Square-up window openings in block cavity walls

3 Jun 2021
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United Kingdom
Hi All,

Short version:
We want to replace the UPVC windows in our house and straighten the window openings through the cavity wall blockwork in the process.

Our house (is the least imaginative design possible):
- 9m x 9m footprint.
- 2 stories with a pitched roof.
- Cavity walls made of blockwork, with rendered exterior.
- 8 big windows, 4 at the front, 4 at the back, arranged symmetrically in a grid with 2 on each floor directly above each other.
- Each window measures 2600mm (w) x 1500mm (h)
- Each window sits in the outer leaf of blockwork on top of a concrete sill.
- The existing windows are UPVC in various states of repair and various ages.

The problem:
-We want to replace the windows so that they are all matching, work perfectly, and are up to modern specifications.
-HOWEVER.. The 4 corners of the house have settled by 10-20mm in the past. We believe the settling was caused by the concentration of loading at the house's corners, partly due to the huge windows, and also by the original drainage and guttering that simply dumped rainwater on to the ground adjacent to each corner. The drainage has been fixed 5-10 years ago and there is no evidence of further settling following this (no fresh cracks etc). The result is that most of the window openings are not perfectly square. To accommodate this, the windows that have been replaced have had plastic packing strips added top and/or bottom, that have been cut at slight angles to provide a level top and bottom for the perfectly square UPVC windows to sit in. Unfortunately this looks rubbish, because the original concrete sills clearly aren't level when viewed next to the level window sat on top of them. Some windows also have stub-sills with packing material separating the level stub-sill from the wonky concrete sill beneath it, and the shadowy gap between the two highlights the misalignment terribly. A similar misalignment is apparent at the top of some windows although this is less noticeable at first glance. Two example pics below. See the differences along the bottom edge from left to right:

WhatsApp Image 2021-06-03 at 12.22.52.jpeg
WhatsApp Image 2021-06-03 at 12.22.22.jpeg

What we want to achieve:

-We want to replace the windows.
-We want to remove any visible misalignment completely, in a way that is durable, affordable, and doesn't leave us without windows in openings for any longer than absolutely necessary.

Ideas so far:
- Leaving current windows in-place, cut off the concrete sills flush with the external walls and level the top of the remaining concrete sill section during the installation of new UPVC windows, complete with UPVC sills . Render over cut concrete sill to finish. This seems somewhat laborious, but not particularly technically challenging. It won't do anything to fix misalignment at the top of the window openings.
- Remove current windows and completely remove existing concrete sills. Replace with new concrete sills, or UPVC sills, or a better/smarter alternative sill material before fitting new UPVC windows. Again, this does nothing for the misalignment at the top of each window opening.
- Do one of the above and clad the first/both floors of the house, using level cladding in the reveals to cover the misalignment above each window.

It would be great to get advice on this issue. I've worked on houses with far more historic settling than this one, but the size of the windows and the contrast between level windows and non-level sills/openings really stands out in this case.

Any help/advice/links would be much appreciated!

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