Quote Originally Posted by Rogue View Post
They say that great minds think alike, but then again they also say that fools seldom differ. This is the first thing that occured to me when I read the post. Can the joist be on top of the floorboards? Basically creating a raised platform at one end of the room? Hopefully Jonathan and I have great minds but I must admit I'm not doing our side any justice at the moment.
Yes in theory, but you'll need to support the joist at the wall at each end using joist hangers and not just terminate it at the wall. Although just having an additional joist on the floorboard will improve the 'bending' aspect by spreading the load, it wont reduce (indeed it will increase) the shear force at the wall and the underlying joist could still fail with the load. The correct solution is to lift the floorboards, add new joists, cutting them back into the wall and then board over with 18mm OSB which will be a lot stronger than the old floor boards. The new joists need to be securely attached to the old ones at no more than 600mm intervals, either drilled and screwed through into the old ones (BIG screws lol) or you can use retaining straps screwed to lower joist and taken over top joist in a notch and back down to lower joist, tensioned, and screwed through into both joists. Tensioning the straps needs some sort of tool...

Incidentally the minimum joist size now acceptable for loft conversions and new builds is 9 x 2 but 10 x 2 is usual as this matches the 10"/250mm insulation depth requirement and allows direct boarding over.