Good evening,
I have black spots in the joints of my skylight in one corner.
Can an expert assess whether this is mold or how I can find out?
If so, what should I do in the short term and long term to address it?
The pictures are heavily zoomed in. The affected area is about the size of a fingernail.
[ATTACH type="full" alt="Corner area with cracks and flaking in wall and floor joints" width="500px">
I have black spots in the joints of my skylight in one corner.
Can an expert assess whether this is mold or how I can find out?
If so, what should I do in the short term and long term to address it?
The pictures are heavily zoomed in. The affected area is about the size of a fingernail.
[ATTACH type="full" alt="Corner area with cracks and flaking in wall and floor joints" width="500px">
D
derdietmar14 Jan 2026 19:50Hello,
I think everything has been done properly; there are no visible signs of poor workmanship in the pictures. Usually, where there is poor workmanship, there are also careless details nearby.
For now, I would just remove the mold using a mold remover from the hardware store. When it gets significantly warmer in spring and no more condensation can form, you can proceed with the repair. Until then, ventilate thoroughly and perhaps use a fan with an extension cord blowing out the window overnight.
Velux also offers plastic window reveals / interior linings. Check if there are suitable inserts for your model. This would be the sustainable and clean solution.
As an alternative repair: first, scrape the acrylic out of the joints, then address the mold. Let everything dry openly and then apply new sealant. Finally, paint over it with latex paint. The cracks are problematic. The framework shifts, so it will probably keep cracking at the corners; this is normal to a certain extent.
Best regards
I think everything has been done properly; there are no visible signs of poor workmanship in the pictures. Usually, where there is poor workmanship, there are also careless details nearby.
For now, I would just remove the mold using a mold remover from the hardware store. When it gets significantly warmer in spring and no more condensation can form, you can proceed with the repair. Until then, ventilate thoroughly and perhaps use a fan with an extension cord blowing out the window overnight.
Velux also offers plastic window reveals / interior linings. Check if there are suitable inserts for your model. This would be the sustainable and clean solution.
As an alternative repair: first, scrape the acrylic out of the joints, then address the mold. Let everything dry openly and then apply new sealant. Finally, paint over it with latex paint. The cracks are problematic. The framework shifts, so it will probably keep cracking at the corners; this is normal to a certain extent.
Best regards
S
stromneuling14 Jan 2026 21:15I think everything has been done properly; there is no visible shoddy workmanship in the pictures. Usually, where there is poor workmanship, you also find sloppy work in other small details nearby. That’s already a good sign :-)
When it’s lighter again, I’ll take a picture of how it looks currently.
Velux also offers plastic reveals / interior linings. Check whether there are suitable inserts for your model. That would be the sustainable and clean solution. Yes, I will look into that, but I think finding someone to install it might be more difficult. I don’t believe I could manage that on my own 😉
Alternatively, for a repair: first, scrape out the acrylic from the joints, then treat the mold. Let everything dry out openly and then apply new joints. I can actually do the jointing quite well.
Question:
a) The mold is visibly only on the joint. Behind, underneath, and around it I see nothing. So my question is, besides scraping out the joints, do I need to do anything else or check further?
b) Should I do this around the entire window or is treating the affected side enough?
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