# Forum Home Renovation Plumbing  Round or Rectangular down pipes? Any difference in flow?

## PaulKaren

Hi all, 
I was just told that round downpipes create better flow than rectangle ones. I know it is a generic question but is this true? Half the reason i am asking is just to determine if the person telling me this was just telling me what i wanted to hear to make the proposed job less hassle for him!  
Also can anyone tell me what is the equivalent of 100 x 75 rectangle downpipe in round PVC? Is 90mm stormwater ok to use or would 100mm dwv be better. If i decide on the PVC it will have to be painted black to match the rest of the gutters etc. 
Thanks for any help.... 
Paul

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## Bros

I know leaves tend not to get suck in round downpipes like they do in rectangular

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## OBBob

I'm sure round would be better from a pure engineering perspective but I doubt there's much in it.  90mm diameter is standard ... and don't think you'd want to move away from the norm.

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## phild01

I'd bet that is right as when water goes down a drain it turns and a round pipe would offer least resistance.  Interesting topic!

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## Whitey180

Not to mention the ease of connection with no pop rivets and no rust issues.

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## r3nov8or

Dunno really, but I like the look of round better. Used 70mm for downpipes (or is it 75?), converted to 90mm once under the house.

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## OBBob

> Interesting topic!

  If you were considering micro hydro on your downpipes it would get very interesting.  
I'd say PVC corners are the biggest flow advantage.

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## elver

Hi, 
Round pipes let more water flow through faster for the same cross sectional area. Round has less surface area against the water so less "friction".   It makes a big difference over long runs.

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## cyclic

> Hi all, 
> I was just told that round downpipes create better flow than rectangle ones. I know it is a generic question but is this true? Half the reason i am asking is just to determine if the person telling me this was just telling me what i wanted to hear to make the proposed job less hassle for him!  
> Also can anyone tell me what is the equivalent of 100 x 75 rectangle downpipe in round PVC? Is 90mm stormwater ok to use or would 100mm dwv be better. If i decide on the PVC it will have to be painted black to match the rest of the gutters etc. 
> Thanks for any help.... 
> Paul

  
100 DWV is best because it is UV resistant, 90mm stormwater is not, so 90 needs to be painted.
100 DWV is also heavier wall and the area of 100 is slightly larger than 100x75 where as 90mm is smaller in area.
100 bends are more of a sweeping bend whereas 90 bends are very sharp
Area of
90 round=6300 
100x75= 7500
100 round=7850
100 DWV is of course considerably more costly but IMO much better in the long run.
Would you not paint any downpipe to match the walls.
Most installers prefer 90mm because it is lighter and easier to handle than 100mm and as I said, less expensive.
Sizing would also depend on how much roof area and how many downpipes are installed.

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## Renopa

Round....you never see rainwater tanks with rectangular pipes, specially those who are wanting to harvest every drop of precious water.   :Smilie:

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