Bay Window Movement

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Posted by Robert Hopps | September 26th, 2018


A very common type of cracking that we see in older style properties with bay windows relates to movement of one type or another. In this blog we will look at the problems that can arise as a result of failure of timber beam or 'bressumer' above the bay window.

Below is a section through a typical Victorian house showing the ground floor and part of the first floor.

In this image we can see that a fairly large amount of the structure in a bay window is in fact formed in timber. Commonly there is a form of timber framework above the bay window frame, timber posts, timber framing for the bay roof and most notably the timber beam/bressumer below the first floor wall.

The bressumer is a large timber or two adjacent timbers that brickwork is built up off of. These timbers sit in the external wall either side of the bay and often have very little covering to the external elements.

As long as the property is well maintained and external weathering is taken care of, the bressumer will function without concern. However if the bay roof leaks for example if the flashing has cracks or is split, dependent on its material, it will allow the timber to become wet.

It stands to reason that timber should be kept dry, particularly when it has such an integral structural role. Wet timber, if allowed to stay wet will progressively rot and lose strength and this can cause the wall above to begin to move downwards. In the below image we can see the extent of the movement of the wall above.

We have included an annotated image and a close up. Clearly, the window sills of the windows above are leaning heavily down, the brickwork is sagging visibly and there has been a fair amount of re-pointing where the brickwork has opened up.

It is not just water damage that can cause a bressumer to fail. If the wall is altered or overloaded in any way this can cause the bressumer to have forces imposed upon it which it was not designed to take. For example some loft conversions (although many impose loads on the party or flank walls) or altered window openings and newer, heavier tiles placed upon the roof.

In a later blog we will look at structural movement of a bay window caused for other reasons. In the meantime, if you have any concerns about your bay window or indeed any part of your property, feel free to give The Hopps Partnership a call.

The Hopps Partnership
Second Floor, 34 - 40 High Street
Wanstead, London E11 2RJ

The Hopps Partnership
70 Cowcross Street
London EC1M 6EJ

020 8502 6323

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The Hopps Partnership