Common rafter sistering

[Jimbow]

Hello Folks
I need to sister new common rafters along side existing ones and maybe do some work with the pole plate configuration i haven't figured this out yet. Some how i have put rafter ties in as i have cut some of the existing ceiling joists that were acting as ties i think. I am thinking that i could put floor joists in hangers off the back of my main floor steels to tie the rafters.

 

The new rafters are 47 x 150 and the existing are 50 x 100m. 

Any suggestions, comments welcome

[Temp]

On 03/08/2020 at 19:52, Jimbow said:

I am thinking that i could put floor joists in hangers off the back of my main floor steels to tie the rafters.

 

Not quite sure I follow that but you can't use hangers in a situation where the joist is in tension. The joist would pull out of the heel of the hanger. Even if it didn't it could just pull the steel sideways.

 

I can't figure out from the photos what's supporting the loft floor if you removed the joists/rafter ties? 

[Temp]

I'm not an SE but I am worried you have broken the triangulation provided by the rafter ties. In such cases i think you normally need collar ties (not enough headroom?) OR add a structural ridge beam (if suitable supporting walls exist for its ends to bear on) OR add Purlins (eg above your new steels and verticals between them) OR some combination of all of these maybe needed.

 

The sort of work you are doing needs Building Control Approval and normally they would ask you to justify the design which is where an SE or similar comes in.