1940s house, plaster over pyrobar, bathroom remodel

[cshoe]

Hi, guys -
Thanks for helping out folks like me who are at a loss about how to proceed.

As you can see in the photos, we're renovating a 1948 bathroom. The exterior wall is brick; the interior walls are pyrobar (gypsum brick - the stuff inside drywall panels, but in 4in x 12in blocks, joined by mortar). The pyrobar is covered with an inch of brown scratch coat + finishing plaster. This is great in most of the house, but when the tub was originally installed, they seated the tub next to the pyrobar blocks and plastered down over the tub flange - and then affixed the tile to the finishing plaster with brown mastic.
Over the next decades, the plaster wicked up the water from the tub flange and degraded, the tiles loosened, and folks wondered why mold was an ever-present reality. So someone just covered it all with a fiberglass surround and moved on with life.
Now our family's in the house, and we'd like to do it right. But what does that mean?

Again, back to the photos... We tore off the surround and old tiles and removed the tub. Most of the finishing plaster came off with the tiles in the water-damaged area, leaving the degraded brown coat that tapers from its original 7/8in depth higher on the wall down to less than 1/4in lower toward the old tub flange area.

So... does anyone have any idea what to do now, especially as regards the long tub wall and valve wall? Our original plan: Install the tub, thinset/screw go board to the existing plaster, tile.
But because the plaster went down an -inch deep- over the tub flange, and because it's so degraded from water damage, we at the very least have to address it somehow.
A local plasterer said, "just structo-lite over the existing brown coat and waterproof!" - is this advisable? If we did this, would we need to remove the existing brown coat to a certain level (above the water-damage line?) or can we just go over it with structo-lite? Do we need wire mesh between the gypsum bricks and the structo-lite (none was originally installed, except over areas of wood, eg. the valve area). Would we use kerdi membrane over the structo-lite (does thinset bond to structo-lite), or just aquadefense it?

We could remove all the brown coat of plaster and get back to the gypsum brick and conceivably attach go board to the gypsum - but -how- to attach the go board? Would thinset work? I tried to adhere go board to a piece of the brick with liquid nails, but it came off with no force required - priming the brick with PVA primer increased the bond, but i could still separate them by pulling hard (the primer came off the gypsum). Screwing into the gypsum would help improve the bond, but the gypsum is so soft that screws seem to offer minimal support. We could restore the brown coat with structo-lite and then thinset+screw go board over the structo-lite? Is SL a strong enough product to hold up go board + tile?

Thanks for any insights or moral support you can offer!

[John Bridge]

Hi Carrie, Welcome aboard.

I've worked on a lot of old bathrooms, but I've never run into any gypsum blocks. I think I'd like you to back up and give us a shot or two of the wall surrounding the area that was tiled.

[cshoe]

Ok! Maybe these photos will help clarify how things were previously, and how they are now.
I understand your confusion about what a gypsum block is. I think in one of these photos you can see the "PYROBAR" stamped on an exposed one.
Thanks for making the attempt to help - appreciate any insights.

At the moment, this is the plan for the long tub wall:
Install (glue+screw) a board along the grey primed strip to screw a tub flange to.
Prime the gypsum blocks with Kwikrete concrete adhesive/bonder.
Structo-lite from the board up to to meet the edge of the brown/scratch coat.
Prime with... mapei eco prim? pva primer?
Install Kerdi membrane.
Tile.

For the foot of the tub, we were going to install go board and prime.

For the valve wall, we were going to put metal lathe over the wood where the new valve/faucet will emerge and structolite, and either kerdi membrane or attempt to adhere go board (prime and thinset?) and tile.

Ugh. Crazy old houses. So interesting, and so...... interesting.

[smifwal]

There is a nother post on flicker or something like that that said the blocks don't contain asbestos but the mortar might, FYI. That being said why not just tear it down and start from scratch. I say gut it and start fresh, provided you can do it safely of course.

I will feel more comfortable with that approach rather that going over something that may or may not hold up over time

I tend to take the scorched Earth approach to remodels sometimes at the detriment of my wallet and the timeline afforded for the project but like I said I don't know no other way that I feel comfortable with

[cshoe]

Hey, Shawn, you're right in some ways - it'd be tidier to just rip things out. But given how this room meets with other rooms, and how the ceilings interface, and how hard it is to reframe and re-attach rooms within those parameters (which we've done before and is super annoying) and how much skill and money and time we have, this is the best we can do. Unless all hell breaks loose, of course, which is also always possible.

Thanks for the warning; we're trying to be responsible & cover things as much and as soon as possible.

[John Bridge]

Well, I'm a mud man, and I know how difficult that work is for novices, but that's what I would do. I would go right back over the wall with mud and then waterproof the surface before tiling.

[cshoe]

It sounds like what we were thinking with the structo-lite plaster.

At some point, we'll need to interface between portland-cement-based product, either mud or thinset, and the gypsum block & plaster that's already there. What kind of primer would you suggest to create a barrier between them? Anything you particularly like?