This bit of steel is a box section for sub sills to go on for sliding windows
It has to be level and as you can see there's a bit of fall to the existing concrete which we will screed and tile over
The sub sills and box section can't be used to screw anything structural to because it'll just crumble
What can you put underneath so it's strong enough to screw the sub sills for the heavy sliding doors that would also be water proof
Then screed will be cemented in behind and in front of it

