We've got 2 of these Ever hard storm water pits with plastic grates ( but I am going to soon change to aluminum steel ones)
these grates are under our alfresco pergola which we are going to get tiled in a couple of months
The tiler told me the pit has to be 15mm higher so it sits flush against the tiles
The floor at the moment is all concrete
The grates are removable but the pits a solid and won't move
Does anyone have any ideas on how to raise it up a bit ?
Photos below with the black and yellow spaces equal 15mm so has to go flush with that

