Sash and case windows

Sash and case windows have two glazed, timber-framed components: the sashes, which slide up and down, and the case, in whose channels they sit.

The channels of the case are formed by the case itself and by two strips of timber called the baton rod and the parting bead.

Sashes are usually hung on ropes called sash cords, which pass over pulleys in the top of the case. Brass chains are sometimes used instead.

The sash cords are tied to lead or iron counterweights hidden in the case. These counterbalance the sashes, making it easier to open and close them, and allowing the sashes to safely stay open in any position.

As the position of both sashes can be widely adjusted – from a small gap to fully open – sash and case windows enable gentle and beneficial ventilation to suit most weather conditions.

Glazing variations

The earliest sash and case windows had very small panes to suit the limited size of glass available.

Better quality crown glass, with its distinctive curved ripples, was used to glaze the larger paned windows that emerged in the 1700s. True crown glass is no longer made, so it’s important to keep any that your property might have.

Cylinder sheet and patent plate glass were later used instead of crown glass. From the mid-1800s, fewer subdivisions and larger, heavier panes of glass became common. Eventually sashes had single large panes of plate glass.

In the late 1800s and early 1900s, small paned sash windows became briefly popular again, as a reaction to the trend towards ever-larger panes. Upper sashes from this period sometimes feature stained glass.

Regular checks will help you to spot any signs of damage or decay so that you can deal with them promptly.

You will have to look carefully at each of your sash and case windows in turn to work out the extent of any defects requiring repair. A problem that affects one window won’t necessarily affect them all.

Most defects you’re likely to find in sash and case windows will be obvious on close visual inspection. Timber decay is less easy to spot, and you will need to test suspect timber with a penknife – sound timber will resist the blade.

What to look for during an inspection

Visible defects:

• visible gap at windowsill
• gaps causing draughts
• misaligned meeting rails (where top of inner sash meets bottom of outer sash)
• joints in sashes opening up
• broken sash cords
• broken or cracked glass
• flaking or missing paint
• timber missing or damaged
• worn sides to sashes
• signs of previous repairs
• missing or defective glazing putty
• missing or defective external mastic
• missing or defective sill bedding mortar

Hidden defects:

• sashes drop or rise of their own accord or drift out of position
• timber decay in windowsill
• timber decay in parting beads
• timber decay in sash frame
• timber decay in hidden parts of case joinery
• debris in weight pockets

Other defects:

• shutters won’t open
• shutters open only with difficulty
• split panels in shutters or lining
• timber decay to shutters or lining
• damp plaster in window recess or behind shutters
• structural defects in the window opening or surrounding wall

Sash and case windows require regular maintenance and repairs to aid their smooth running and avoid problems like timber decay, broken sash cords and lost putty or mastic.

Timber decay

Where paint breaks down, the timber parts are directly exposed to the weather and become vulnerable to decay.

Rot is often localised, however, and in most cases is simple to repair. Tackling the problem early will minimise how much material must be replaced. Repainting is also essential to better protect the window.

The windowsill is usually the area most prone to decay. A musty smell inside the weight box signals the presence of damp and the potential for timber decay.

Read about repairing sash windows.

Find out about painting sash windows.

Poor operation

Windows may not open or close properly for a number of reasons:

• windows may become painted shut
• broken sash cords prevent sash weights counterbalancing the sashes
• wearing of the sides of the sashes or cases – caused by operating the window with only one side properly balanced

Broken or worn sash cords

Sash cords may break or wear out over time. The attached counterweights will fall to the bottom of the case if the cords snap, making it difficult to open the window.

Learn about replacing sash cords.

Loss or decay of putty and mastic

Putty used to hold the glass in place will become hard and crack when paint fails on the sashes. Water may seep in and decay the timbers if the putty isn’t replaced and repainted. Traditional linseed oil putty should be used for repairs.

Sand mastic has been traditionally used to fill the gap between the case and surrounding stonework. Over time this will also dry out, harden and drop out, and will need to be replaced every so often. Mastic shouldn't be painted.

Regular cleaning of the glass and timber will improve the appearance of your sash windows and keep them working well. It also gives you the chance to spot any defects early so that you can repair them before they become problems.

Stay safe when cleaning windows:

• keep both feet firmly on the floor
• do not use chairs or stools
• do not overreach

The upper sash should be drawn down to a comfortable height – it will then be easy to clean the inner face. Clean the outer face with extreme care. Don't press hard as old glass can be very thin.

Simplex hinges are fitted to the lower sashes of many windows. These allow you to clean both the inner and outer faces of the sash from inside. You may wish to install Simplex hinges for easier cleaning where these aren’t already fitted.

Use water and a clean cloth or brush for cleaning. Be careful not to scratch the glass with any grit you might pick up, especially when cleaning off bird mess.

Sash and case windows can usually be repaired with relative ease, and regular maintenance will prolong their life by many years.

Modern timber can rarely match the quality and durability of the slow-grown softwoods originally used to make these windows. So it’s best to retain as much of the existing timber as possible wherever repair is required.

Repairing timber decay

New timber can be spliced in to replace areas of localised decay. But care must be taken to select compatible wood with similar qualities to the original. Severely rotten windowsills may need to be replaced completely using new matching timber, or sometimes just the front part may need to be renewed (half sill repair).

Painting

External and internal window paintwork must be kept in good condition to protect against the effects of weather on the outside and condensation inside. Windows usually need external repainting every five years, depending on exposure and the paint manufacturer’s guidelines.

Painting of the sashes and case must follow a particular order to avoid sealing the window shut.

Learn all about painting sash windows, including how to prevent them sticking and how to remove old paint.

Paint removal

Paint layers that hinder the window’s operation can be removed using a number of methods. Care must be taken to avoid damage to timber, glass, putty and surrounding masonry whichever option is used.

Replacing sand mastic

Traditional mastic made of a mix of burnt sand and linseed oil is a long-lasting, durable material and is still readily available. The seal can occasionally be repaired, but when split or partially missing it should be replaced, cutting out the defective mastic and replacing entirely.

Replacing sash cords

Cotton sash cords and braids will become worn over time and require renewal. This is best done before the cord breaks.

Find out about replacing sash cords.

Replacing glazing

Only original glazing that is missing or beyond repair should be replaced, as many types of traditional glass are no longer produced in the UK. Replacement glass should match the original as closely as possible. Salvaged glass may be an option, and replica cylinder glass is also available.

Repairing broken ironmongery

You should aim to reuse surviving original ironmongery such as cord clamps, sash lifts and sash fasteners, which are usually of a very high quality. Broken ironmongery can often be repaired. Paint can be removed fairly easily where it has hidden or impeded the use of ironmongery.

Reproduction fittings can be used if original ironmongery is missing or beyond repair.

Extra sash locks can be fitted to the meeting rails to improve security of the window when closed. Timber blocks and/or sash stops can also be fitted to restrict opening beyond a certain point.

You should check whether you must first apply for planning permission or any other type of consent before making any changes to sash and case windows. Contact your planning authority to find out more.

Listed building consent

Plans to carry out any work that may change the appearance of windows in listed buildings will require listed building consent.

Works requiring listed building consent include plans to change:

• window frame material or operating method
• timber astragal (glazing bar) profile or the arrangement of panes
• glazing panes to double glazed units
• paint colour or finish

Plans to install secondary glazing or special ventilation arrangements are also likely to require listed building consent.

Your home may be unlisted but within a conservation area where planning controls over window alterations are in effect. Your planning authority may also have planning policies controlling alterations to windows in other locations such as flatted properties or properties along important routes. These may include acceptable paint colours for external joinery.

Learn more about listed building and conservation area consent.

The sash and case window was born in the late 1600s, when pulleys and weights were first applied to timber sashes to balance the weight of the individual frames. The new style of window soon caught on and was installed across Scotland from the 1680s onwards.

Very early sash and case windows from this time have chunky astragals or glazing bars (timber members) subdividing sashes into very small panes. This design suited the limited size of poor quality glass that was available.

Bigger, better glazing

Larger pane sizes and slimmer sash members became feasible in the 1700s. By mid-century, the familiar Georgian window with six panes to each sash had become commonplace. Better quality crown glass, with its distinctive curved ripples, was used to glaze these windows. True crown glass is no longer made, so it’s important to keep any that you have in your property.

Horizontally proportioned panes, known as lying panes, were also popular in the early 1800s. Projecting horns also began to appear on sashes in the 19th century. These are small, shaped extensions to the vertical sides of each sash, used to strengthen the joints at these points.

Single pane sashes

Cylinder sheet and patent plate glass began to be used instead of crown glass. From the mid-1800s onwards fewer subdivisions and larger, heavier panes of glass became common. Eventually sashes were glazed with single large panes of plate glass. Sash frames and joints were strengthened to suit.

In the late 1800s and early 1900s, small paned sash windows, sometimes with chunky astragals, became briefly popular again. This was a reaction to the trend towards ever-larger panes. Windows of this period often combine a multiple small paned upper sash – very often with projecting horns – with a single or two-pane lower sash. Upper sashes from this time, and into the 1930s, sometimes feature stained glass.

Sash and case windows reigned supreme in all types of buildings – from castle to croft – for more than 250 years. They finally fell out of widespread use in new buildings in Scotland in the 1950s.

Get in touch if you have any questions. We will be happy to help.

• Technical Research Team
• 0131 668 8951
• technicalresearch@hes.scot

Professional institutions

The Wood Window Alliance

Publications

Inform guides

An introduction to conservation and maintenance of traditional buildings for complete beginners.

Maintaining Sash and Case Windows
Timber Window Shutters

Short guides

Information on building conservation topics for home owners, trades people and building professionals.

Sash and Case Windows – A Short Guide for Homeowners

Guides for practitioners

Information and advice for building professionals, developers, planners and conservation officers.

3 – The Conservation of Timber Sash and Case Windows

Research reports

Detailed reports of research carried out by Historic Environment Scotland on a variety of building conservation topics.

The Historical and Technical Development of Sash and Case Windows in Scotland

Technical Papers

Detailed technical reports of research carried out or commissioned by Historic Environment Scotland.

1 – Thermal Performance of Traditional Windows
9 – Slim profile double glazing
23 – Thermal assessment of internal shutters and window film applied to traditional single glazed sash and case windows

Other resources

The Building Conservation Directory