First time poster, posting on behalf of the homeowner. Sorry for the big post, and if I'm posting in the wrong place! Sadly this house will have to be demolished or stripped back to brick due to the amount of problems it has, so we just need to find out if it's safe for the foreseeable future, or if this room will have to be boarded up. Fixing it permanently isn't a practical solution in this case so that's not on the cards. This part of the house dates back to at least the C17th, but save for the exterior walls and a wood post & lintel doorway spanning 3m directly under the carpet strip, which is in great condition, we have no idea how much of the original structure remains; it was only converted for habitation in 1972, previously being a pub, and originally most likely a smithy.
We were surprised to find plywood boards under the carpet. Even more surprising is that it sounds as if there is a 1m wide stretch along the length of the room from the doorway with something metal sounding underneath the plywood, with considerable sag where it meets the wall, and doorframe (pictured). There's clearly less density in this stretch; the rest of the room sounds like plywood over floorboard or something similarly solid/sturdy. These sag points are reflected in the downstairs ceiling, but to a far lesser extent that isn't very visible in photos. The sag has obviously happened since the carpet got put in as it has pulled away from its fastenings on the right. Carpet has not been updated recently.
My theory is that, since this was originally one building, and the post/lintel setup resembles an old barn door setup (hinge marks on the posts), that there was originally a stairway in this section that wasn't sufficiently re-floored when taken out.
Rising damp has been a problem in all walls of this room since at least 1984, so the main concern is that it has now travelled down and started eating away at the supporting beams (as was suggested would happen in 1984).
Sag is 1.5in over 60cm, I propped a level up with a tape measure to help visualise this.
If no one here can ascertain anything from what I've provided (which isn't much, I know) do you think I'm good to take up the plywood and investigate further, or do we need to get a pro in? Thanks.