A 1/4" per foot is standard, therefore in twenty feet the furthest point would be 5" below the high point. You can make it flatter (3 or 4 inch drop) but it would require greater perfection in concrete placement, perhaps beyond reason.
That depends on the roof material. Metal can be as low a 1/12. Fibergglass shingles should be at least 3/12.
20ft
225 bricks. 16in * 16in = 256 sq. in. 20ft = 240in. 240in * 240 in = 57,600 sq. in. 57,600 sq. in. / 256 sq. in. = 225. 225 bricks is your answer.
20ft. x 20ft.
20 ft= 6096 mm (6096 * 6096) / (450 * 450) = 183.512178 184 slabs
20ft
The answer will depend on the size of the tiles.
CBM for 20ft container?
5 yds - One yard of concrete will cover approximately 81 square feet 4" deep.
20ft subtracted by 10in = 10
400 square feet