Loft Conversion + Plywood Diaphragm

[Vinny7]

I am looking at a loft conversion. The new stairs will mean the removal of ceiling joints and a timber purlin spanning about 2.8m. The existing ceiling ties appear to counter act the horizontal trust at eaves level. I wany to fix deeper rafters to the existing and also form a diaphragm fixing a plywood deck to the underside of the rafters. This will be fixed to a ties rafter at each end, would this scheme be adequate to resist the trust at eaves?

 

[jike]

I would try to run the stairs parallel to the rafters and ties so that perhaps only a couple of ceiling joists need to be headered out and reinforced.

Your calculations of forces and existing member and connection capacities will prove whether it is feasible and what needs to be reinforced. What do your calculations indicate?

 

[Vinny7]

The analysis indicates a 3.6m Dp diaphragm can transfer the axial force in the rafters to either side of the opening. This then has to be resisted by the trussed rafters. The horizontal thrust force at the eaves will need to be taken by a tooth plate connectors into the ceiling ties. I think the new deeper rafters may be difficult to install at the roof including wallplate and ridge boards are already in place. I may use packing blocks on the wall plate. working around the eaves may be tight.