Cavity between patio door and internal slab

[Ritzuk]

After removing old carpet I found a rotten sheet of chipboard up against this door which had no support between the inside concrete slab foundation and the outer brick skin that the door frame sits on. Looking down the cavity it looks like the builders chucked a random bit of wood down there and screwed it to the chipboard above which is why I guess moisture rose up and wrecked the chipboard.

What's the best way to put some support in here? Its a cavity so I assume im not supposed to just pour cement down there? I found the same at the front door but luckily that was being replaced so the fitter put a cavity tray type thing in there. No idea what to do here?

 

[Mr Chibs]

If it was me, I'd remove wood, as it will rot over time.

Fill cavity with some pieces of PIR insulation (celotex) and use expanding foam to fill any gaps. (some fill with expanding foam, if doing this, do it in layers, not all in one go).

When dry cut flush to top of joists/slab and cover over with floorboards/flooring.

 

[noseall]

View attachment 299449
After removing old carpet I found a rotten sheet of chipboard up against this door which had no support between the inside concrete slab foundation and the outer brick skin that the door frame sits on. Looking down the cavity it looks like the builders chucked a random bit of wood down there and screwed it to the chipboard above which is why I guess moisture rose up and wrecked the chipboard.

What's the best way to put some support in here? Its a cavity so I assume im not supposed to just pour cement down there? I found the same at the front door but luckily that was being replaced so the fitter put a cavity tray type thing in there. No idea what to do here?

There are currently 6 billion posts on the very subject, on this forum. Search is your friend.
Type in the word threshold and my username.