Replacement Windows

Maintenance of timber windows

We all know prevention is better than cure, so regular maintenance – treating small instances of rot before it takes over, painting and checking the glazing, can avoid many problems down the road.

If the casement itself shows signs of rot that cannot be treated locally, but the frame itself still has its integrity, then just replacing the casements can be relatively cost effective and potentially much safer than considering a uPVC replacement – even if planning permission were to be granted which is unlikely in the conservation area.

windowsIf the frames themselves are losing integrity, then replacement may be necessary. We would advise hiring a structural engineer to check how bad they are, and to suggest if any remedial work is viable before considering replacement. OTRA is happy to receive recommendations of good local companies so we can publish the details here – please do not rely on the cold calling, door to door salespersons or the cheapest quote from Checkatrade!

Why these houses may require a different approach – beware!

If replacement is necessary, it may be money well spent to involve building control rather than rely on the advice of the less scrupulous window companies who can self regulate. Typical window installers are accredited by the organisation FENSA. This means that they can basically bypass Building Control and certify the installation themselves. While they may be very good fitters often they do not understand the issues with older buildings and do not carry out the kind of survey necessary to detect the integrity of the old brick and deteriorating lime mortar that is supporting an entire bay and gable.

The bay windows are a structural element and are basically a frame that is supporting all the masonry above. When a fitter replaces an entire bay window, theyt will typically prop up the internal ceiling before removing the structural window. They do this with props holding up a plank that runs parallel to the window and so perpendicular to the joists above. However John Smith style houses often have joists which are parallel to the front of a bay rather than perpendicular to it, and unless they check, the fitter might assume otherwise and place the plank parallel with and between two joists, so that when the old supporting  window is removed, all the weight of the bay above rests on the old lathe and plaster ceiling. This has actually happened (see below)

Repair or Replace?

Experience has shown us that even with timber frame replacement, modern building regulations would often require/advise to reinforce the structure with an extra lintel.

Sadly many typical window installers are able to use their FENSA accreditation to bypass building control and self certificate. This is usually not an issue with newer buildings, but have no clue about the inherent issues with older properties with bay windows. 100 years after being built the heavy bay and gable structure is supported only by old clay bricks and disintegrating lime mortar.

Even then just the trauma of removal and replacing can be problematic.

Some links from Historic England regarding Windows:Repair may be a much better option than replacement.

Replacement Windows

As you can see from our planning permission guide, unless covered by like for like works, replacing windows that are in the public view do require planning permission. If in doubt please contact us for informal advice or speak to the council for more formal advice. In some special cases permission for uPVC windows has been granted, however it is very likely that there would be many objections unless a good case can be made for them. If possible, repairing original timber frame or replacing like for like is the preferred option. Note that the original windows were single glazed of course, so sadly a double glazed replacement would not be considered as like for like.


Although uPVC may appear to be economically viable, it is important to to bear in mind that such windows have a much more limited lifespan. Most companies offer a mere 15 year warranty. However timber frame widows, provided they are of good quality and well maintained can last much longer – there are plenty of examples in the Triangle of windows still going strong after a century or more – and may well outlast the rest of the house. 

However regular paiting can be crucial. After a few years the paint can still look OK from a distance, but small cracks can mean that water gets in and strt the rotting process so it’s worth checking every few years.

Note also that modern timber framed windows will often be “factory finished”, and although theoretically  paint finish would could last many more years than traditional hand painting, it is still worth checking for cracks regularly.

Construction – Bay Windows

Baypole and spreader plates

The original timber frames were built to be part of the bay construction. The top timber of a frame which follows the contour of a bay, in conjunction with a lintel, would support the brickwork and gable above.  The lower timber would spread the load evenly right across the course of bricks beneath.

Structural issues

Apart from the looks, there are potentially huge issues with uPVC bay windows. They generally rely on load bearing poles at the two front corners of the bay. This is literally a steel pole with a “spreader” plate at the top and bottom to bear the load, and this is where a big problem can arise, especially with older houses. The plates are often inadequate to spread the load in the way the timber frame did, and so only act as a “partial” lintel above in conjunction with any existing timber lintel (which after a 100 or so years may well have lost some of its integrity.)

Corner bricks, aka squints, cracking due to inadequate spreader plates.

The baypoles and spreader plates fitted by window companies are mostly fine for new builds, which use modern bricks, cement mortar and steel lintels. But if you think about a typical house in the Oakmount Triangle with soft clay bricks held together with old (often crumbly) lime mortar, you will realise this is a very different kettle of fish. Without spreading the load adequately on bricks and lime mortar beneath the windows, the bricks could crack and the foundations may be compromised.

An Oakmount Triangle Windows Horror Story:

A great reason to use timber framed windows

After cracks started to appear inside and outside the bays, we contacted the original fitter, Anglian Home Improvements, (who had installed uPVC windows before we moved in and before conservation status).

They initially offered to send a plasterer round to fill the cracks, but after we insisted on a structural engineer’s report, they finally agreed to replace ground floor windows under warranty and carry out structural repairs (claiming from their own insurance company).

A year later the situation was worse, the gables had dropped by 15cm and the squints below the ground floor baypoles started to crack (see above).

The structural engineer, along with Southampton Building Control, determined Anglian had installed the windows with the wrong baypoles (aluminium instead of load-bearing steel), inadequate spreader plates (in some cases no spreader plates at all) and no regard for the need for lintels. Also they had propped the ceilings incorrectly, assuming the joists ran into the bays (see above) and so more cracks appeared in the ceilings.

All three bays were now slipping away from the house and a further successful claim against Anglian was made resulting in rebuilding the bays from the ground up to the roof (cost to Zurich Insurance: £75000).Anglian 2

Anglian Windows 1

Update: Anglian Home Improvements are still installing windows in the Triangle with small square spreader plates and no lintels.

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