A local website has this table for calculating the size of downpipe needed for various roof sizes. The roof size is stated to be the "plan area" . Why should the roof pitch be relevant, assuming that the rain falls roughly vertically?
Maximum plan area of roof served by downpipe (m2)
degrees Roof pitch 0 - 25 26 - 35 36 - 45 46 - 55
65mm Round 60 50 40 35 80mm Round 100 80 70 60 100mm Round 155 130 110 90Why should the roof pitch be relevant, assuming that the rain
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:
PS
or pages 45-46 of
document:
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.
I don't think you are wrong as such. It is more a matter of assumptions.
IIRC[1] the assumption made for sizing downpipes is that you have different downpipes draining different sides of a pitched roof [2]. So in your 2nd diagramme (the one with the low pitch) a lot more rain hits the RHS than hits the LHS. If they all drain into the same downpipe it averages out OK. But if not.....
Another way of looking at it is that rain gauges have horizontal tops. So all the figures for rainfall are based on that. But it is a fact that rain don't always fall horizontally. (And the only time I visited NZ, in Feb 2004, there was certainly some *very* non-vertical rainfall). So looking at your 1st diagramme, the RHS of the roof collects much more rain than would hit the horizontal area under just the RHS of the roof. Less hits the LHS, but again that is assumed to drain into a different pipe. I think.
If you have downpipes which drain both sides of the roof then it may be that you don't need to make the adjustment - but I don't know if the regs. would allow for that.
E&OE of course :)
[1] a big "if" as I think I met this for A-level applied maths in 1969! [2] very common in the UK at least
vertically?
document:
I think you are correct. If a given volume of water falls..liters per hour per square meter..the wind and the roof pitch is irrelevant, since its only the projected area of the roof into the horizontal plane that matters.
And rainfall is not specified at right angles to the direction of fall. Its per unit area of (horizonatal) land.
HOWEVER that flow on a typical gable type roof will not be distributed evenly: In the limit with '45 degree' rain on a 45 degree roof, it will all end up on the upwind side...;-)
An in my case, with one valley, that downpipe is the most massively used as it collects from a far larger area.
However even there, a 4" pipe is adequate..just!
Yes I'd not allowed for the downpipes on each side having different amounts of water. However the roof that I'm wanting downpipes for is a hip roof, and the spouting runs all the way around the roof. So in theory I don't have to worry about the pitch. There are wide eaves, so if there is a torrential downpour the water can overflow the spouting without a problem unless someone is standing underneath!
I am not sure about that theory. Even if the gutters are laid level all the way around (which reduces their capacity - and is not allowed in NZ from 5.3.1 of the Clause E document?) the water from the "rainy" side will meet one downpipe before the others. But this is getting beyond my level of incompetence into "act like a politician" territory ;)
...especially if that someone is the BCO
I think that's a valid point actually.
I've ended up with one downpipe per section, as I hate running guttering under dormer windows.
Guttering should always slope towards the downpipe as well. If you have two downpipes, bend it up between them.
The builder insisted on putting the gutter horizontal, despite my objections at the time. It's not my building, and I was merely helping him. So now it breeds mosquitoes. The gutter he installed was galvanised iron (despite my objections) , and now it's rusty and leaking. So we are about to replace it with plastic sloping guttering.
At present we have three downpipes. Only one is connected to the water tank. How stupid is that.
ughly vertically?
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
ughly vertically?
I haven't done the maths but maybe "plan area" is incorrect. If they mean actual roof area then a 45 degree pitched roof will have a greater area than a flat roof and the rain collected per unit area will be lower.
MBQ
vertically?
document:
Having actually read the relevant bit, it seems clear that what they are talking about is the peak flow off a *section* of roof, not the whole roof.
Which will indeed vary as the pitch, as the facing section of roof, will 'rob' rain from the opposite slope.
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)
But I could well be wrong!
workings (possibly, temporarily) at
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
[g]
Indeed. Seen that here when the downpipes get leaves in.
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