Damp underlay and carpet in window bay

[willjohnh]

My mother is in the process of trying to sell her parents old victorian terraced house and we are having trouble working out why the underlay and carpet in the ground floor bay window keeps getting wet.

The carpet sits on an underlay, which sits on a layer of brown floor lining paper, which in-turn sits on old oak floorboards.

Each time we notice the wetness in the carpet, we check each layer and only the carpet and underlay are getting wet. The 'brown paper' is dry, as are the floorboards, joists and space under the flooring (when I say dry, I mean not wet). There are airbricks at the front of the bay (we've had a second put in) so in this weather, whilst the sub-floor contains cold damp air as you would expect, there seems to be a good air movement under the floor when I have the boards at the back of the room up.

We've had a few joists replaced in the bay which had rotted at the ends where they sat on a step of brickwork and the new ones all have DPM. The floor is probably original and we've certainly done nothing to it in the last 15 years. The property has been rented and we think this is an old ongoing issue as the previous tenant was hiding the damp with furniture.

I am fairly confident given how dry all the timbers (joists and floorboards) are, that the moisture is not transferring up (through materials) from below. I've also checked the ceiling above (as it's almost as if something is dripping down onto the carpet) but see no sign in the plasterwork on the ceiling or walls of any leaks.

The issue is intermittent and I'm going round after every heavy rainfall and not always finding the problem. There is no signs of damp in the walls and these have been treated with damp-proof into the cavity and damp-proof rendered and painted.

So far in searching for a solution (and fixing some other issues) we have :
- Resolved an issue with the flashing on the roof of the bay whihc has slipped (and had the roofer come back and double check his work)
- Had a new downpipe and soakaway installed out the front of the bay to take away water coming off the bay roof (again, they've been back and checked the soakaway and the downpipe membrane is still intact ).
- Had some of the brickwork in the bay repointed where we had some heli-bars put in to address some structural movement.

The floor and airbricks sit well above a level where a rising water-table would get to them.

I am now at the point of laying toilet paper on every level of flooring and along the window sills (maybe it's dripping off the sills onto the floor directly) to try and isolate the sources of the water, but have yet to have the problem re-occur to be able to pin it down.

A couple of the floorboards in the bay and the carpet (which the new owner will replace) do not all go as far as the skirting board, so air is getting from the airbrick into the room pretty freely. Could it be that air coming in is meeting and air in the room condensing into the carpet?

Any other suggestions on how to isolate the issue or possible causes are very welcome.

 

[ree]

OP,
Short, concise questions are best - I've not read your long post but from the pics:
The limited exterior pic doesn't show a DPC - is there a DPC?
The bay brickwork appears to be solid - no cavity?
Interior pics are masked by the curtains.
The air brick has nothing to do with the damp carpet.
The ramshackle method of supporting the bay joists needs doing correctly - the present arrangement could even cause a local collapse?
.

 

[JohnD]

The wet carpet looks like the water is running or dripping from above.

The roofs of bays often leak

Or it might be the window frames.

Take the curtains out of the way, and photograph the windowframe inside and out

Stand back and take wider pics

And the roof especially the gutter and downpipes.