Many factors such as poor design, moisture, vandalism, insect
attack, and lack of maintenance can contribute to window deterioration,
but moisture is the primary contributing factor
in wooden window decay.
All window units should be inspected to see if water is entering
around the edges of the frame and, if so, the joints or seams
should be caulked to eliminate this danger.
The glazing putty should be checked for cracked,
loose, or missing sections which allow water to saturate the
wood, especially at the joints. The back putty on the interior
side of the pane should also be inspected, because it creates
a seal which prevents condensation from running down into the
The sill should be examined to insure that
it slopes downward away from the building and allows water to
drain off. In addition, it may be advisable to cut a dripline
along the underside of the sill. This almost invisible treatment
will insure proper water runoff, particularly if the bottom
of the sill is flat. Any conditions, including poor original
design, which permit water to come in contact with the wood
or to puddle on the sill must be corrected as they contribute
to deterioration of the window.
One clue to the location of areas of excessive moisture is
the condition of the paint; therefore, each
window should be examined for areas of paint failure. Since
excessive moisture is detrimental to the paint bond, areas of
paint blistering, cracking, flaking, and peeling usually identify
points of water penetration, moisture saturation, and potential
deterioration. Failure of the paint should not, however, be
mistakenly interpreted as a sign that the wood is in poor condition
and hence, irreparable. Wood is frequently in sound physical
condition beneath unsightly paint.
After noting areas of paint failure, the next step is to inspect
the condition of the wood, particularly at
the points identified during the paint examination. Each window
should be examined for operational soundness beginning with
the lower portions of the frame and sash. Exterior rainwater
and interior condensation can flow downward along the window,
entering and collecting at points where the flow is blocked.
The sill, joints between the sill and jamb, corners of the bottom
rails and muntin joints are typical points where water collects
and deterioration begins.
The operation of the window (continuous opening
and closing over the years and seasonal temperature changes)
weakens the joints, causing movement and slight separation.
This process makes the joints more vulnerable to water which
is readily absorbed into the endgrain of the wood.
If severe deterioration exists in these areas, it will usually
be apparent on visual inspection, but other less severely deteriorated
areas of the wood may be tested by two traditional methods using
a small ice pick.
An ice pick or an awl may be used to test
wood for soundness. The technique is simply
to jab the pick into a wetted wood surface at an angle and pry
up a small section of the wood. Sound wood will separate in
long fibrous splinters, but decayed wood will lift up in short
irregular pieces due to the breakdown of fiber strength.
Another method of testing for soundness consists of pushing
a sharp object into the wood, perpendicular
to the surface. If deterioration has begun from the hidden side
of a member and the core is badly decayed, the visible surface
may appear to be sound wood. Pressure on the probe can force
it through an apparently sound skin to penetrate deeply into
decayed wood. This technique is especially useful for checking
sills where visual access to the underside is restricted.
Following the inspection and analysis of the results, the scope
of the necessary repairs will be evident and a plan for the
rehabilitation can be formulated. Generally the actions necessary
to return a window to "like new" condition will fall
into three broad categories:
These categories will be discussed in the following sections
and will be referred to respectively as Repair Class I, Repair
Class II, and Repair Class III. Each successive repair class
represents an increasing level of difficulty, expense, and work
time. Note that most of the points mentioned in Repair Class
I are routine maintenance items and should be provided in a
regular maintenance program for any building.
The neglect of these routine items can contribute to many common
window problems. Before undertaking any of the repairs mentioned
in the following sections all sources of moisture penetration
should be identified and eliminated, and all existing decay
fungi destroyed in order to arrest the deterioration process.
Many commercially available fungicides and wood preservatives
are toxic, so it is extremely important to follow the manufacturer's
recommendations for application, and store all chemical materials
away from children and animals. After fungicidal and preservative
treatment the windows may be stabilized, retained, and restored
with every expectation for a long service life.