This is something I am asked about frequently. It depends on the style of window you have and the thickness of the openers or sashes. Unless I am taking out double glazed units and replacing them, the rebates will need to be altered to accommodate the extra thicknesses of the double glazed units. The thinnest units available are 11mm, which have 3mm glass, a 5 mm spacer, and 3mm glass. These are known as a 3-5-3. The next size up, which incidentally is cheaper, is a 4-6-4: 4mm glass, a 6mm spacer, and 4mm glass, giving a total thickness of 14mm.
This extra thickness needs to be accommodated in the rebate which has been manufactured to accommodate 4mm glass or on older box sash windows 3mm glass. This alteration to the rebate can come with problems, to begin with it may weaken the actual frame that the glass sits in i.e.: the opener or sash. Timber beading as opposed to putty has to be used to keep them secure, extra width may be required to cover up the edging around the double glazed unit. This can throw your eye towards this and can take away the originality of your period windows, also if you have original glass it may well be drawn glass as opposed to float which has a regular flat appearance this again can throw your eye towards this change of glass, and to be frank it doesn’t look right. Another consideration is the extra weight particularly with box sash windows this has to be counteracted with new sash weights which can be costly particularly if the pocket that the weights go into are too small to take the extra length of a bigger steel/iron weight and a lead weight has to be used instead.
You may have gathered at this point that I am no fan of double glazed units, single pieces of glass have an approximate life span of 800 years, double glazed units 15 perhaps 20 years before they ‘blow’ and condensation then becomes a problem in the space between the glass. Replacement really does mean replacement every 15/20 yrs! Indeed with modern plastic windows the guarantee more often than not does not cover the glass units, only the plastic and window furniture.
So what’s the solution? There are ways to overcome the inherent issues that you may be experiencing with draughts and maybe sound from outside, the first one is to perhaps consider investing in some thick curtains, they really do make a difference, or perhaps the fitting of acoustic and thermal seals, rubber strips that are fitted after a suitable groove has been machined on the edges of your openers or if you have box sash windows, brush seals fitted to parting and sash beads. Another viable option is secondary glazing, either fixed to the inside of the window frame, or sliding or the manufacture and fitting of a second window within the revel that opens inwards this option is common in many parts of Europe and was also employed in this country in the late 1800s in the centre of towns and cities.
An annual visual check and action on potential weak spots, coupled with a 5-7 year cycle of re-painting, will be sufficient to protect your timber windows for many years. The oldest windows I worked on were installed in the 1700s. They were ‘tired’ but because they had been included in a maintenance programme over the centuries (the windows were part of a church estate), they were in remarkably good condition.
As a matter of course I measure the moisture content of your windows, starting at the cill and moving up every 6” until I get a reading of between 12 and 18 percent. This will give me the information I need to asses the extent of potential wet rot areas. On average no more than 10% of any given window will be rotten, including the cill. Repair rather than replacement will be economically viable in all but the most exceptional cases of damaged of frames and openers. Of all the windows in a year that I assess, I see perhaps five percent that are beyond economical repair. In these few cases, it is cheaper for me to manufacture and fit like-for-like wooden replacements.
What is wet rot?
Essentially, it’s a fungal attack on timber that has high moisture content for an extended period of time. There are two main types: white and brown, with several less common types also in existence. You can easily identify both white wet rot and brown wet rot. The former has the appearance of fine threads and often a soft white tissue structure. This type is usually found between openers and cills or transoms. Brown rot appears as cracked cube structures, often brown and brittle, found on exposed timber and joints. Of the two, white rot is probably more destructive as sometimes the fine hairs can travel onto masonry.
Treatment is the same for both: cut it out and replace the affected area with new timber. If the spores from white rot have penetrated the brick this will need to be treated with an appropriate chemical solution.
Window glossary: Here you can see the common terms used when describing windows.
The number one cause of rot, it cracks over time, usually starting in the corners, water gets in, and the damage starts. Water sits in the rebate where it has no means of drying out sufficiently, until next winter arrives, bringing more rain and, for good measure, a frost, which freezes the water between the glass and putty. This then expands, letting in more water, and the cycle repeats. These events will set in motion wet rot behind the putty in the rebate. If the damage is in the corner of the window, the joints between the bottom rail and stiles, mullions, and glazing bars start to suffer as well. It would be wise to check the putty on your windows once a year; it only takes five minutes, and if it is showing signs of degrading then get it raked out and a fresh bead put back in. You can tell if new putty is needed by seeing if there are any cracks in the putty or if it has come away from the glass, in which case you often see moss or dirt between the glass and putty.
Other causes of wet rot in openers can be as described above for window frames: poor quality paints, problems regarding the throat, shrubs, and leaking gutters.
Below highlighted in red are the most common areas vulnerable to wet rot.
There are very few instances where a window frame has become damaged by rot any significant distance above the cill. Such rare instances have four main causes: when water gets into the drip bar or the transom; when a closed opener sits tight on the transom leading to capillary action; when the opener loses the throating, usually the result of swelling, so someone shaves off the bottom rail so that the opener fits again and doesn’t put in a new throat; or finally when the throat has become filled with layers of paint over the years. There is a reason why the throating should be present all around the opener: it is to stop capillary action taking place.
The most common area where you will see rot is in the corners of the cill and in the first 6” on the frame jamb. It can be caused by the above mentioned faults, or by the pooling of water in the corners coupled with a poor paint surface. If the paint cracks at this crucial joint and water seeps in, the water has no where to go and no means of drying out, perfect conditions to allow wet rot to take hold.
Other causes of wet rot can be: poor quality paints allowing moisture penetration; painting timber with a high moisture content; timber of inferior quality (usually not applicable to pre 1970s frames); shrubs growing too close or against the timber; and gutters that leak.
Replacement of rotten cills to a former Victorian school, the grooves in the brickwork were made by pupils in the 1800s that used the bricks as a sharpening stone for their pencils, a marine primer was used to protect the new cills until a painter can finish in the desired gloss.
Refurbishment of a bay window…
Secondary glazing manufactured and fitted to a timber bay window. This format of secondary glazing is common in Europe. Essentially, a second window is fitted into the reveal. The secondary windows open inwards so the original bay window remains intact with its windows opening outwards. This type of secondary glazing is less conspicuous than aluminium sliding secondary glazing and it works well in keeping out draughts as well as traffic noise, which is particularly useful if you live on a main road.
These images show the process of repairing a rotted corner in an otherwise sound hardwood window frame. This rot occurred almost certainly due to the failure of the rubber draught seal that is fitted to this type of window. Over time, the rubbers can become compressed and tired, which weakens the seal, allowing water to ingress and resulting in the rot. You can replace this seal relatively cheaply. It can be purchased in rolls of 50m and you simply cut it to length, then fit it after removing the old seal. Be sure though to get the correct profile (the shape of the cross section). Most suppliers will send a sample on request so that you can make sure. Catch this problem before it starts and you can save a lot of money and effort in having to replace or repair window frames and cills.
These pictures show repairs to a bay window: replacing the cill, machining out the rotten
timber on the bottom rail and the stile to a fixed window, and fitting timber cheeks.
This shows a repair to the corner of a window cill that had been chewed by a previous
owner’s dog, showing the method of routing out the damaged piece and the
This is the refurbishment of a fan light (the glazing above a door). This particular fan light had rotted because of the failure of the putty, which is a common cause of rot in timber frames and widows. It is important to check the putty, particularly on the bottom of the glazing and on the corners. If it’s cracked then water will get behind and slowly but surely the timber will decay.
This was a recent repair to a conservatory roof, undertaken without taking the polycarbonate roof off, using 45 degree scarfing joints and finishing in white gloss.
This is a window I repaired on a Victorian house near Southampton.
It is well worth the time and effort to repair timber windows and frames, not only from a financial perspective but also from an aesthetic point of view. There are many fine examples of traditional timber windows, whether the timeless purity of traditional box sashes or straight forward timber casements, that complement the building’s overall appearance. There are also unfortunately many examples of the outcome of replacement with plastic; it just doesn’t work on older properties. Often you see two identical properties next to each other, one with plastic windows, the other retaining the original windows. No prizes for guessing which one has that special appeal.
Recently, a friend considered putting their house on the market and sought the advice of an estate agent, asking them whether to replace “these old wooden sash windows with plastic double glazed ones”.
The estate agent replied that if he did then the value of the house would be reduced by about £10,000. The house incidentally was valued at £170,000. The agent was suggesting quite strongly that the original windows would be a significant selling point.
Repairs to timber windows are not an exact science and each repair is different. It can be a straight forward cill replacement, or the scarping-in of fresh timber once the rot has been cut out, or copying mouldings on finer examples. Sometimes I can come across surprises; what may appear straight forward becomes major wood surgery. Over time home owners use fillers then sand this down and paint, and over the years this repair becomes lost. It’s only when the timber next to this temporary repair starts to rot that the earlier filling is found.
Most, but not all, cases of rot in timber windows and frames will be wet rot. There are various types of wet rot and it is caused by timber having a high moisture content for an extended period of time. Vulnerable areas are window cills, bottom rails on openers, and the bottom of stiles (the vertical members on a window). Almost without exception the rot is due to paint degrading or putty becoming cracked and letting in water. Another problem is capillary action between the bottom rail and the cill, where water can sit unnoticed for years. Also, the degrading of glue or pegs on the joints can be an issue, again effecting the bottom members of windows. I usually cut out effected areas, scarp-in replacements, and introduce a timber tennon known as a free tennon into joints to stiffen them, followed by replacing the damaged putty.
It would be prudent to take a look at why the rot has occurred. More often than not the problem is the guttering above or a bush that has grown too close, but it may be just the passage of time. If your windows are old, for example 50, 80 or even 100 years or more, then it’s likely to be no more than wear and tear. A mention of paint would not go amiss. I do not usually get involved in painting but do recommend that you use marine paints on your external joinery, so a trip to a chandlers will need to be made to get the right paint. It will cost a bit more but in my experience as a boat builder (and one-time live-on-board) it out lasts the usual brands available.
I am happy to visit and take a look at your windows with a view to repairs. It may well save you £1000s in replacement costs.
This is a good example of wet rot showing just what can go on unseen this board formed part of a window surround and the gutter above had a leak this event took place over a period of two years resulting in the surround having to be replaced and the leak sorted. A rubber gutter seal at £1.50 and 15 minutes would have prevented this!