Gypsum board on a sloping roof, apply felt plaster.

Hello, I have a question, I want to apply felt plaster on a green drywall panel. Now I have already noticed during tiling and previously when applying a liquid membrane in the lower area that the gray liquid membrane turns brown because of the green panel, so if I apply a white felt plaster, how will the color show through then? I would like to keep the top color in beige/yellow. Are there primers in yellow that go under the plaster or something?

Hello apprentice,

what kind of felt plaster is that supposed to be? Basically, you can felt many types of plaster: gypsum plaster, lime plaster, lime-cement plaster.
Is the wall located in the splash water area?

In my bathrooms, I have double-layered the ceilings with green plasterboard, stretched a complete fabric reinforcement over it, and then applied about 10 mm thick gypsum plaster, which I smoothed with a mirror finish by felting. Overhead work is fine. After drying, I just briefly went over it with sanding mesh and finished. Nothing shows through, and there are not even any moisture problems or hairline cracks directly above the shower. With such a coating, you can do without magic additives because the classic method of plaster carrier + "thick" plaster also works on plasterboard.

If you only want to apply a thin layer of some kind of premium plaster, you should strictly follow the manufacturer's specifications for the plasterboard substrate. Then you may need expensive burn-in barriers or () primers so that nothing shines through. But this is not clearly evident from your information.

I want to apply the lime-cement plaster from Schäfer-Krusemark. Partly with the spray area, the roof slope runs past the shower. Actually, a plasterer would do the work, but he hasn't been on site yet and since I want to provide the material, I want to get some information beforehand about what I need. 3-5mm application, more is possible on the dormer cheeks, not because of the window, that's about how much it should be. So you mean there should be something from the manufacturer if a primer is necessary? I will have a look, thanks for the info.

In my opinion, that is likely to cause problems. The manufacturer requires a minimum application thickness of 15 mm, with 10 mm allowed in a few places. So 3-5 mm is not feasible. A special substrate treatment is only described for HWL boards, but in the case of GK, there should be something similar in my opinion. It’s best to inquire with the manufacturer. The technical data sheet is easy to find via Google.

But what is the point anyway – plaster on GK in a partial splash water area? I don’t have GK walls, but solid walls, all with lime-cement base plaster and white lime plaster as the top coat. In the splash water area, I have sealed rigorously in two layers, nicely incorporating sealing tapes, then tiled everything. With millimeter-thin plaster in the splash water zone – and on GK at that – you won’t have any pleasure. Who designed that?

Somehow it says at Schäfer-Krusemark "Filzputz 600: thin-layer lime-cement plaster" and I can't imagine that this has to be applied in 10-15mm thickness. There is also a primer from Krusemark to prevent "bleeding in GK boards," so much for that on the manufacturer's page. It is not a direct splash area; I was thinking more of rising water vapor. A shower cabin will be installed.

moin apprentice,

"Filzputz 600" is of course something different from "Schäfer-Krusemark lime-cement plaster". The exact material designation is always helpful for technical inquiries in the forum.

But again I see problems: the Filzputz is supposed to be applied according to the manufacturer's specifications on a mineral substrate. A gypsum plasterboard is anything but mineral. They recommend either their base plaster or their great ETICS system as a substrate. But even there, the Filzputz does not come directly onto the insulation boards with which the poor houses are plastered. The prescribed layering sequence is: reinforcing mortar - reinforcement - primer - top plaster. Then you can smear the stuff on Styrofoam or mineral wool. So everything other than a simple and merely millimeter-thin structure. Calculate what all that costs!

Before the "plasterer" without gypsum shows up at your place, you should download the technical datasheets and make sure he also makes his offer according to these regulations. If he wants to apply the plaster directly on a primer, you can intervene.

Unfortunately, I don't know how your bathroom is constructed, how big it is, what ventilation options exist, etc. But if it is an "average" bathroom of maybe 6-8 sqm, in the attic, where everything upwards is nailed shut with vapor barriers, in the worst case no window, then prepare for a lot of steam. In my walk-in shower (open without cabin and splash protection) I can easily saturate 12 sqm x 3.60 m height with steam during extensive showering if the window is not open. The steam goes upwards, is usually hindered by a vapor retarder and buffered in the ceiling plaster (this also includes 12.5 mm gypsum board). The surplus bounces back, condenses on the glass and tiles, which serve as condensation surfaces. The rest now seeks the other surfaces, like plaster. If that is very thin, it cannot buffer anything. It is saturated just from the steam alone, causing material tensions in the plaster. That is why such a plaster must be formulated and reinforced. How well the system then really works, you will see after one year of bathroom use. Light plasters can discolor with daily steam exposure (e.g., yellow tint, dark spots that no longer go away) or even flake off. The steam load is especially difficult to manage in winter, because actually one would have to ventilate by shock ventilation consistently to let the steam out. If that doesn't happen, the steam lingers in all corners for a long time and certainly finds all microscopic holes and cracks in your vapor barriers, where it causes damage in mineral wool & co. that is hard to detect.

In small bathrooms, I would forego plaster near the shower, seal everything with an elastic system and tile consistently, especially if it is a gypsum board miracle. Then you have a target condensation surface, where the condensate eventually lands on the tiled floor and then in the floor drain.