I am replacing the guttering on a flat roof in London using deep fill
gutters . It is about 80sq mtr. It is going to be diffucult to give it more
than one 75mm downpipe.

Is that likely to be adequate?

croxley wrote:
I am replacing the guttering on a flat roof in London using deep fill
gutters . It is about 80sq mtr. It is going to be diffucult to give it more
than one 75mm downpipe.

Is that likely to be adequate?


I cant remember but the Osma site - google it - has specs.

On Aug 22, 8:36 am, The Natural Philosopher wrote:
croxley wrote:
I am replacing the guttering on a flat roof in London using deep fill
gutters . It is about 80sq mtr. It is going to be diffucult to give it more
than one 75mm downpipe.

Is that likely to be adequate?

I cant remember but the Osma site - google it - has specs.

http://content.wavin.com/WAXGB.NSF/pages/RWRoofAreaCalculationsEN/$FILE/RainwaterGuide5.pdf

In that case it says that Roundline should do the job for a 79 sq
metre roof. I assume that Roundline is 75mm, I can't seem to find that
easily.

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.

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.

--
Robin

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.

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.

I am not qualified to argue but FWIW Approved Document H - Drainage and
Waste Disposal
(http://www.planningportal.gov.uk/eng...000000382.html)
also specifies, on page 40 para 1.3, a calculation which takes account
of the pitch.

For the OP there is also on page 40 table 2 showing 65m2 as the max
effective area for a 75mm outlet (subject to some other points). But
I've no idea of the practicalities/risks etc: I was just skimming the
document for something else. And I may have got it all wrong.

--
Robin

"Robin" pleaseaskinthegroup@becauseIhavegotfedupwithspama ndstuff.co.ru
wrote in message . ..
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.

I would have said that such calculations would have to assume "worst case".
Worst case here is maximum water, ie the rain hitting perpendicular to the
roof, whatever its pitch. Therefore the roof area is required rather than
the plan area.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

Thanks, I think I'd better accomodate two downpipes somehow

John
"Matty F" wrote in message
ups.com...
On Aug 22, 8:36 am, The Natural Philosopher wrote:
croxley wrote:
I am replacing the guttering on a flat roof in London using deep fill
gutters . It is about 80sq mtr. It is going to be diffucult to give it
more
than one 75mm downpipe.

Is that likely to be adequate?

I cant remember but the Osma site - google it - has specs.

http://content.wavin.com/WAXGB.NSF/pages/RWRoofAreaCalculationsEN/$FILE/RainwaterGuide5.pdf

In that case it says that Roundline should do the job for a 79 sq
metre roof. I assume that Roundline is 75mm, I can't seem to find that
easily.

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.