Repair or replace? How to decide what is best for your London sash windows
London homeowners often contact us knowing that they need to have work done to their sash windows, but are unsure if they need to be repaired or replaced. Often both options are possible so the question becomes which is the better option for you, in terms of longevity, cost-effectiveness and property values. Here we will run through the different repair and replacement options, the pros and cons of each, and give some tips and pointers on how you can decide which is more suited for your property.
In other circumstances, due to extensive rot or water ingress, only replacement will be viable. We’ll cover the signs to look for to determine if a replacement is your only choice.
We will also discuss what you are allowed to do, if you are in a conservation area such as the Battersea Park Conservation Area in Wandsworth Borough Council.
What repair options are available?
Repairs can range from functional fixes that make a sound window work properly again, through to structural timber repair – and which you need depends on whether the problem is how the window operates, or the condition of the timber itself.
Draught proofing and overhaul
If your windows are stiff or do not open, they may need easing and adjusting, if they are draughty then fitting new weatherpile draught excluders will be best. It is possible to combine a number of these different repairs into a single service.
A draught proofing and overhaul service generally involves removing the existing sashes from the frame and planing them down at the sides to remove any build-up of paint that has accumulated over the years. This will allow the sashes to slide more easily within the box frame. While the sashes are removed, that is the best time to clean the channels they run up and down in, again to remove any build-up of paint, dust or debris.
Your sashes also may not run smoothly at the moment because they may be incorrectly balanced or the sash cords have become detached from the counterbalance weights. In a draught proofing and overhauling repair service, we can hang the sashes on brand new cords and ensure that they are correctly balanced.
Installing new staff and parting beads with pre-fitted weatherpile draught excluders will help to reduce the cold air coming into your property through gaps between the sashes and the box frames.
Rot repair and splice repair
A draught proofing and overhaul service is a functional repair, ensuring that the window operates smoothly, locks correctly and doesn’t let cold air in. If there are more structural issues with the timber of your window, the timber itself can be repaired. Rot in a timber window can be repaired in two ways, both involve cutting out and removing the rotten areas of timber, and then either splicing in a new section of timber, or using a material like RepairCare DryFlex 4 Epoxy Resin.
This work is most suited where there are small, localised areas of decay in an otherwise sound frame. If the rot or decay is more extensive, this option can start to become a false economy. If the rot you can see on your windows is extensive, it is likely that it goes much deeper than you think and any repair becomes more expensive and less effective, at this point a replacement becomes the more financially prudent option.
There is also the option of a fill-repair, which is using filler where the existing timber is deteriorating. This generally does not work well, as the timber will be decaying due to it containing moisture, and most fillers will trap that moisture into the timber. Fill repairs will often make your window look better for a short period of time, but underneath it is actually making the problem worse.
What replacement options are available?
Replacing your sash windows will often be the most effective option available to you, but it doesn’t always mean replacing the entire window. There are two replacement options, a sash replacement where the sliding sashes are replaced but the surrounding box frame remains in place, and a complete replacement where the entire window is swapped for new.
Sash replacement
Timber box sash windows are generally fitted in a rebate in the brickwork – known as a checked reveal, the width of each frame jamb is around 100mm but only around 15mm is visible from outside. This means that the vast majority of the frame is tucked behind the brick, and sheltered from the weather. As long as the frame has remained sealed to the brickwork or masonry, and water hasn’t been able to penetrate between the two, the box frame will almost always be in a good condition.
If the frame is in a good condition, then often there isn’t any need to replace it and that is why it is possible to carry out a sash replacement service.
A sash is the part of the window that slides up and down, and it sits in the frame. In a sash replacement service, new double-glazed sashes are manufactured in our workshop and fitted into the existing box frame. In conjunction with this, the window is draught sealed, rebalanced and new cords are fitted.
This service is ideal for you if your frame is in a good condition and you want the benefits of double glazing without the added expense of replacing the entire frame. Our sash replacement service is around £600 – £1,000 cheaper than a complete replacement service, it can be done in around half a day and isn’t nearly as dusty or messy.
One compromise that you have to accept with a sash replacement over a complete replacement is that the new sashes are fitted into what may be a slightly out-of-square frame. If you visit our showroom in Fulham, you will see that the brand new sashes in a brand new box frame can be moved with one finger because everything is perfectly square. With a sash replacement service, the sashes will slide much better than they did before, but not as perfectly as a complete replacement. And that is the decision you have to weigh up, whether that perfect sliding motion is worth the additional cost.
Complete replacement
This is the most comprehensive option that you can choose from, and as the name suggests it involves removing all timber and glass parts of the window and replacing with new. This includes the sashes and box frames, but also the architraves, linings and window boards.
This service is most suited to you if your windows are rotten beyond reasonable repair, or if you simply want the best possible product. With all parts of your windows being brand new, you have complete confidence that every aspect will stand the test of time. You won’t be concerned whether a fill repair has just covered up a problem, or that having a sash replacement may mean that further repairs are needed to the frame in years to come.
This option also gives the highest level of insulation. Replacing all parts of the window means that we can design and engineer the window to incredible precision and tolerances. This means that your windows will be as air-tight as possible, and with a U-value as low as 1.2W/m2K.
When does replacement become the only sensible option?
On first inspection, it may not immediately be clear whether repair is realistic. Often windows will not open, which means inspecting large parts of the frames won’t be possible. Furthermore, rot can be hidden under layers of paint and filler by previous tradespeople. In some circumstances, it may be obvious that a window is beyond reasonable repair with clearly decaying timber with holes in frames and sashes.
However, more often the point at which replacement is the only option is a tipping point, where the cost of repairs and the amount of time that those repairs will last does not make economic sense compared to replacement. Whilst rot repairs, splice repairs and draught proofing can work very well, you will still be left largely with timber that is more than 100 years old and will likely find yourself needing further repairs in other areas of the window in 3 – 5 years’ time.
In our professional opinion, once rot is established and widespread in your windows then it is more than likely that your best option is to replace the window, either with a sash replacement or complete replacement.
What if your home is in a conservation area?
If your home is in a conservation area, there are extra controls on what you can do to your windows – and that can influence the repair-or-replace decision.
Repairs are usually straightforward. Like-for-like repair of your existing windows generally does not need permission. (Listed buildings are the exception – they can require consent even for repair work, so treat those as a separate case.)
Replacement depends on your specific area. The deciding factor is whether an Article 4 Direction is in force, which some conservation areas have and others do not. Where one applies to windows, even a like-for-like replacement needs planning permission; where it does not, a sympathetic timber replacement may be permitted development. Flats and listed buildings will generally need permission regardless.
Because this varies from street to street, we always check the position with your local authority before any replacement work – so you never carry out work that needs consent without it.
Repair vs replacement for London sash windows – A comparison
| Service Option | Best suited to | Glazing outcome | Relative cost | Disruption | Longevity |
| Draught proofing & overhaul | A sound frame and sound sashes that just don’t operate well or that let in draughts | Keeps existing glazing (usually single glazing) | Lowest | Low – half a day | Improves operation now; timber is unchanged, so age-related issues remain |
| Rot / splice repair | Small, localised areas of decay in an otherwise sound frame | Keeps existing glazing (usually single glazing) | Low, but rises sharply if rot is more extensive than it looks | Low to moderate | Good for isolated repairs; a false economy where decay is widespread |
| Sash replacement | A sound box frame where you want double glazing, without the cost of a full replacement | New double-glazed sashes | Mid – around £600 – £1,000 cheaper than complete replacement | Moderate – around half a day, relatively clean | New sashes in an existing frame; excellent, though frame may be slightly out-of-square |
| Complete replacement | Windows rotten beyond repair, or where you want the best possible result | New double-glazed sashes, compliant with Approved Document L Building Regulations | Highest | Highest – most involved and messiest | Longest – every component new, engineered to the tightest tolerances |
Contact us
If you would like to know more or to arrange a free home visit for one of our expert surveyors to inspect your windows with you, please contact us.
