By
the end of winter many people living inland in SA are looking at their wood
floors and wondering why there are such large gaps between the floorboards and
our phones start ringing off the hook.
This occurs not primarily because of poor installation but because this
is what wood does and this has been particularly noticeable this year because
of the prolonged hot and dry spell that suddenly arrived after winter as the current
period of drought set in.
Even
homes of 60-100 years with wood floors will find that there may even be
permanent gaps between floorboards which have become filled with dirt over the
years but even these gaps will get larger in winter months.
Wood
is naturally hygroscopic and contracts when it loses moisture and expands when
it absorbs moisture, even the dead piece of tree that has become your
floorboard. As the atmospheric humidity
decreases, so moisture is pulled from the wood until it reaches a level of
equilibrium. As the wood dries out, it shrinks and thus gaps occur between
floorboards and your windows and doors close more easily. This happens to all
wood, not just floorboards and includes all types of wood floors whether they
are laminate, engineered or solid wood.
The
extremely dry Hiveld winters make this cracking or splitting between
floorboards quite pronounced and it often causes height variations between
boards to further aggravate the problem and can even cause splits in the
boards.
Sometimes
floors do not show an even gapping between floorboards because boards become
bonded together by adhesive or surface finish that has penetrated between the
boards and this causes the shrinkage to be transferred across several boards at
once until a large gap opens at the weakest point. This gap is the sum of all
the gaps that would have occurred between the group of boards and is known as rafting
or panelling.
As
the humidity levels increase, the wood absorbs moisture in order to get back to
its equilibrium level and as it absorbs moisture, so it expands. Generally this
goes on in an unnoticeable fashion but sudden increases in humidity can also cause
extreme expansion problems and you may even see your floor developing small
ripples in it as the gaps close and the wood continues to expand. This is the
reason for leaving expansion gaps around the room which are covered by the
skirtings and quarter rounds.
Different
species and types of floor behave differently – a wide board solid floor will
expand/shrink more than a narrower board; engineered floors less than solid; oak
will settle down after one to two seasons whereas maple continues to shrink and
swell with every seasonal change.
It
is important to understand a little of the science behind this, as here in
Johannesburg we have seen humidity levels range from 6% to 80% over a week
period as rains have come and are currently around 25% at the time of writing. At the coast,
humidity levels remain more constant and it is much easier for a wood floor to
remain balanced.
WOOD MOISTURE
A
living tree uses water to convey food between the roots and leaves and
typically can contain more than 50% water. Some of this water is contained
between the cells (known as free water) and the rest inside the cells
themselves (known as bound water). Free water can generally be removed
naturally once a tree is cut and this process is known as air-drying and will
reduce the moisture content to somewhere between 25-35% (the fibre saturation
point) depending on the tree. The bound
water is held in the cell by hydrogen bonding and can only be removed by heat
and this process is known as kiln drying and will reduce the overall moisture
level in the wood to between 8-12%, depending on its application. When the
bound water leaves the cell, the wood shrinks.
Wood
does not shrink or swell evenly in all directions and will move more in the
direction parallel to the growth rings (width of the floorboard generally).
The
moisture content of wood below the fibre saturation point becomes a function of
relative humidity and surrounding temperature and wood will absorb or lose
moisture until it reaches and equilibrium moisture content (EMC). The changes
in wood are usually gradual and short term fluctuations in humidity would
affect the surface only. The following table illustrates typical wood EMC
levels at various temperatures and external relative humidity’s:
Moisture Content of wood in equilibrium at stated dry
bulb temperature & relative humidity levels
% Relative Humidity
|
0°C
|
5°C
|
10°C
|
15°C
|
20°C
|
25°C
|
30°C
|
35°C
|
40°C
|
5
|
1.4
|
1.4
|
1.4
|
1.3
|
1.3
|
1.3
|
1.2
|
1.2
|
1.2
|
10
|
2.6
|
2.6
|
2.6
|
2.5
|
2.5
|
2.4
|
2.3
|
2.3
|
2.3
|
15
|
3.7
|
3.7
|
3.6
|
3.6
|
3.6
|
3.5
|
3.4
|
3.4
|
3.3
|
20
|
4.6
|
4.6
|
4.6
|
4.6
|
4.5
|
4.4
|
4.3
|
4.3
|
4.2
|
25
|
5.5
|
5.5
|
5.5
|
5.4
|
5.4
|
5.3
|
5.1
|
5.1
|
5.0
|
30
|
6.3
|
6.3
|
6.3
|
6.2
|
6.2
|
6.1
|
5.9
|
5.9
|
5.8
|
35
|
7.1
|
7.1
|
7.1
|
7.0
|
6.9
|
6.8
|
6.7
|
6.6
|
6.5
|
40
|
7.9
|
7.9
|
7.9
|
7.8
|
7.7
|
7.6
|
7.4
|
7.3
|
7.2
|
45
|
8.7
|
8.7
|
8.7
|
8.6
|
8.6
|
8.3
|
8.1
|
8.0
|
7.9
|
50
|
9.5
|
9.5
|
9.5
|
9.4
|
9.2
|
9.1
|
8.9
|
8.8
|
8.7
|
55
|
10.4
|
10.4
|
10.3
|
10.2
|
10.1
|
9.9
|
9.7
|
9.6
|
9.5
|
60
|
11.3
|
11.3
|
11.2
|
11.1
|
11.0
|
10.8
|
10.5
|
10.4
|
10.3
|
65
|
12.4
|
12.3
|
12.3
|
12.1
|
12.0
|
11.7
|
11.5
|
11.4
|
11.2
|
70
|
13.5
|
13.5
|
13.4
|
13.3
|
13.1
|
12.9
|
12.6
|
12.4
|
12.3
|
75
|
14.9
|
14.9
|
14.8
|
14.6
|
14.4
|
14.2
|
13.9
|
13.8
|
13.6
|
80
|
16.5
|
16.5
|
16.4
|
16.2
|
16.0
|
15.7
|
15.4
|
15.2
|
15.1
|
85
|
18.5
|
18.5
|
18.4
|
18.2
|
17.9
|
17.7
|
17.3
|
17.1
|
17.0
|
90
|
21.0
|
21.0
|
20.9
|
20.7
|
20.5
|
20.2
|
19.8
|
19.6
|
19.5
|
95
|
24.3
|
24.3
|
24.3
|
24.1
|
23.9
|
23.6
|
23.3
|
23.1
|
22.9
|
Source: The Wood Handbook” Wood as an Engineering
Material 1999”, Forest Products Library USDA Forest Service
Moisture
content changes can be retarded by sealers and sub-floors such as ply wood as
well as recommended vapour barriers.
GAP PREVENTION:
In
SA, many floor installers are neglectful over taking due care over job site
conditions – it is critical that attention is paid to installation MC checks on
the floors and substrate of the site as well as due checks on RH levels
combined with correct acclimation of floors to the particular sites. One
building's internal humidity levels can be very different to the next.
Likewise,
most homeowners are not aware of the fact that ideal humidity levels in the
home for the protection of their floor should ideally be around the 30-50%
level which means using humidifiers in winter on the Hiveld and theoretically,
a dehumidifier should be used for those moister times in summer. This does
become unduly onerous on the home owner as one does not want to spend one’s
time checking RH levels every day but as a rule of thumb, if your skin feels
good, so does your floor. When your nose, throat or skin feels dry then your
floor is busy shrinking and needs moisturising!
Regardless
of what moisture environment a floor has been exposed to, removal or
replacement of the floor because of gaps
is usually unnecessary as the replacement flooring material is more than likely
to have a different MC to that of the existing flooring that has already begun
acclimating to the house.
The
best solution is to leave the floor in place for at least one or two full
season cycles whereafter it is most likely stable. Any repairs then felt necessary should be
undertaken and gaps filled by wedging or filling and then resanding as
required. Filling is problematic if the floors are still moving as it can be
squeezed out of the gaps when the boards expand again, creating further
unsightly issues.
Factors
to consider when laying a floor are as follows:
·
Engineered
flooring is generally more stable than solid wood.
·
Narrower
boards will shrink less than wide boards for a given change in MC – a 180mm
board will shrink/expand twice that of 90mm width. More joints between narrower
boards also allows more spaces to distribute movement.
·
Species
– some species of wood are dimensionally more stable than others.
·
Quarter
sawn vs flat sawn floorboards differ in expansion because of the position that
they came out of the tree when sawn.
·
Address
the moisture issues - one must accept the physics that there is a direct
relationship between temperature, moisture and the resultant Relative Humidity
level. Bringing cold dry air into a house and warming it up will further reduce
the RH level. To increase this RH level,
one must reduce the ventilation or add moisture by means of a humidifier.
Adding too much moisture will create further issues and the primary indicator
of this will be condensation on the windows.
So
what is the real answer to this problem, bearing in mind that wood does what
wood does and we cannot alter the laws of physics – ignore fashion and use
narrower floorboards or opt for engineered floors or accept that winter weather
coupled with uncharacteristic dry drought weather causes extreme seasonal
gapping which should go away with time as the floor settles.
Wood
is a natural product and develops its own character and shape with age which
you most likely admired in older houses as part of their charm. Allow it to
happen in your own home as the floors follow the laws of nature.
REFERENCES:
“Behaviour of Flooring” – NOFMA -
National Wood Flooring Association
Wood Handbook – Ch. 4 “Moisture Relations
& Physical Properties of Wood” Forest Products Laboratory, Glass &
Zelinka
