You building one like mine. I put the base of 3/4 ply down and then a 12 inch void with the loft ontop. There are 8 trays each a 8x4 plywood size.
The floor is industrial galvanized grill flooring about 150 quid each. needed 3 for a 24 loft.
The birds don't like it and if I was doing it again I would have turned them upside down as there is more to stand on.
The floor is fab little dust inside the loft and I cleaqn out the bottoms every 8 weeks or so.
Fantastic.