Historic Preservation - Technical Procedures
Repairing Weather Checks In A Wood Window Sill
Ohj - John Leeke, 3-31-94
Doors And Windows
Repairing Weather Checks In A Wood Window Sill
REPAIRING WEATHER CHECKS IN A WOOD WINDOW SILL
A. This procedure includes guidance on repairing a wood
window sill that shows weathering of the type described
in 1.01 B. and C. below.
B. Weather checks are cracks in the wood that develop when
bare wood is exposed to the weather. They begin as small
hairline cracks, but exposure to the sun dries out the
inner wood causing the crack to widen. Rainwater and
freeze/thaw cycles further exacerbate the problem, making
the checks wider and deeper.
C. Weather checks are typically found on the South and West
sides of a building where the sun has severely dried out
the wood. They can range in size from hairline to 1/4"
wide and 3/8" deep.
D. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO).
E. See also 06300-01-R for additional guidance on epoxy
1.02 SYSTEM DESCRIPTION
A. A wood window sill in good condition is free from decay
and sloped away from the building to shed water. The
connection between sill and jamb is tight and well
caulked. The sub-sill should have a drip on the bottom
that prevents water from entering the building under the
A. Inspect the sills every two years for breaks in the
joints or paint film. Spot prime, paint and provide
sealant as needed.
B. Provide ventilation between storm windows and sill by
leaving a narrow gap where the two meet.
C. Remove any impervious coverings (such as sheet metal)
that may have been installed over the sill.
D. Clean clogged gutters.
E. Cut back large encroaching plants and shrubbery at least
2 feet from the sill (10 feet for trees) to allow
adequate air flow between the building and the plants.
A. Abatron, Inc.
5501 95th Ave.
Kenosha, WI 53144
414/653-2000 or FAX 414/653-2019
B. Roux Laboratories
5344 Overmyer Dr.
Jacksonville, FL 32205
A. Epoxy consolidant such as "LiquidWood" (Abatron, Inc.),
or approved equal.
1. Consists of two clear liquids (a resin and a
hardener) mixed together in equal volumes.
2. The mixture is poured or brushed on to the surface
where it penetrates and hardens.
3. Acts as primer for epoxy filler.
B. Epoxy paste filler such as "WoodEpox" (Abatron, Inc.), or
1. A two-part putty-like filler (a resin paste and a
hardener paste) mixed together in equal volumes.
2. Hardens within 1-2 hours, is lightweight, non-
shrinking, heat and weather-resistant.
3. Applied like a putty; will fill gaps and holes of
any thickness and shape.
C. Oil-base or acrylic latex paint or high performance
D. Rubber Gloves
E. Disposable vinyl gloves: Available from drug store or
pharmaceutical supply distributor in 50 count or larger
F. Polyethylene sheeting
A. Specially ground scraper
B. Plastic bottles (narrow spouted), like those used for
hair dye, to apply the consolidant; having many on hand
is recommended. Cleaning of the bottles for reuse is
C. Applicator bottles: Available from drug store and sold
for hair dye application usually in 8 fl. oz. size; Also
available in bulk from Roux Laboratories. Roux Color
Applicators lend themselves more easily to cleaning and
D. Rags of different sizes to wipe up spills before epoxy
has a chance to harden, small rags are recommended for
quick one time uses such as wiping off spouts and caps.
E. Thin wooden sticks, approximately 8" long for scooping
out paste and mixing consolidant.
F. Goggles and a respirator for protection from fumes.
G. Putty knives for application of filler
H. Needle nose pliers to pull out hardened epoxy
I. 1/8"x8"x12" Masonite boards for mixing paste filler
J. Carbon dioxide fire extinguisher: Curing epoxy creates
heat that may cause fire
K. Stiff, fiber bristle brushes
L. Paint brushes
A. Look for breaks in the paint film that may indicate
checks below the surface.
B. DO NOT confuse weather checks with a split sill. Weather
checks are deep but do not extend through the entire
depth of the sill.
A. Surface Preparation:
1. Remove loose paint and build-up from the surface of
2. Clean out all cracks of any dust and debris using a
special scraper and a stiff, fiber bristle brush.
3. Dry the sill out; cover it loosely with
polyethylene sheeting and allow to sit until a low
moisture level is achieved in the wood. This may
take anywhere from one week to one month.
NOTE: The cracks must be at their widest position
when they are filled. Therefore, it is important
that the wood be thoroughly dry before proceeding
with the repair.
3.03 ERECTION, INSTALLATION, APPLICATION
A. Prime the edges of the checks with epoxy consolidant;
inject consolidant into each check using a narrow spouted
squeeze bottle; this provides a surface that the filler
can adhere easily to. Avoid getting consolidant all over
the sill surface.
NOTE: MOST OF THE CONSOLIDANT SHOULD SOAK INTO THE WOOD
IF IT IS DRY.
B. After the consolidant has cured, apply epoxy paste filler
using a putty knife; fill each check completely.
NOTE: DO NOT COVER THE ENTIRE TOP SURFACE OF THE SILL
WITH EPOXY. THIS MAY LIMIT THE WOOD'S ABILITY TO DRY OUT
OR CAUSE IT TO BECOME TOO BRITTLE AND CRACK WITH THERMAL
1. For very deep or narrow checks, spread filler with
a very loose consistency into the check using a
putty knife. Force it deep into the check with the
end of the knife.
2. Then apply a stiffer epoxy, again forcing the mix
into the check with a putty knife.
NOTE: THIS METHOD SHOULD FORCE OUT ANY AIR POCKETS
OR VOIDS THAT MAY BE PRESENT.
3. The epoxy should raise slightly above the surface.
C. Allow the filler to set, but before it hardens, trim off
any excess so that it is flush with the surface using a
very sharp hook-type paint scraper.
Note: If the filler shrinks below the surface of the
sill while setting, a second application of epoxy putty
will be necessary.
D. Sand the surface to prepare it for painting.
E. Apply a paintable water-repellent before priming if
F. Prime the surface and apply two top coats paint using top
quality exterior house paint.
NOTE: BE SURE TO PAINT THE UNDERSIDE OF THE SILL.
END OF SECTION