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 90

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
which also has more such figures (for UK rainfalls of course).



--
R

PS

displacement-activity
or pages 45-46 of
http://www.dbh.govt.nz/UserFiles/Fil...face-water.pdf
for your more local figures?
/displacement-activity
--
R

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

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
--
R

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


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!

On Jun 30, 10:50 pm, "neverwas" wrote:
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.....

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!

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.

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

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!

....especially if that someone is the BCO

--
R

neverwas wrote:
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.

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


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.

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!

...especially if that someone is the BCO

On Jul 1, 12:38 am, The Natural Philosopher
wrote:
neverwas wrote:
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.

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

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.

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.

At present we have three downpipes. Only one is connected to the water
tank. How stupid is that.

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

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.


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

Man at B&Q wrote:
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.


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
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.

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

--
R

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

[g]

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]