ᐅ Mineral cast window sills warp or deform over time

Hello,

I would like to know if anyone has experience with a similar case or expertise in this area. My search in forums and on the internet has so far yielded no results.

In our new build, the window sills are warping—especially on the south side.

The window sills were installed in February while the house was still in the shell stage. The sills were fixed on the left and right sides to the aerated concrete blocks using wooden wedges and sprayed with foam from underneath. The interior plaster was applied around March. By May, the screed was in place and the underfloor heating commissioning program had been completed.

Recently, we noticed that the window sills, mainly on the south side, are bending upwards in the middle. It can be seen that the interior plaster, which has been in place for a long time, is still perfectly straight. This means the window sills warped only after the plaster was applied and after the foam had fully hardened.

It is also noticeable that this only affects the shorter window sills of about 1.10 meters (3 feet 7 inches) in length and only when they are on the south side. There are larger window sills on the south side and smaller ones on other sides that are not as severely affected. The upward bend in the middle of those sills can only be guessed at and amounts to about 1–2 millimeters (0.04–0.08 inches). For the smaller south-facing window sills, the upward bend in the middle is about 8–10 millimeters (0.3–0.4 inches).

According to our construction supervisor, the case was discussed with the stonemason, but there is no explanation—“We have never seen anything like this with window sills before.”

Now, the three mainly affected window sills are supposed to be replaced.

My concern is that since the cause of the problem cannot be explained, the same behavior might occur again after some time. In the worst case, the window sills could crack. Stone is not exactly known for its elasticity.

What I currently do not know is the brand of the mineral cast window sills and the type of foam that was used.

I would really appreciate any comments. Best regards

My first suspicion is that they were installed sideways without any gaps, and when the sun shines on them, they want to expand but can't, which causes them to bend. However, I find the extent of this quite extreme and have never seen it like this before.

Thank you for your assessment – that sounds plausible. What speaks against it: I would assume that the plaster on the left and right is much softer and would likely give way due to the expansion. However, there is no visible crack or anything similar.

I thought the benches were installed before the interior plastering? Weren’t they plastered in and are now pressing sideways against the masonry?

Yes, exactly, they were installed before the interior plastering. The window sills definitely had some space on both the left and right sides within the wall opening and were initially secured there with wooden wedges. During plastering, the wedges were removed, but I’m not sure whether the gap between the window sill and the wall was then filled with plaster or if a joint was left. I rather suspect that plaster was simply poured in. Unfortunately, I don’t have a good picture. Only the attached photo—there you can make out the wooden wedge next to the window sill.

Well, it is a bit strange. Once the wooden wedges are removed, there is naturally a gap. And if the gypsum plaster got into that gap, I would also expect to see some damage to the plaster. But who knows, maybe several factors are at play here. On one hand, the mineral cast mixture could be of poor quality (too much resin, not properly cured?), and on the other hand, the gap could have been plastered over. I would definitely recommend making a complaint. I just read that they are already being replaced. When reinstalling, it might be a good idea to ensure the gaps are properly maintained.