how to fill big gap underneath door threshold between hallway and step

[al97]

Hi, I am planning on putting tiles at the end of my hallway by the door that goes out into a lean-to sort building. We ripped up the doormat style carpet that was there and there is a really cold draught and on further inspection have found that underneath the door threshold there is a gap in the concrete between the hallway and the concrete step that is in the lean-to. I put a tape measure in and the gap is 2 inches wide and then when putting a tape measure down the gap, it is 25 inches deep.

Can I fill the gap with anything? Any help/advice will be greatly appreciated!

 

[Mr Chibs]

Presume you meant 2.5" deep not 25"?

You could fill with a mix of sharp sand and cement 4:1 and not too much water.
That said you might introduce damp if there is no DPC/DPM in there.

Expanding foam would do the job, and mitigate damp bridging, you can chop back the excess once dry with knife or saw.

Think you might need some self levelling compound on the floor before you tile, it looks like the surface of the moon.