I had a similar problem that was resolved last winter by Ardfern Boatyard in Scotland. The easiest way is to copy their subsequent findings.
We have been looking further into how to go about repairs to Lady of Hartys coachroof. It isn’t a straightforward fix I’m afraid. We took a core sample and there is no delamination of the deck. We rigged up a lift to the coachroof to make sure that we could reinstate the shape. That was ok. Down below we were investigating the possibility of fitting in an additional fore and aft beam to stiffen and support the deck. We took off the trim on the bulkhead just forward of the mast step and found the bulkhead timber to be soft. Photograph attached. This position would explain why the coachroof area immediately forward of the mast step was depressed. The way the interior has been put together complicates the repair as to access requires removal of the sliding door which requires removal of a panel in the heads etc etc. It is impossible to estimate the cost at the moment as we are finding out what needs opened up as we go along. Only once we have done this and seen what is required to reinforce and repair could we supply a price. I would suspect however that the joinery work will be a large part of the cost. If you could get back to me and let me know how you wish us to proceed and if you have any questions, please get in touch.
I will send you a couple of pictures to your email and can send some of the completed work after they scarfed in a new piece of beam. Let me know if you require them or if I can be of further help.
Keith Lewin, Lady of Harty (1978, Hull 570).