ᐅ Roof Tiles – Analysis of the Roof and Dormer Watertightness

Dear house-building forum,

During my current conversion of an attic into living space, I have started a discussion with the carpentry contractor regarding the roof's airtightness.
The roof structure is as follows:
- Existing rafters reinforced from 14 to 20 cm (5.5 to 8 inches)
- 3 cm (1 inch) rough tongue-and-groove boarding with a bracing effect
- 5 cm (2 inches) wood fiberboard as added roof insulation and first waterproofing layer
- Vapor-permeable self-adhesive roofing membrane as the second waterproofing layer
- Roof battens with nail sealing strips
- Roof tiles

The sealing membrane used was Siga Majcoat or Braas Divoroll with the corresponding adhesive tapes. After a rather dry summer, I only recently had some rainy days on site, during which I removed a few roof tiles over the dormers and took thermal images from the inside. I have attached the results. The carpentry contractor claims this is normal because the membrane is vapor-permeable and will eventually saturate, causing water to form underneath. This explanation doesn’t fully convince me at the moment, especially since I wouldn’t expect such staining. I would appreciate your opinions on this.

Furthermore, no windows have been installed yet, so interior and exterior conditions currently have similar humidity levels, which is relevant to condensation formation.

I only used the thermal camera to locate potential damp areas more quickly and easily. The spots in question also felt wet to the touch.

I look forward to your responses! Thank you in advance!
Best regards
Prost2000

What is the roof pitch of the dormer (it looks very shallow in the pictures) and which roof tiles were used?

Regards, SvenP

Hello Sven,

Thank you for your efforts so far.
I don't have the roof tiles in mind right now; I would need to check them. However, that shouldn’t be a major issue since I want to examine the waterproofing layers underneath. The roof itself has a pitch of 34°, and the dormer is indeed quite flat, about 5° pitch. But during my tests, water was still draining reasonably well from the waterproof membrane there.
The dormers themselves are metal-clad—maybe I should have mentioned that. I removed a few roof tiles above the dormers and found some damp spots. My concern is that in a few years, a few damaged roof tiles (which can easily happen without me noticing) could cause mold to develop inside the insulation due to moisture on the interior side. However, it will likely take much longer for that moisture to penetrate all the way inside. At the moment, I suspect that the transition between the main roof and the dormer allows water ingress.

To put it briefly, what bothers me most is the fact that I have a separate charge on the invoice for a "double waterproofing of the dormer due to low pitch," and yet this appears not to be watertight—which I also have to pay for!

Thank you very much for your help and best regards,
Prost2000

An underlay membrane does not have to be waterproof, only water-resistant. According to roofing industry standards, a tiled roof with a pitch below 10 degrees is not permitted, and simplified, a pitch below 16 degrees requires additional measures.

If your dormers have a roof pitch under 10 degrees covered with tiles, and there is only an underlay membrane underneath, then that is fundamentally incorrect.

Best regards, Sven

Thanks for the reply!

The dormers are clad in metal, not tiles; the tiles are on the main roof slope, not on the dormers themselves! But due to the shallow pitch, I was supposed to pay for double waterproofing on the dormers, which I currently don’t see. It can’t be that if 2-3 roof tiles are removed (later on due to damage), I already have damp spots inside after just one day, as the insulation felt will start to mold.

The problem is that it takes ages until I can actually see this from the inside, behind the insulation felt, the double drywall layers.

Thanks and best regards!
prost2000