A mews floor can be simple or complex. Mine happens to be complex.
I contoured the floor to drain from the center to the perimeter, and the perimeter to drain to one far corner. I did this with crushed granite. Over this I placed a layer of geotextile fabric to hold everything in place and still allow water to drain through. Next went a layer of 1/2", galvanized, hardware cloth. Over this went a 4"-6" layer of pea gravel. As the gravel will dull the hawk's talons I bought a prepackaged square of cheap outdoor carpet from Home Depot and used this around the perch. So far, so good
The geotextile fabric will help to keep the soil and the pea gravel separated over the years. The carpet washes satisfactorily and the effluent drains well.