How it started
View attachment 1189351
How it’s going
Why would the shelf inside a wall cupboard on an internal wall, high up on that wall, rot away? Rot requires the wood (chipboard in this case) to be wet.
It can't be dampness through the wall as it is not external.
It can't be rising damp as that will only rise maximum of three feet up the wall.
It can't be leaking pipes because the plumbing for the bathroom is above the other internal wall.
It can't be a roof leak as there is another floor above the ground floor that would be affected first.
It can't be dampness because of condensation as that would affect the external walls first. Also black mould would be evident in every picture she has shared.
I could go on and on. My post grad dissertation was on penetrating dampness, condensation, and it's effects on residential dwellings. There is a couple of chapters on the various wood decays, dry(which bizarrely needs the wood to be wet!) and wet rot.
Building surveyor (retired) here. I think Jack might be lying. It is probably due to overloading the shelves. I would love to see a picture of the supposed "rotten" shelf. So many people use the word "rotten" to describe something that is anything but.