Ply subfloor unsupported at doorway

[wassupjg]

18mm ply subfloor next to patio door threshold unsupported as joists fall short of 400mm width area (left of green line; cavity with rockwool), any way to support from underneath to stop plywood flexing when stepped on?

 

[Alastairreid]

Take the insulation out, line the cavity with dpm and backfill with concrete.

 

[noseall]

18mm ply subfloor next to patio door threshold unsupported as joists fall short of 400mm width area (left of green line; cavity with rockwool), any way to support from underneath to stop plywood flexing when stepped on?

A piece of 70 x 50 dropped into the cav, with a stapled piece of DPC on the cold side, foamed in place. Connected to both pieces of ply after, should take out the flex.
Otherwise its remove the Celotex insulation that's there and secure a piece of timber against the masonry by screwing long screws through the wall. Or do what Alistair suggests.

It's an odd set up. Why the hangers and not the joists being allowed to travel all the way up to the threshold? They could have been protected from cold bridging with a makeshift closer.