Replacing A Wood Window Sill

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.


We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.




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

3. Submittals

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).


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
window assembly.


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)


A. Prybar

B. Wide blade putty knives

C. Back saw

D. Chisel

E. Hacksaw

F. Router

G. Caulk

H. Carpenter's end-cutting pliers



A. Method #1 (This allows one to save the old sill for posterity and/or duplication.)

1. Remove stool and apron from interior of window. Pry
apron off and tap stool out from under jambs.

2. 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

3. 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.

4. 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.

5. 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

6. Remove exposed portions of nails that secured the dadoes.

B. Method #2: (This method is for total sill replacement and discard of the old sill.)

1. Remove stool and apron from interior of window.

2. After removing any lower interior window trim, using a reciprocating saw cut the deteriorated sill approximately in half in the transverse plane (perpendicular, or across the grain), so that a 36��� long sill would now be comprised of two 18��� long pieces. A fine-tooth metal hacksaw-type blade is best for this.

3. Carefully pry up on the center of the cut sill, moving it up and down and forward and back, loosening the remaining nails while taking care not to damage the remaining window frame while you remove the damaged portion. Use cedar shims as fulcrums to protect remaining frame millwork from damage.

4. Retain samples of pieces to be removed for use a template for duplication. Be prepared to completely destroy the old deteriorated portion piece-by-piece to protect the rest of the window frame.

5. Utilize the reciprocating saw to cut any nails off in place while being cautious of undamaged window trim and the surrounding wood. Utilize the carpenter���s pliers to remove stubborn nails. Skip to Section 3.02 below.


If required, new sub-sill must be installed first. Rout
a drip groove to underside of sub-sill to prevent water from
entering wall from under window.

A. Cut sill to match original and sand sill before
installation. Bevel ends slightly to ease installation.

B. Nail sill into casing from underneath. Countersink nail
and fill hole with putty and seal. Seal edge of sill
with caulk at jamb connection.

C. Prime and paint (see 06300-01-S, 06300-02-R and 09900-07-
S for guidance on painting wood).