The grade of drain pipe liable to receive solid substances shall be as follows:
Six inches (6") in diameter, one-quarter inch (1/4") per lineal foot.
Nine inches (9") and over in diameter, one-eighth inch (1/8") per lineal foot.
Drain tiling for surface drainage, to receive water only, one-eighth inch (1/8") per lineal foot.
No deviation from these grades will be permitted in the streets, but within the lot lines, permission will be granted to increase the grade upon written application of the property owner, approved by the Commissioner of Public Works.