Techniques > Systems
> Windows > Conservation
a) Decayed Wood:
Very small areas of decay which do not affect the structural
integrity of the joinery can be cut back to sound wood and filled
with a flexible external filler. There is a flexible epoxy filler
on the market which does the job extremely well. Larger repairs
should be carried out using timber of a similar species to that
of the original, which in the case of sills is often oak. Softwood
for repairs or replacements should be in a naturally durable
species such as Douglas Fir. If a less durable timber is used,
it should be vacuum impregnated with preservative. All new wood
should be properly seasoned.
Decayed sections should be cut back to sound wood and new
pieces of timber should be glued and screwed in place using
external quality wood glue and non-ferrous (eg brass or stainless
steel) screws. If the ends of the sill have decayed, it will
probably be necessary to replace the whole sill. This can
be difficult as the best join with the box frame is achieved
if the new sill is inserted from underneath. To allow this,
either internal plaster and brickwork or the external (stone)
sill may need to be removed. If neither of these is possible
(if, for example, there are elaborate architraves or shutters)
it may be better to cut off the bottom of the inner lining,
ensuring that the sill is glued and screwed to the pulley
and outer linings. In extremes, it may be necessary to insert
the sill in two pieces, halved in the middle with an overlapping
joint, glued and screwed.
c) Box frames:
Repairs generally involve replacing the bottom sections of
the outer linings or of the pulley stiles. This can normally
be done in situ and the joints should be formed in such a
way as to throw water out of, rather than into, the joint.
Joints should, where possible, be both glued and screwed.
Joints between the bottom rail and the stiles of a sash are
often loose. Sometimes all that is necessary is to reglue
the joints; at other times the bottom rail or the stiles may
need to be repaired or replaced.
If a sash is to be replaced, the old glass should be removed
and re-used in the new sash. If a sash is to be repaired the
old glass should be removed before doing so. There are various
traditional techniques for this, but they are time-consuming
and not always satisfactory. An electrical infra-red lamp
has been developed in Sweden and is available in this country.
This softens the putty without either scorching the timber
or over-heating the glass. When the putty is softened it can
be cut out and the glass carefully removed, labelled and put
safely aside for refitting after the repairs.
e) Replacing a Sash Cord:
Even if the other cords appear sound, it is usually sensible
to replace them all. This can be done as follows:
- The lower sash is first removed by carefully taking off
the staff beads, which will free the sash.
- Disconnect the sash cords from the sash and tie a knot
in the cord to prevent the sash weight dropping to the bottom
of the box.
- Take out the parting beads and the upper sash will now
be free to be removed, restraining the sash cords as with
the lower sash.
- Take out the pockets, which are loose pieces of wood normally
held in place by the parting beads; by moving the wagtail
from side to side and letting down the cords it should be
possible to find the weights in the box.
- If any of the glass has been changed since the windows
were hung, the sashes should be weighed at this stage.
- A new length of cord (waxed sashcord is best) is weighted
with a nail, threaded through a pulley and allowed to drop
until it can be reached through the pocket.
- The weight should be tied to it with a secure knot (bowline
or similar), with make-weights added as necessary to balance
the weight of the sash, and replaced in the box.
- When all the cords have been inserted in this way, the
free end of a cord is then nailed with three flat headed
nails into the groove in the side of the upper sash, adjusting
the lengths of the cords so that the weights do not quite
touch the bottom of the box when the sash is at its highest
- When the upper sash has been hung the pockets can be
put back and the parting bead replaced (or renewed if damaged).
- The lower sash can then be rehung, finally replacing (or
renewing if damaged) the staff bead, ensuring a firm, but
not tight, fit.
Guide to the Repair of Sash Windows [PDF file],
Building and Maintenance Guides, Planning Department of
the London Borough of Islington and Roger Mears Architects
Repairing Wooden Windows, Built Environment Team,
Environmental Services, Technical Leaflet No.TL05, Ediion 1,
March 2002, North Wilshire District Council, Monkton Park, Chippenham,
Wiltshire, SN15 1ER, UK [PDF file]
Illustrations from Townsend, A and Clarke, M (1988) Repair of
Wood Windows SPAB Technical Pamphlet 13, London: SPAB.
The traditional window frame material is wood, because of its
availability and ease of milling into the complex shapes required
to make windows. Wood is favored in many residential applications
because of its appearance and traditional place in house design.
From a thermal point of view, wood-framed windows perform well
with frame U-factors in the range of 0.3 to 0.5 Btu/hr-sq. ft-°F.
Wood is not intrinsically the most durable window frame material,
because of its susceptibility to rot, but well-built and well-maintained
wood windows can have a very long life. Paint protects the exterior
surface and allows an easy change in color schemes.
Collaborative, College of Architecture and Landscape Architecture,
Twin Cities Campus, University of Minnesota.