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Design & Conservation Guide For Timber Sash Windows in London

The majority of historic buildings in London, whether Georgian, Victorian, or Edwardian, have timber sash windows. Sash windows first appeared in Britain in the latter half of the seventeenth century and their appearance underwent considerable change over the next 200 years. Before the introduction of the sash window, most windows were casements, often with leaded lights. The development of the sash window was paralleled by improvements in glass making, which lead to increases in the clarity of the glass and available pane sizes. 

Sash Window Design:


Sash windows are almost always made of timber, usually softwood, although in grander houses hardwood, normally oak, was sometimes used. The sashes slide vertically in a box frame, counterbalanced by lead or cast iron weights suspended on sash cords. Early windows had small panes separated by thick glazing bars with robust profiles, but these were refined continually, becoming extremely delicate by the end of the eighteenth century, with much variety in the profiles used. Glass was originally expensive to manufacture and was only available in small sizes. Improvements in glass making allowed ever larger pane sizes so that it became possible to do without glazing bars altogether in the latter part of the nineteenth century. Until the middle of the nineteenth century all joinery was made using only hand tools, so that there are minute irregularities in each moulding. The importance of the original windows to the fabric of a historic building cannot be too highly stressed. The original glass and the small variations in the joinery, resulting from original manufacture and from maintenance, wear and use, give character, from both the outside and the inside of the building. Replacement joinery and glass will never exactly match the original. It is important to consider careful repair rather than replacement. 

General Repair:

Repair of sash windows is a relatively easy process and should always be considered as the first option. Only if the boxes or the sashes have deteriorated beyond repair should replacement be considered. The quality of timber and workmanship in earlier days was generally superior to that which is used today. Where replacement is deemed appropriate, new windows should always be in materials to match the original, never in metal or plastic (uPVC), particularly where a building is listed or located within a Conservation Area. It should be noted that even repair may require Listed Building Consent in some circumstances and consultation should occur with Council’s Conservation Officer prior to proceeding with such works. On some occasions the opportunity can be taken to reinstate missing original features or glazing bar patterns. Prior to circa 1840 windows did not have horns on the sashes; these were introduced as a means of strengthening the sash as glass sizes increased and glazing bars were omitted. When replacing multi-paned sashes in houses before this time (and some after) always ensure that there are no horns on replacement sashes. Before carrying out any work to windows, examine the glass carefully. Where old glass survives every effort should be made to retain it, as it is an attractive and irreplaceable feature, often unwittingly sacrificed in the past when windows were repaired or replaced. 

Determining Repairs Required:

Before carrying out repairs to historic windows, frames and sashes should be examined carefully to establish the extent of decay. This can be done using a penknife or other sharp implement to locate rotten timber. The most common repairs required to sash windows are the repair or replacement of sills, repairs to box linings and the repair or replacement of either the bottom rail or a glazing bar of a sash. When analysing a window, check for decay at the ends of the sill, at the bottom of the box frame and in the joints and bottom rails of the sashes. The decay is most likely to be caused by wet rot, but in certain instances woodworm or dry rot could have affected the box frame. If dry rot is suspected advice should be sought from a specialist. 

Carrying Out Repairs:

Timber Decay:

Timber decay Very small areas of decay which do not affect the structural integrity of the joinery can be cut back to sound timber and filled with a flexible external filler. 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 (also known as Columbian or British Columbian pine). If a less durable timber is used, it should be vacuum impregnated with preservative. All new wood should be properly seasoned. 


Hardwood Cills:

Decayed sections of sill should be cut back to sound timber and new pieces of timber should be glued and screwed in place using external quality wood glue and nonferrous (e.g. 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. Under extreme circumstances, it may be necessary to insert the sill in two pieces, halved in the middle with an overlapping joint, glued and screwed.



Repairs generally involve replacing the bottom sections of the outer linings or 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 re-glue the joints; at other times the bottom rail or the stiles may need to be repaired or replaced. As noted above, if a sash is to be replaced, the historic glass should be removed and reused in the new sash. If a sash is to be repaired the historic glass should be removed before doing so. There are various traditional techniques for doing this, but they are time consuming and not always satisfactory. 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.


Replacing a Sash Cord:

Even if the other cords appear sound, it is usually sensible to replace them all. The lower sash is firstly 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 timber 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 sash cord is best) is then 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 sashes, 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 point. When the upper sash has been hung the pockets can be put back and the parting bead replaced (or renewed if damage). The lower sash can then be re-hung, finally replacing (or renewing if damaged) the staff bead, ensuring a firm, but not tight, fit. 


Draught Proofing:

Historic windows can become draughty and sash windows are no exception. This is often seen as a difficult problem to overcome and a reason for replacing either the sashes or even the whole window, with the loss of many fine original windows and their glass. This is not necessary. Sash windows can be draught stripped in a satisfactory manner, with the added bonus of reducing fuel bills, improving soundproofing and making the windows easier to operate. It is possible to carry out a reasonable do-it-yourself job, but fitting is much better carried out by a specialist. Shutters remain an excellent means of reducing heat losses in historic buildings. Closing shutters on single glazed windows will cut the heat loss through it by over 50 per cent. Shutters also provide additional privacy, security and help with noise pollution. Curtains and blinds can also help reduce heat loss through single glazed windows. Heavy curtains will reduce heat loss through a window by around 40 per cent. Blinds can be just as effective, insulating blinds with a reflective surface facing outwards can reduce heat loss by over 50 per cent. 


Secondary Glazing:

Consent for secondary glazing is only required in listed buildings and is usually granted provided that it causes no harm to any architectural detailing (such as internal shutters) and the meeting rail aligns with that of the existing window. Inadequate ventilation with secondary glazing can lead to condensation which may cause damage to the window.


Double Glazing:

There is increasing pressure for windows to be replaced with double-glazed windows. Slim double-glazing might be acceptable provided that the new windows accurately replicate the appearance of the original windows in terms of material, profile and detailing. In this regard, the original windows must be beyond repair to justify their removal. Where the principle of replacement double glazed units is accepted they should be painted timber, double-hung sash windows (usually without horns for a pre circa 1840 property and with for a post 1840 property), with a slim profile and narrow integral glazing bars with a putty finish. If double glazing is considered appropriate, the Council asks for a new slim double glazing technology with a total glazing thickness of 10mm (3mm glass, 4mm gas, 3mm glass) to be used. These windows are likely to weigh more than the single glazed windows being replaced and so the weights may also need to be replaced. Windows should use solid glazing bars to replicate any existing original windows and the timber profiles should be matched to the existing profiles. The unit can be puttied into conventional size timber sashes and glazing bars, leaving no signs of double glazing. More recently developed vacuum glazing technology has a total glazing thickness of only 6mm. A drawback is that each pane requires a 12mm protection cap which has an undesirable visual impact. However, this product has been used in circumstances where an historic window is to be retained but only the glass replaced. Replacing the glass to existing windows is not always possible as additional weight will need to be added to the lead sash weights and sometimes there is not space within the sash box to do this.


Paint Striping & Painting: 

In the course of repairing and decorating windows it may be necessary to strip the paint. This should not be done as a matter of course, as the paint itself is not only part of the historic fabric, but also an important part of the protection against decay; as much original paint should be left on as possible. The paint originally used will almost certainly have been linseed oil based and will have had a significant lead content, both of which assist durability of the timber. Modern paints have very different properties from traditional paints, often forming an impervious skin, and as a result can be more likely to entrap moisture in the timber, increasing the chances of decay. Unless the building is listed Grade I or II* (of which there are very few in Islington) lead paint may not be used in redecoration. Linseed oil paints are available but there are also modern paints which are suitable for use in external joinery. These are known as vapour permeable or microporous paints and most manufacturers now make their own version. In stripping old paint always remember that the dust from it may contain lead and it should therefore be damped down, kept well out of the range of children and disposed of safely. Do not use a blow-lamp for stripping lead paint.

Please note that all the information provided above has been copied from Islington council website to ensure that any information we provide is in-line with current conservation guidelines:


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