Okay… so I chatted further with my builder this morning. He is actually recommending we tackle this issue properly and don’t simply opt for a quick skim over or the dropped ceiling route (which I had him start without realising the excessive loss of height).
The issues we have are the current loft joists are only 70mm thick, they are bowed and weak bending easily under foot … as a result our loft is not really safe for walking on or storage and one of the ceilings (second bedroom) is sagging badly.
Another factor for consideration is we would like a dorma loft extension adding a couple of years down the line, so any work that is or isn’t carried out now will have certain implications.
His recommended route is now the following:
Cross batten the existing joists with timber battens approx 150–170mm thick, these battens will be fixed to the top of our current joists across the whole loft space. A 250x50 timber cross beam frame (as far as I understood) will be screwed into the party walls for the cross battens to sit on, as opposed to the battens actually going into the party wall brick work (which will require party wall agreements).
The battens will then be boarded to create a new loft floor… which he is suggesting will be of good enough quality to serve as our dorma loft conversion floor in the future, would this be correct?? So in effect we’re laying the dorma loft conversion flooring now.
Can I be sure that any future loft conversion company will be satisfied with this floor he creates and will it be fit for purpose?
The original bowed joists will then be screwed up and into the cross battens, relieving as much of the bowing as possible (but not all).
Our second bedroom ceiling is currently plyboard with plaster (not lath and plaster) … he will remove the old plaster but use the plyboard and replaster onto it and the now secure joists, skim and finish.