On Aug 22, 10:40 am, "Robin"
pleaseaskinthegr...@becauseIhavegotfedupwithspama ndstuff.co.ru
wrote:
I am puzzled why the site says "Multiply the roof plan area by the
roof pitch factor" in order to get the actual roof area. Why do you
need to know the actual roof area for rainfall calculations? Surely
the plan area should be used.

Because rain does not always fall vertically? Then the actual roof area
matches the ground area from which the roof intercepts the rain (I
think). For example, imagine a very steeply pitched roof and driving
rain.

Imagine a gable roof with a slope of 45 degrees, and the rain falling
at an angle of 45 degrees on one side. Only one side of the roof will
collect water, i.e. 50% of the total roof area.
If the rain is falling vertically on such a roof then more water will
be collected - about 70% of the roof area, i.e. 100% of the plan
area..

The only time more water will be collected than the plan area is on a
flat roof that slopes towards the rain. For calculating spouting
sizes, that circumstance can be ignored. It's more important to know
how many bends there are in the downpipe.

I believe the site has made a mistake. If you are buying roofing or
roofing paint, by all means use the roof pitch factor to work out the
size of roof, but not to calculate the amount of water falling on it.