Sagging bay window - probable cause found?

Our 1930's 45 degree bay fronted semi has been suffering from a sagging bay since we bought it in May (well, its been suffering from it for longer than that it appears!). This was the cause of some cladding to pop off a while back (and a posting about what to stick it back on with).
Firstly an explanation of the symptoms:
Upstairs small (2mm?) crack showing in plaster at edge of the bay. After removing plaster can see a 1cm gap at the top (window ledge height) going to 0cm at the bottom. This had obviously been replastered in the past.
Downstairs, the inside window ledge dips down slightly (enough to notice by eye) at back of the corners where the large wooden supports go. Window sill is otherwise flat and level.
Cladding (which is fitted against bottom of external window sill) popped off in 1 place. Suspect that its happened before on other corner due to slight differing of mortar colour.
Now, after taking a good look at the area exposed by the popped cladding I believe I have now sourced the cause of the sagging. The underside of the exterior window sill is very rotten. I can very easily push something into it. The front part (which extends past the cladding) is okay, but gets worse as you get further back towards the brickwork. From what I can see, the bit exterior wooden sill (part of the whole window frame) sits directly on the front wall of the cavity. If this sill has rotten, then the sill is slowly being pressed/compacted into the brickwork as the wood rots further.
Does this sound like a reasonable explanation? If so, would replacing the whole window be a suitable solution? I'm not thinking about curing the slight drop in the upper bay as this appears to be stable - and once the source of the dropping has been solved, it shouldn't move any further. In the year we've been there, we've not noticed any further dropping - the popped off cladding was just leaning off since we got it - so its not suddenly popped that off.
I can't see that replacing the sill is going to be easy because its part of the window frame, and would probably need the windows to be removed anyway to get access. Removing windows means propping up the bay... whilst that's being done, seems easy to put in new windows!
We had hoped on replacing the windows (old wooden frames with poor aluminium double glazing) at some point - but if the structure of the house pretty much depends on the windows being replaced, then it may be more urgent and important to get it done sooner rather than later.
Any thoughts?
Thanks
David
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removal of the glass in most cases. Send for the window fitters !!!!!!! I put hardwood bay windows in my last house, stained them, beautiful !!!!!!!! everybody complimented on them. This house has upvc plastic and not a patch on wood. mike
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David Hearn wrote:

In houses of that era the woodwork of the window frame is structural. Its not unknown for them to collapse, happened to a friend of mine. Also replacement doubleglazing obviously needs to have this taken into account e.g with steel reinforcing bars inserted. This is not obvious though to some cowboy companies.
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Yes, a most ambiguous post.
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N. Thornton wrote:

What was ambiguous about it?
Thanks
D
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You don't say what is built into the exterior angles of the lower window to stop the second floor falling in on you when you eventually remove the bottom window.
Windows are not egineered to take the weight of the building, they just provide cheap infill in the manner of wattle and daub or brick panels in timber framed (i.e. Tudor) houses. Fitted into a wall they reqire lintles.
With a wooden bay they may have the sides extended all the way to the top. With replacements this problem could be solved by just cutting the old bits out and putting in replacements that are going to sag after the fitters have been paid. Probably after the guarantee has expired or the firm that supplied them has changed hands/gone broke/found out what the fitters did.
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Does this link help?
http://www.axp.mdx.ac.uk/~john49/epi3.htm
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Michael McNeil wrote:

Hence why we'll be using a company which has experience doing other bays in our locality (preferably recommended by someone we known) and have been trading under the same name for at least as long as their guarantee period.
Put it this way, the majority of houses in our road have PCVu windows and all have bays. A couple have signs of bay drops being repaired (repointing etc) so we're well aware of potential problems.
As for windows not being engineered to take load - the URL in the other reply explains a lot about problems with dropped bays and new windows - however points out the cause as being "the metal inserts in the window frame elements 'X' were not fitted, or fitted incorrectly. ...Unfortunately over time, with a constant load, PVC creeps, slowly lowering the upper bay. This is why the metal inserts are needed." PVCu windows aren't incompatible with bays - they just have to make sure that adequate provisions are made for taking the weight.
Incidentally, our bay is already sagging due to the wood the vertical supports sit on being rotten - hence why we need to replace that part, which is likely to require removal of old windows.
D
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