Sliding sash windows were very popular in the 1700s and 1800s, and millions of these windows are still in service today.
Sliding sashes work well if maintained correctly, but many don't work properly today due to needing minor repair or to having been improperly painted. Wrong painting technique is probably the number 1 cause of sash window defects.
Anatomy of a Sash Window at bottom of page.
Pulley & Spring Mechanisms
Vertical sliding window sashes fall down unless they are counterbalanced. There are 3 methods of dealing with this:
Early sash windows & very small windows may use no counterbalance mechanism. To keep a window open it is propped with a stick.
This does not work well or safely with medium and large sashes, but is practical for very small windows.
A pulley, cord & weight on each side of each sash is the original equipment with most sash windows. This method works well. The sash weight is properly counterbalanced, leaving only friction to stop sashes moving by themselves. Reliability of such systems is very good, although most are now so old that unrepaired faults are common. In most cases this is just a broken cord.
The weight is built into a pocket in the wood frame, and is usually lead or cast iron.
Sash springs are quicker to fit, but suffer many defects.
- Sashes often won't stay where they're put, as the spring force inevitably varies with sash position
- Sash springs don't last well. About 50% failure in 20 years is what I've observed so far.
- When they fail, the window can become a blunt finger guillotine.
- Repeat fault fixing down the line is likely to result in greater total cost than doing a proper job with cord & weights in the first place.
Usually caused by buildup of paint between sash and frame. The beading needs to be removed to clear this.
Also sometimes caused by the beading being up against the sash, where there needs to be a slight gap. Beading needs removing and refitting to solve this.
This is usually caused by painting over the small gap between sash & frame. Running a thin knife blade all around this gap usually frees the window. The excess paint will then need removal to enable proper free running.
Probably the least common cause of jamming is distortion of the frame, usually caused by structural movement. In most such cases the sashes can be planed a little to fit the new space.
The usually linseed glazing putty should be slightly overpainted, so the paint just goes onto the edge of the glass. Otherwise the putty will fail prematurely.
Sliding channels and edges of sashes should be painted only when really necessary, as paint buildup quickly makes the window unusable. Unseen areas of woodwork that are part of the sliding system should be painted with primer only. Adding undercoat and topcoat is a recipe for difficulty in use and jamming.
Avoid painting over the slight gap between sash and frame. Doing so will cause the window to seize.
Wood windows need regular painting to avoid rot. If this is neglected, rot sets in. Rotting sashes are often replaced at significant expense, but in reality most such windows can be repaired for a much lower price.
The cill, bottom sash rail and bottom corners are the most prone to rot. Small patches of rot can usually be repaired with car body filler once all rotten wood is removed. Another option is to let in a new piece of wood. Extensive rot will require replacement of a section of wood.
Life Expectancy depends mainly on type of timber and maintenance.
Historic windows were mostly made from forest felled softwoods. These woods grew slowly enough to last well when properly painted.
Today's softwood often comes from planted sources, with much higher growth rates and thus less durable timber. Windows made with such wood tend not to last so well.
Painting is important, and should not be delayed or avoided. As long as paintwork is seen to when required, softwood windows can happily last centuries. Fail to maintain them and they can rot away in several years.
New double glazed wooden windows are designed for dg and don't have the same issues as retrofitting. The one visible issue is that providing shade for the dg seals requires thicker glazing bars.
While it is possible to fit dg to sliding sash windows, there are enough issues with retrofitting to make it not usually done.
- Much greater weight of glass means the counterbalance weights need increasing.
- In some cases there may not be enough space to increase the weights as much as desired.
- DG unit seals will be exposed to direct sun with original thin glazing bars on multi-pane windows, which reduces sealed unit life expectancy
- Sealed dg units should not be puttied in place. Using a flexible sealant that does not couple the glass to the frame leaves the wooden sashes weaker.
- Relatively thin dg units are needed to fit the shallower profile of sg windows, and the resulting exterior profile will be different.
Secondary glazing is a simpler option.
Pros & Cons of Sliding Sash Windows
- Access to the window exterior is difficult for windows on 2nd floor or above
- Window cleaners tend to avoid houses with windows with many small panes of glass
- Lack of widespread awareness of the need for a little skill when painting leads many to paint their windows so they jam shut.
- Ability to open only the top leads to around 1 degree C more cooling in summer than with windows without top lights.
- Window corners do not protrude when open, eliminating a minor risk of injury to children.
- Faulty sash mechanisms can smash fingers.
- Wooden sash windows have a mean lifetime measured in centuries.
UPVC Replacement Windows
- Upvc sashes are not in keeping with the character of historic buildings. Building appearance can be marred and in some cases building value reduced by replacement with upvc.
- PVC replacement windows are less repairable, not proven to last, and don't normally pay back their financial or energy cost in heating savings
- Double glazing in any frame type reduces noise significantly.
- Uovc does not need regular painting, reducing maintenance costs. However the failure of sealed units adds a different maintenance cost.
- Upvc is often preferred in properties where appearance is judged unimportant. The main appeal is absence of repainting, plus reduced noise and heat loss.
Wooden sash windows are famous for rattles & draughts. Both may be eliminated by fitting draughtproofing.
A simple effective method is to route a groove in the sides, top & bottom of the sashes and insert a lightweight plastic or brush strip. Where the 2 sashes meet in the centre, the strip is placed in one of the mating faces of the sashes.
Rattle & draught are caused by excess gap between beading & sash. The original way to control the problem was to refit the beading, reducing this clearance. Modern draught excluders have the advantage that they will continue to be effective if the beading clearance changes, as is prone to occur.
Sash cord is available from any builder's merchants. It is more flexible than ordinary twisted rope, and has a rounder profile. It is also better able to cope with repeated small radius flexing.
There are 2 main types, waxed cotton and synthetic. Cotton lasts several times as long as synthetics.
When replacing broken pulleys, it is wise to avoid the relatively popular cheap plastic pulleys, which can have a very short life.
Quality pulleys are available from: See Suppliers
When needing to increase counterbalance weights, eg due to fitting thicker new glass, the weights may be added to with lead, or cast iron replaced with lead.
When reinstating removed pulley systems, the weights might be missing. If so, weights can be purchased new, bought from a reclaim yard or cast from scrap lead.
In some cases there is enough room for cast iron weights, which are larger than lead due to lower density.
Sash windows were mostly glazed with 1/8" glass (apx 3mm thick), and original non-float glass has slight imperfections giving slight movement to the image as the eye is moved across the glass. Fitting new glass upsets the weight balance and loses the character of the original glass, so fitting old glass is an option sometimes chosen. Horticultural glass is about the same as Victorian window glass, 3mm and non-float type.
Sashes with coloured glass borders usually used 2mm coloured glass, and some sashes used 2mm glass for all panes. Replacing 2mm with new 4mm glass doubles the glass weight, and upsets the weight balance considerably. If 4mm glass is used, the weights need enlarging to match the new sash weight, and there often isn't the room. 2mm sheet glass is less easy to find than 3mm.
Today's building regs require a minimum of 4mm glass for windows when glass is replaced. Such requirements are likely to be waived for listed buildings.
In the 1800s sash windows were mostly green, brown, black or broken white. The modern fashion for white is quite different to the original colour schemes.
If considering restoring a house to its original colour scheme, a look at some hand tinted photos or drawings from the period show the sort of schemes usual at the time. Such pictures are usually enough to put people off the idea of recreating these schemes today.
Traditional sash catches are easily opened from the outside, and are not sufficient security for ground floor windows, or first floor windows that can be accessed from an extension roof.
There is a choice of security fixings available.
Screw-in bolts (pic) are cheap but a complete pain to use, and only really suitable for windows that are very rarely opened.
There are a couple of other types of sliding sash window rarely found.
- Vertical sliding sashes where each sash slides away from the centre and out of the visible frame into the wall cavity when opened.
- Full opening is achieved, but an untied cavity wall is needed.
- Horizontal sliding sashes.
- These are much less free-running than counterbalanced vertical sliding sashes, and debris in the channel can easily cause jamming, paint scraping or rot.