Windows are an important feature of many historic buildings. Where original examples exist they should if at all possible, be repaired rather than replaced. If replacement is unavoidable, then the new windows should be historically accurate. Modern replacements in metal or plastic frames are definitely not appropriate in historic buildings and the relative improvement of comfort with the double-glazing is very little in an older building where the walls can account for the major proportion of the heat loss. Whilst still maintaining the existing windows, the improved performance of modern windows can be achieved in a number of alternative ways which have less impact on the historic fabric. These include the use of draught stripping, secondary double glazing (with glazing bars lined through with the existing) and the refurbishment of original internal shutters.

Repairs

With all work to windows it is important to assess what you are dealing with historically before you start. The centre pages of this leaflet will hopefully provide a useful overview of this, whilst the publications listed at the end give more detailed further guidance.

Windows themselves are composite structures of timber, glass and metal, each part of which can be repaired as a separate element when required. Whenever possible the original fixtures and fittings of historic windows (hinges, catches, pulleys, handles, stays etc.) should be kept for refurbishment and re-use on the window being repaired.

Glass

When carrying out repairs to historic windows it is important to save for re-use any old glass, which can be carefully removed from old frames by a skilled joiner. This has irreplaceable qualities of unevenness due to the original process of manufacture: Cylinder glass was produced by opening out a large blown cylinder of glass whilst Crown glass was produced by spinning al large disc. The centre of this disc, where they pontil was attached, is the bullseye which was generally used in an out of sight location or recycled rather than used as a fake picturesque " add on", as its is today. The flaws in historic glass catch the light and show off defects which cannot be found in modern glass.

Horticultural glass is a modern substitute that exhibits such defects to some degree and can be used in repairs. When reglazing sash windows, care should be taken to adjust the sash weights for any change in the balance of weights caused by different thickness of glass. In general upper sashes should be slightly lighter than the weights, whilst lower sashes should be slightly heavier. Any glazing should be undertaken using a traditional linseed oil putty rather than modern compounds. Timber beading, which increase the apparent thickness of glazing bars, is particularly unwelcome.

Frames

Careful repairs to timberwork, maintaining the maximum amount of historic fabric, are preferable to new work. New sections of framing can be pieced in to match and glazing bars replaced, using existing parts as a pattern. It should be noted that casement frames were traditionally inset within their overall frames, rather than projecting proud of their surrounds as in the 'weathered' casements so popular amongst window manufacturers.

Sashes can be easily removed from their frames by removing the staff beads on the inside. Sash cords can easily be replaced, access being gained to the sash weights via the removable pockets at the bottom of the pulley stiles. Replacement cords can be in jute, cotton or nylon. THE stiles of sashes should not be painted but lubricated with wax, and not eased after a building has remained empty for some time as heat and ventilation can often do the trick.

The appropriate finish for historic windows after the 17th century is generally paint, most often white, and certainly not a modern stain. For complete authenticity on a grade I or II* listed building, permission can be obtained to use a traditional lead based paint.

Windows Surrounds

When renovating a window by repair it is important to remedy at the same time any defects in the surrounding fabric that may have initiated the original decay in the window. In this way the repaired material can be guaranteed a longer life. It is thus important to detail the surrounds to replacement or repaired windows properly.

On rendered walls a lay board (or pentice) is often fixed above the window head to throw off the rain and give a pleasing shadow line. Moulded surrounds should be carefully repaired or reinstated using the original as a pattern.

Sill details are also important to maintain their projection varying with different window types. Timber sills are particularly vulnerable to rot and if replaced should be of Oak or other durable native hardwood.

a) Windows originated in timber-framed houses as openings within the frame provided to let light in: the diamond or square cross section mullions of early windows generally followed the lines of the building's timber framing. They were originally unglazed and usually had an internal timer sliding (or hinged) shutter, the grooves for this being the only remaining clue to their former existence.
b) In the sixteenth century, along with the ceiling of rooms, and the provision of floors and chimneys to former open halls windows began to be glazed. Small diamond shaped panes of glass, known as quarries were set in lead cames in fixed or opening iron casement frames within the overall mullioned framework. These usually had ovolo mouldings providing a ledge for fixing the lights using nails.
c) In the seventeenth century the mullion an transom type of window was developed. Usually in oak with leaded light glazing it represents a transitional type towards more vertically orientated windows. The basic pattern was still in use in the nineteenth century but in painted softwood with nine large panes of glass.
d) In the eighteenth century, with the greater availability of softwood and larger panes of glass, the side hung timber casement developed from mullioned windows as a cheaper form of construction. Usually with two lights it was less suitable for 'classical' buildings but was commonly used in domestic and 'gothic' style buildings where a central mullion was acceptable.
e) Another more domestic type of window form is the 'Yorkshire sash', a horizontally sliding version of the norm, which did not require the elaborate counterweight system of vertical sashes. This, as its name suggest, was more prevalent in the north of England.
f) From the late seventeenth century the double hung vertical sliding sash was increasingly used, usually in painted softwood. This presented a more vertically orientated opening in keeping with the fashionable classical proportions being adopted for buildings. Early examples have quite thick ovolo moulded glazing bars and exposed sash boxes containing the counterweights, set flush with the outer face of the wall. Sometimes only one of the sashes was hung.
g) The London Building Acts of 1709 and 1774 required firstly that sashes be set back 4" from the wall face and secondly rebated behind the wall face as a fire precaution. These styles of sash boxes both spread as fashions to the provinces some 20 years later. Gradually through the eighteenth century panes or glass became bigger and glazing bars thinner, using lamb's tongue or ogee mouldings.

By the end of the eighteenth century sash windows were also produced with rounded or gothic heads, whilst by the 1830s the margin lights became fashionable.

h) In the nineteenth century sash windows generally had fewer but larger panes, as a consequence of improved methods of manufacture, together with even thinner glazing bars. This left the sash frame vulnerable to stress and rot particularly at the corners, and resulted in the introduction of horned sashes with a stronger through tenon joint from mid-century onwards.

New Windows

The complete replacement of windows should only be a last resort and in a Listed Building this will require consent. If this is done, particular attention should be paid to the use of original mouldings and accurate setting out to the original pattern. With replacement casements the inclusion of additional night vents unbalances a window's elevation and is inappropriate. Most historic windows have already survived periods measured in hundreds of years, although only constructed of deal, a slow grown pine. If properly repaired and reasonably maintained they can be expected to last at least as long again.

Additional or redesigned windows in a listed building, or an extension to it should always follow the design criteria already mentioned and be based on authentic historic precedents.

Advice

If you require further advice on this topic please contact our Conservation Team.

A list of joiners able to carry out repairs or make purpose made replacement windows is kept by the Council. Please note that these are not recommended companies and that you must satisfy yourself that they are able to meet your requirements before placing an order.

This information is based on parts of the following publications:
The Georgian Group Guides: No.1 Windows (1991)
The Repair of Wood Windows: SPAB Technical Information Sheet (1992)
English Heritage:Listed Building Guidance Leaflet:Sash Windows
Period House:Guide to Windows and Doors (1995)