Hanging new floor joists below existing joists

[Tinytotems]

Hi,

I'm planning to convert my loft but there's not masses of height available so I'd like to hang the new joists from the top of the external supporting walls rather than site them on top. The problem is, what happens to the existing joists that are tied into the rafters? Can I attach the rafters to the new stud wall supports then remove the existing joists?

Also, I've got large timber purlins that support the rafters which are bedded into the party wall on one side and the gable wall on the other (see photos). I want to remove the gable wall as the house will be extended. What is the best way to manage this? Will I need to replace the purlin with a steel beam (8m span to the new gable wall)? I'd like to avoid messing with the party wall, if possible. Another complication is that there are interim supports made of blockwork that sit on the internal walls, one of which needs to be removed.

Any advice is much appreciated.

 

[Tinytotems]

I've actually got the answer about the existing joists from a structural engineer - he said I need to beef up the ridge beam then the existing joists can be removed. He said it was the best and cheapest solution.