insulation
techniques.
.. Improper subfloor materials. Nail-holding capability is
an imperative consideration in floor installation. If the subfloor
does not hold nails, cracks can occur from less-than-abnormal
moisture absorption or heavy traffic.
Subfloor Materials for-
Wood Joist Construction=== Either kiln dried boards of No.
1 or No. 2 Common Pine or other dense, Group 1 softwood, or exterior
sheathing grade performance rated plywood 5/8" (18/32")
or ¾" (23/32"). Also, ¾" (23/32")
OSB is comparable.
Concrete Slab Construction=== ¾" or thicker sheathing
grade exterior plywood over the appropriate vapor retarder.
For ¾" T & G parquet installation over joist
construction=== The primary subfloor as recommended for joist
construction (See Above) should be overlaid with ¼"
or heavier plywood (offsetting seams or on a diagonal)
and nailed in 4" to 6"grids to the subfloor.
Wood-fiber composite panels, commonly referred to as fiberboard,
wafer board, MDF, particle board, or others, are widely considered
to be unsatisfactory subfloors materials for solid tongue &
groove hardwood flooring.
These types of materials do not hold fasteners well. Also,
composition panels will expand from moisture absorption like
most any wood product, but do not shrink appreciably when the
moisture dries out, leaving floors which are not flat. Hardwood
floors depend on lasting nail retention in the subfloor in order
to perform well over the life of the home-perhaps 100 years or
more. Inadequate nail-holding characteristics should be avoided
at all costs.
.. Heavy vehicular movement, such as fork lifts or trucks
in public buildings, factories and commercial buildings.
Flooring systems designed for normal foot traffic often cannot
support heavy loadings without shifting.
ASSESSING CAUSES FOR CRACKS- PARQUET
- -
Patterned floors, parquets, assemble from ¾"
flooring are usually glued to the subfloor with a mastic. If
the subfloor is concrete, a vapor retarder is required under
the flooring (See Installing Hardwood Flooring). Absence of an
adequate vapor retarder can result in moisture changes with associated
movement of the parquet pieces causing cracks.
For mastic applied parquet, since no piece is permanently
anchored to the subfloor, is that the floor units move in no
particular pattern when they expand. Somewhere near the center
of a room, however, one or two sections of the parquet pattern
act as the anchor, and all other pieces will move away from anchor
points toward the walls. It's for this reason that cork is placed
in the required perimeter expansion space to support the flooring
at the walls. This is a special cork with a very resilient binding
resin, which acts as a compression spring. It should be cut in
small pieces to fit snugly between walls and each unit of parquet.
If it is not used, permanent cracks near the walls of the room
may develop and will be larger than normal cracks in the center
as seasonal drying occurs since there is nothing to push the
parquet back as it dried.
Good floor mastics allow slight movement of parquet units
without breaking the bond, and in fact stretch somewhat under
pressure. Some types will re-tack even after breaking loose.
If a large area of parquet units appear (or sound) loose, expansion
has probably moved the units too far for the mastic to retain
its bond, or perhaps an inappropriate mastic was used.
Parquet with abnormally large cracks during the heating season
has no doubt gone through the post-installation high-moisture
cycle described earlier for nail-down products, and the proof
is established by moisture checks and measurements, much as described
for strip or plank floors. (Since many parquet patterns alternate
grain direction, movement will occur in all directions, but only
50% as much in any direction as plank or strip.)
SOLUTIONS-CRACKS BETWEEN BOARDS -
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Regardless of what moisture environment a floor has been exposed
to, or exists when inspected, removal and replacement of a wood
floor to alleviate cracks is usually both unnecessary and self-defeating.
New flooring material is likely to have a different moisture
content than the flooring already acclimated and in place in
the home.
All parties involved are usually well advised to leave the
offending floor in place and effect repairs if possible. Once
an existing floor has been acclimated to a home's environment,
it is far more likely to remain stable and, with professional
repairs, can regain the appearance it had when new with no loss
of service.
NORMAL CRACKS: If truly
normal, in the sense the cracks close up in the summer months,
no repairs are practical. Any filler used to fill up cracks when
they appear-i.e., when the floor is dry-will be pushed out as
the wood expands when it picks up moisture. Also, these spaces
are necessary for humid season expansion and so if filled can
cause so much stress the flooring may buckle after filling. Fillers,
some of which are as hard as the wood during the expansion cycle
can also crush and damage edges of boards. Thus, fillers may
cause uglier cracks than those Mother Nature forced on the floors.
ABNORMAL CRACKS: Even floors
with have gone through a very high period of moisture absorption,
then dried to leave abnormal cracks, can be repaired by a professional
so that the cracks very nearly disappear.
In this case, it may be possible to properly fill any crack 3/32"
or less, depending on residue in the crack and movement between
pieces, and/or seasonal change.
In the process of expanding and re-shrinking, grain direction
in boards may play a role in the evenness of the resulting surface.
Boards with vertical (quartered) grain may become lower than
adjacent flat grain boards, thus requiring re-sanding.
NOTE: If the floor is cupped or crowned, these solutions may
not be effective. Write NOFMA for solutions for Cupped Floors.
Repairing Cracks:
After determining which cracks are abnormal, the best time
to initiate repair is midway between the seasonal extremes of
heating and cooling. For much of the USA this would occur near
the months of April or October.
First
The flooring should be assessed for movement between strips.
Strips may require face nailing particularly near the groove
edge to eliminate movement. (Face nail into the open grain of
Oak and Ash so the nail holes, when filled, do not show. The
filler may be scratched with a utility knife to imitate grain
to further camouflage the hole.)
Next, thoroughly clean the crack. Old filler and trash should
be scraped and vacuumed away.
Filler is next applied down into the crack to the tongue level.
For latex filler in the wider cracks, shrinkage of the filler
may require more than one application.
After filling, coloring (stains), screening, and re-coating
may follow where surface finish is applied. If refinishing the
floor is determined to be necessary at the mid-season inspection,
filling should properly occur after required face nailing and
the first sanding cut.
For Waxed Floors where wax has been embedded in cracks, even
after cleaning the filler may not adhere to the strip edges.
Test an area first.
For the occasional larger than 3/32" crack, most often
traversing across the entire room and associated with an underfloor
support or subfloor panel edge, some options for repair include:
Replacing a run of flooring with oversized strips or cutting
an appropriate sized shim and gluing it into the cracks.
When properly repaired, cracks are generally lost sight of
and filler should remain in the cracks after repeated seasonal
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