Exterior window sill subsequently added in WDVS

Good evening,

we have a bay window that extends 1.5m outward and has a narrow terrace door on one side. This is only opened for cleaning or ventilation, not for going outside.
There was supposed to be a windowsill there. We have aluminum windowsills on all the other (non-floor-to-ceiling) windows.

Is it still possible to install a windowsill here afterwards? On the others, these extend a bit into the plaster or the ETICS. That is difficult here.


The window faces north and is relatively sheltered since the house wall is immediately to the left. So this window will not get much rain.
The bay window is single-story; the wall on the left side goes up two stories.

Sure. However, I wouldn't use aluminum, as it quickly gets scratches and dents. Use something made of artificial stone, set in a mortar bed, seal with color-matching silicone (make sure it is for outdoor use, it usually contains silicate).

But how do you then connect to/into the [WDVS]?

Well, we have aluminum window sills everywhere else. Since you don't go out there anyway, I don't really see the risk of dents or scratches.

I do not know your specific situation, can only tell you that aluminum is usually not used there. But anyway. Whether aluminum or stoneware: it should fit precisely and then be sealed with silicone containing silica. I cannot name any products here, but any hardware store will help you with that.

short answer: no.

Neither is the insulation walkable, nor will you get the connection done already, because they are plastered in.

L-shaped brackets with special thermal bridge screws are attached TO the masonry, the insulation goes around that, then the window sill, then plastering.

then the load is on the masonry and everyone is happy.

the sealing of the insulation on top has already been done by the builder, I would still apply a bitumen membrane with bitumen cold adhesive, which is available in many widths as a wall barrier membrane. cut to size accordingly and then stick it on with the cold adhesive from NEM. you can throw away the brush afterwards and wear gloves while doing it.

you can then have aluminum corrugated sheet edged and cut to length. but all this will look more or less semi-cool, you won't get it nicely fitted. then seal the side gaps with silicone.

the problem is that the cutout is not right-angled.

what is coming in front of the window?

otherwise, just leave the roofing felt there for now, it is sanded and doesn’t look bad at first and better than now.

No one is supposed to step on it afterwards either. I won’t do it myself but my general contractor will. As mentioned, he wants to just "glue in" the aluminum windowsill. So without plastering the sides. That’s actually not professional, but the question is whether in this case it might be sufficient since it’s only half a meter long and close to the corner and the window faces north anyway.
There will be a flower bed in front of it later.