How to repair and upgrade sash windows

Most defects you’re likely to encounter with sash and case windows will be apparent on close visual inspection. Timber decay is one problem that's less easy to spot.

It’s usually possible to restore even the most rundown windows. Most old sash windows were made from highly durable heartwood timber of a quality that’s now difficult to find. So it makes sense to keep and repair original wood as much as possible.

Maintaining sash windows is fairly straightforward:

• a competent joiner can be engaged to repair sash windows as necessary
• sashes can be removed from the window frame to replace broken or worn sash cords
• external window paint must be kept in good condition to protect joinery and putty from the elements

Sash windows can be upgraded to improve energy efficiency.

Upgrading options include:

• repairing, improving or adding shutters
• fitting draught proofing to windows
• adding secondary glazing

Most defects you’re likely to encounter with sash and case windows will be apparent on close visual inspection.

Timber decay is one problem that is less easy to spot. If you suspect decay, e.g. a rotten windowsill, test the timber with a penknife. Sound timber will resist the blade.

A musty smell inside the weight box signals the presence of damp and the potential for timber decay. Open weight box pocket piece covers to check for this and to examine the condition of weights and ensure that the box is free of debris.

Decay may occur behind shutters, either as dampness in plaster or wood rot in the window case.

Sash windows should run smoothly. To check their operation, open sash fasteners and any other locking devices and operate both sashes, sliding for the full length of travel in each case. Note any stiffness or resistance as well as any signs of sashes dropping out of the control of the counterbalancing weights.

Paint may cause sashes to stick together or to the surrounding joinery. Find out how to paint windows to prevent them sticking.

Draughts around sash windows aid the ventilation of a traditional building. Some degree of ventilation is necessary for the health of both the building and its inhabitants. Where windows aren’t in good condition, draughts can be excessive and may impact on energy efficiency and comfort. Learn about fitting draught proofing.

Most sash and case windows can be readily repaired. It’s usually possible to restore even the most rundown windows.

Most old sash windows were made from highly durable heartwood timber of a quality that’s now difficult to find. It makes sense to keep and repair original wood rather than replacing it wholesale with new wood, which may be more prone to decay.

It’s unlikely that you would carry out repairs to windows yourself. But you should inspect the window carefully before employing a joiner. This will help you to agree on the work required and how much it will cost.

External window paint must be kept in good condition to protect joinery and putty from the elements.

Sash and case windows must usually have their exterior repainted every five years. Repainting windows in good time, before paintwork has begun to break down, will minimise the preparation required.

Window sashes can usually be prepared and repainted in situ. If a more thorough job is required, sashes can be taken out fairly easily. The process of removing and replacing sashes is the same as when replacing sash cords.

You should select a suitable paint for outside use. You may need to apply for permissions for work involving a change in the colour of exterior window paint.

Follow manufacturers’ recommendations about:

• suitability and use of primers
• applying shellac knot solution to knots and resinous patches
• how many undercoats and finish coats are needed

When painting, ensure that the putty is completely covered and the glass to putty joint is sealed with paint. But avoid spreading the paint too far onto the glass surface and make sure you maintain a straight sight line.

Painting preparation

All surfaces must be prepared for repainting, taking care not to damage the wood. Bear in mind that the layers of paint are a record of the decorative history of the building. This would be entirely lost by stripping.

Where existing paint is in good condition:

• roughen the surface with sandpaper to help the new paint stick to it
• remove dirt and grease using sugar soap or washing up liquid in water

Loose and flaking paint should be removed either:

• using sandpaper
• with a thin bladed scraper

Sash lifts and other fittings can be removed if required. But leave the sash fastener in place if it’s in good condition – realigning this later is often tricky.

Paint stripping

Sometimes all or part of the existing paint must be stripped back to bare timber.

Paint stripping may be necessary where:

• windows have been badly neglected and paint has broken down
• paint has been poorly applied and the finish is rough and unsightly
• paint is thickly built up to the stage where mouldings are hidden
• the smooth running of the sashes is affected by multiple coats of paint

Caution should be exercised when stripping paint to avoid damage to timber, glass, putty and surrounding masonry. Protective measures may also be necessary as older paints may contain lead.

Alternative methods of paint removal:

• chemical paint strippers – aggressive caustic strippers are not recommended. More suitable chemical paint removers for hand application, e.g. dichloromethane and methanol solvent based products, are available
• mechanical sanding and scraping – care must be taken to avoid gouging the wood and eroding decorative details
• heat – using a blowtorch is not recommended. A hot air gun can be effective but a baffle must be used to protect the glass as it’s likely to crack even under this gentle heat

How to paint windows to prevent them sticking

1. Pull the top sash right down, push the bottom sash up past it.
2. Paint three sides of the top and bottom meeting rails, the lower half of the stiles and glazing bars, and the parts of the lower sash that you can reach.
3. Paint the inner sill and the lowest 75mm only of the pulley stiles.
4. Let the paint dry.
5. Swap the position of the sashes, i.e. bottom sash right down and top sash halfway up.
6. Paint the remaining parts of both sashes.
7. Paint the top half of the pulley stiles.
8. Do not paint the parts of the pulley stiles that are hidden by the sashes when they’re closed.
9. Let the paint dry, then paint the surrounding woodwork.

Closing internal shutters and heavy curtains can vastly reduce heat loss from sash and case windows.

Wooden window shutters were often installed alongside sash windows and help to improve security and the energy efficiency of a traditional building. Shutters have fallen out of fashion in more recent times, however, and many have been fixed in place or removed.

Original shutters may need a little help to work smoothly and effectively once again.

Shutter repairs and improvements

Tasks may include:

• freeing shutters sealed in their boxes by years of over-painting – use a craft knife to carefully cut through and scrape back paint at shutter edges
• freeing shutters fixed to their housing – gently unscrew or slowly withdraw any screws or nails fitted in the past
• resolving slight movement that has led to shutters getting tight and rubbing – realign hinges slightly or lightly plane the shutter edges
• realigning shutters that no longer line up properly where they meet – unscrew shutters from their housing to access the affected hinges
• treatment of dry rot – but only after you have first found and dealt with the source of moisture
• filling cracks or splits in shutter panels – use wood filler for small cracks and slivers of wood for wider cracks, and then sand smooth

Shutter reinstatement

You may wish to invest in custom-made replacement shutters where the original shutters no longer exist.

A skilled joiner should be able to make and install new shutters to complement the individual style and period of the house. Surviving original joinery in your property (and in comparable buildings nearby) should inform the design of replacement shutters.

Where very draughty sash and case windows affect comfort and energy efficiency, draught proofing is a cheap and discreet solution.

Several proprietary systems are available, some of which also make the window easier to slide up and down.

Draught proofing is a cost-effective way to:

• reduce heat loss
• reduce dust ingress
• reduce noise from outside
• improve comfort

You may need permissions for work to draught proof historic windows.

Find more detailed information about draught proofing in our Sash and Case Windows: A short guide for homeowners.

Adding secondary glazing can help to make sash and case windows more energy efficient in appropriate circumstances.

Secondary glazing can be fitted internally using either:

• side hung glazing
• horizontally sliding windows
• vertically sliding windows

Secondary glazing benefits include:

• reduced heat loss
• reduced dust ingress
• very good sound proofing

Where the building is listed, this alteration would require consent from your planning authority.

Find out more about permissions for works to sash windows.

Double glazing

Research has shown that replacing existing windows with double glazed units is not as cost effective as other energy efficiency measures. Your existing windows can be upgraded for less money to give you the same benefits while still retaining the original features and style that give character to your home.

Replacing the panes of glass in existing multi-paned sashes with double glazed panes is usually possible with the right type of slim profile double glazed panes.