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Historic Preservation - Technical Procedures
Replacing A Wood Window Sill
Hspg Prepared For Nps - Sero
Doors And Windows
Replacing A Wood Window Sill
REPLACING A WOOD WINDOW SILL
A. This procedure includes guidance on replacing a severely
deteriorated wood window sill.
B. To arrest deterioration, repair sill with epoxy
consolidant (see 08610-06-R and 06300-01-R for guidance).
If sill is beyond repair, it must be replaced (see
procedure outlined below).
C. 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).
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. Window apron - A flat broad piece of finished lumber or
trim placed directly under a window sill.
B. Window stool - A horizontal board on a window sill which
forms a base on which the casing rests.
A. Lumber for new sill and subsill (match species, size and
grain direction of original)
B. Wide blade putty knives
C. Back saw
3.01 ERECTION, INSTALLATION, APPLICATION
A. Remove stool and apron from interior of window. Pry
apron off and tap stool out from under jambs.
1. Insert the blade of a wide putty knife between the
apron and the wall; carefully tap a prybar into the
same gap, allowing the knife blade to protect the
2. Using the wall as a fulcrum, work the apron away
from the wall until a nail is visible, hold the gap
open with a piece of blocking or another prybar.
3. Continue working at each nail location until the
next nail is exposed. When all of the nails have
been exposed, the apron should easily lift off.
C. Attempt to tap sill out of place. If this is not
possible, measure sill thoroughly for replacement then
saw or chisel sill out carefully. Follow same directions
to remove sub-sill, if required.
NOTE: The sill may be nailed at rail or under
D. Remove exposed portions of nails that secured the dadoes.
E. If required, new sub-sill must be installed first. Rout
a drip to underside of sub-sill to prevent water from
entering wall from under window.
F. Cut sill to match original and sand sill before
installation. Bevel ends slightly to ease installation.
G. Nail sill into casing from underneath. Countersink nail
and fill hole with putty and seal. Seal edge of sill
with caulk at jamb connection.
H. Prime and paint (see 06300-01-S, 06300-02-R and 09900-07-
S for guidance on painting wood).
END OF SECTION