Running floor over cavity under doorway

[Towtruck]

I am doing an extension which has a block and beam sub-floor and I'm about to lay insulation/screed over the top.

There is a still a cavity under the external doorway however, and I'm not sure how to bridge over it. Here's the current situation for reference: [url=https://ibb.co/eHK61R][/URL]

In order to meet existing floor level in the lounge (being extended to lounge/diner), I'll be laying down roughly 70mm celotox with 75mm screed, final finish will be tiled. Presumably if just run this all over the cavity space it will crack as nothing is supporting it.

I'll be running perimeter/upstand insulation (25mm celotex) around all the external edges and damp-proofing below and around the insulation at the openings. Although where the screed meets the doorframe I will perhas just use some thin foam, so the tile edges are fully supported.

Should I fill the cavity with a loose concrete mix before laying the insulation? Could I maybe just add some re-enforcing mesh in the screed to cantilever it over the cavity? Not sure how to best approach this so any advice would be much appreciated!

Cheers
Dan

 

[Blagard]

One possibility is to use a DPM to line the cavity before packing it with a weak concrete mix. Close off the ends of the DPM to form what is essentially a "bag" so you keep the concrete where you want it. Trim the DPM back as necessary afterwards

 

[Towtruck]

Cheers mate, this is what i’ll go for!