george (dicegeorge) wrote:


neverwas wrote:
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.

My physics is beyond rusty so may I ask if they collect the same
amount of water if and only if the angle from the vertical of the
rainfall is less than 90 degrees minus the angle of the pitch[1]?

A simple "gedanken" experiment: take a flat roof and driving rain which
approaches horizontal. The flat roof - and rain gauges - "see" next to
no rain/m2. But a pitched roof with the same plan area which presents a
face to the wind sees a lot (assuming the roof stays on).

Generalising, in the case of:

o a symmetrical roof with
o plan area A and
o a pitch of T
o rain falling at R from the vertical

the equivalent *horizontal* cross-section [2] presented to the rain E is

A for R=(90-T)
A (1 + tan(T)/tan(90-R)) for R(90-T)

But I could well be wrong!

workings (possibly, temporarily) at
http://www.all129.pwp.blueyonder.co.uk/pages/roof1.xls


[1] and of course with symmetrical roofs
[2] since rainfalls are measured in terms of the horizontal area

If the angle of the rain is greater than the angle of the roof
and the direction is into the roof
then all the rain will land on one side...

However in an unusual thunderstorm here at my house
I would expect the gutters to overflow,
and the ground around the house to be deluged with water,
and the walls of the house to be wet,
but as this happens maybe one day out of a hundred
the other 99 days will dry the walls out
and most of the surface water will flow away
leaving the normal drainage to work the other 99 days...

Wood rot wont set in after one day's soaking at rare intervals


Indeed. Seen that here when the downpipes get leaves in.

[g]