How many tiles do I need...

To work out the number of tiles you need: Multiply the width of your room or roof by the depth, this will give you sq.in, then divide by the the coverage of the tiles.
Example:   For a roof ( 1 side) 17" x 10" this would give you 170 sq.in, divide by the tile coverage (Viscountess Roof Slates) 59 sq.in and round up to the next whole number, in this case 3.