How Strong is Wallpaper? and Other Questions

Can anyone who knows French/old houses, please help me?

I am working under the assumption that wallpaper cannot hold up a three-storey house: not even if there are five or more layers of it. I hope I am right, because:

I’ve been working in a first-storey bedroom we call the ‘big cupboard room’. At some point the room height has been reduced by about 100cm to borrow enough height to squeeze a room above it, leading to a low void, like an eaves cupboard. The room was originally the same height as the salon (390cm). The original moulded cornice is still visible within the void next to the later room on the second floor, and shows that the division between the bedroom and the ‘big cupboard’ is an original 17th Century feature (perhaps a salle de toilette?), and not a later change as we had assumed

(I only know this from crouching in this void, armed with a torch to get a better look at the cornices. A weird little space with a rough seat/bench built onto the wall, it contains huge cobwebs, a cast iron saucepan lid and a couple of very old mummified shallots. Strange? I thought so)

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Into the void. Something’s hanging from the ceiling mid-pic. Only just noticed

How is your spacial awareness doing so far? I know, a diagram might help

Anyway, the original tall double doors are still in place in the ‘big cupboard room’ in keeping with a higher ceiling. But was the entire first floor of that ‘house’ once two high-ceilinged rooms with mouldings and tiled floors? This makes me wonder if it was not originally a house, but a commercial premises of some sort

Now the paper stripping comes in: the plaster in this butchered room is bulging along the adjoining wall to next door. A crack has broken through all the layers of wallpaper, and there are at least five, probably six layers (have I just answered my original question?)

I’ve removed some paper the original plaster shows an outline about 75cm square, by the looks of it, filled in before any of the layers of paper were applied. The rough filling is dropping out and someone has later (badly) skimmed over it. I scraped away to get a better view of the indent. I might have said that it was a window, but the house is part of a terrace of houses and this is an adjoining wall. Plus, a window would not be so close to the floor of this room and right up against the fireplace

It’s a terrible picture and I’ll try to get something better, but the top of the indent is roughly 1/4 of the way down and the bottom 3/4 of the way down the photo. The right edge lines up pretty much with the mantelpiece. The crack is not visible here

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So what could this unwanted ‘feature’ be? It’s definitely original to the house