My roof (2 story, 1925 square foot house) was just replaced and I heard
the project manager and crew chief debating whether I needed one or two
downspouts for the bottom roof. On the front of the house, everything
from the ridge forward drains toward the front. That includes a large
dormer (2nd story), whose downspout drains directly onto the lower roof.
Water from that flows down about 5 feet of 50 year asphalt composition
shingles into the gutter on the lower roof where it meets the runoff
from the lower roof. The debate was whether one or two downspouts are
needed from the lower gutter and the project manager decided on one, at
one side. I wonder if this is enough. I also wonder if there shouldn't
be an extension of downspout from the dormer roof directly to the lower
gutter so all that water doesn't run down one part of the lower roof for
5 feet.

The total area of roof forward of the ridge is something like 700 square
feet and all that going down one downspout (standard size aluminum, I
figure around 7 square inch cross section) seems like a lot to expect of
one downspout in a heavy rain. What is the situation? Is there a website
I can go to to figure out downspout requirements for a given roof area?
Thanks for any help.

Dan