Offset floor from top of wall

[trent900]

Hi, I'm new here. My name's Jon, I'm from Nottingham, and I work in engineering but have limited DIY experience...which is a dangerous combo.

I'm buying a house which has a weird attic conversion caused by a high first floor ceiling. We plan to redo the conversion by lowering the ceiling by 500mm. This means the floor joists will no longer be parallel to the top of the walls, and unable to take the roof spreading load in direct tension. The house is a late victorian brick semi with a simple ridged roof. Both gables are masonry but I'm not sure if the party wall is single or double skin.

Our builder is very confident about being able to redirect the spreading load through the floor using bracing brackets. I'm not so convinced, because I can't find off-the-shelf components that would transfer the load into a floor that's offset by as much as 500mm. Do they exist?

Our other two backup options are effectively a square of steels to connect the two wallplates at the current floor height allowing the floor in the middle to drop, and tie rods between the rafters up near the gable. I wonder if this last solution would require sister beams for the rafters to stiffen them.

Any comments? For example: 'you must be insane'?

Thanks

 

[tony1851]

When the ceiling ties are removed and a lower floor fitted, spread of the rafters can be prevented by diagonal timbers connected to each rafter and floor joist.

 

[RonnyRaygun]

When the ceiling ties are removed and a lower floor fitted, spread of the rafters can be prevented by diagonal timbers connected to each rafter and floor joist.

And a vertical to triangulate the restraint. It can be incorporated into the dormer walls.

 

[trent900]

Ok that makes sense - the diagonal connects the floor up so that if the rafters were to spread they would also need to collapse down, and the vertical prevents that happening. So I lose a bit of internal space under each of the eaves, but hey we can call that 'storage' and timber restraints are cheaper than custom steels.

Thanks very much!