On Jun 30, 11:34*am, Matty F wrote:
On Jun 30, 12:56 am, "neverwas" wrote:

Why should the roof pitch be relevant, assuming that the rain falls roughly vertically?

You have answered your own question really. *If the rain does not fall
vertically the effective area intersected could be the "pitch" area.
See page 40 in the approved document:http://www.planningportal.gov.uk/upl...F_ADH_2002.pdf

I don't understand. Perhaps a mathematician can explain.
The two documents that you quote (and my original question) both imply
that a flat roof collects around 60% of the rain that a pitched roof
with an angle of 45 degrees will collect.
(Pitched roof : A two-sided sloped roof having a gable at both ends)

I have drawn three roofs with identical plan area, and rain at an
angle of 45 degrees. It is my contention that all of the roofs collect
the same amount of water. And of course if the rain is vertical they
will all collect the same amount of water.
Please can someone tell me where I am going wrong in my calculations.
I am saying that the pitch is irrelevant to the amount of rain a roof
will collect, apart from an exceedingly steep roof and rain that is
almost horizontal.

http://i42.tinypic.com/5w9qth.jpg


A physicist explains:

They DO collect the same amount of water. BUT the steep pitched roof
potentially puts all the water into one gutter whereas the shallower
slopes distribute it more evenly between the two gutters.

Robert