flat roof gutter

[croxley]

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?

[The Natural Philosopher]

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

[Matty F]

$FILE/RainwaterGuide5.pdfIn 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.

[Robin]

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.

[Matty F]

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.

[Robin]

I am not qualified to argue but FWIW Approved Document H - Drainage and Waste Disposal

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.

[Bob Mannix]

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.

[croxley]

$FILE/RainwaterGuide5.pdf>> In that case it says that Roundline should do the job for a 79 sq