B.l. Rooftop Disconnection
When rooftop downspouts are directed to a pervious area that allows for infiltration, filtration, and increased time of concentration, the rooftop may qualify as completely or partially DIA and a portion of the impervious rooftop area may be excluded from the calculation of total impervious area.
A rooftop is considered to be completely or partially disconnected if it meets the requirements listed below:
• The contributing area of rooftop to each disconnected discharge is 500 square feet or less, and
• The soil, in proximity of the roof water discharge area, is not designated as hydrologic soil group “D” or equivalent, and
• The overland flow path from roof water discharge area has a positive slope of 5% or less.
For designs that meet these requirements, the portion of the roof that may be considered disconnected depends on the length of the overland path as designated in Table B.l.
Table B.1: Partial Rooftop Disconnection
| |
Length of Pervious Flow Path * | Roof Area Treated as Disconnected |
Table B.1: Partial Rooftop Disconnection
| |
Length of Pervious Flow Path * | Roof Area Treated as Disconnected |
(ft) | (% of contributing area) |
0-14 | 0 |
15-29 | 20 |
30-44 | 40 |
45-59 | 60 |
60-74 | 80 |
75 or more | 100 |
* Flow path cannot include impervious surfaces and must be at least 15 feet from any impervious surfaces. | |
B.2. Pavement Disconnection