Just checking strength of fixings for strengthening ceiling for storage

[Happy Campers]

Looking to use space above bathroom for storage, its 2.8 wide (beam length) and about 4m long

Currently has 3x2 "beams" at 18" centres holding up the bathroom ceiling - so its not really usable for storing stuff

Plan is to add 6x2 C16 joists with 47x147 hangers bolted to the walls - slight overkill maybe, but plan to store books and stuff, so don't want to under engineer. Plus that keeps the 9" of insulation under the boards

Not sure what bolts to use, was planning on M10 Thunderbolt, but do I need 2 per hanger or 4? Or something else?

 

[blup]

Sit them above the existing joists (allowing for deflection) so the wall plate and any load bearing wall will take the weight. Or bolt new wall plate to gable end and fit joist hangars with galvanised nails.

 

[Happy Campers]

Thanks blup, that's the plan

Found out that M10 Thunderbolts have a shear strength of 4300kg, so I reckon 1 pair per hanger should be plenty - sounds like M8 would probably work fine but will stick to M10