Downpipe capacity

[seasickstevie]

I'm curious as to what is the maximum capacity of a standard size downpipe.

My front downpipe takes the front of mine and next door (two semis).

The total area of the two front roofs - and I've calculated at the horizontal plane, not the pitched surface - is 80 sq.m of rainfall.

The original cast iron was 3" dia. but every builders yard I've been to, they say just use 2-1/2".

I feel it's a hell of a lot of surface water in a heavy downpour to go down one thin pipe. Surely there must be a recognised maximum surface area for standard sizes.

After all, we're told to be expecting more extreme weather with climate change. If 3" was considered necessary 150 years ago (when the houses were built) it strikes me as folly to reduce it now.

 

[noseall]

It is not the so much the bore of the down-pipe that is the problem, more to do with the lack of downpipes.

You would be better off introducing another drop (or two) and re-fettle your drains to suit. The drains will easily deal with the additional drops.

 

[Nige F]

Thanks Noseall Polypipe have an exellent calculation site - i know youre busy with efflorescence so i googled for you

 

[alittlerespect]

Hi

As a rule of thumb a 100mm gutter with 68mm dia. downpipe placed at the centre of the gutter run will drain an effective roof area of 110m2, however if the downpipe is located at an end of the gutter this would reduce the effective roof area to 55m2.

So depending on where your running outlet is located you will be able to determine whether or not you have a potential problem.

Regards

 

[seasickstevie]

Thanks for replies.

Nige, would you have a link to the calculation part of the Polypipe site? I found some references on Terrain, but not a calculation as such.

Alittlerespect. A plan of the two houses is a 'U' shape with a 3.5m section in the middle spanning two houses with the outlet at one end (mine).
The two sides of the 'U' are outriggers which both run onto the short middle section. So it's kind of in the middle, but at the end of one section, with three lengths in all, if you follow what I mean.

So is it at the end, or in the middle? Either way, I think 68mm is a bit optimistic. And of course if a second drop where to be introduced, it'd need to go next door - which patently isn't going to happen!

 

[Nige F]

http://download.polypipe.com/bp/product_guides/rainwater_design_system_selection.pdf Blimey I managed to cut + paste that