Down pipes – Square/Round?

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  • This topic has 4 replies, 4 voices, and was last updated 24 years ago by Avatar photohj.
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    • #273033
      Avatar photoMasterPlumbers

        I am having a new colourbond roof installed very soon and need to make a descision on the down
        pipes. I would prefer round as it matches the style and period of the house, but I am being told that
        square has more water flow capacity.
        Ccan you give the pro’s and cons on this subject in order for me to make a descision.

      • #286755
        Avatar photoSylvanLMP

          Lets be a little professional and think like a 1st YEAR APPRENTICE

          Lets say you have A 4″ 4″ DOWN SPOUT leader 44 =16 Sq inches CORRECT?

          NOW let go with a 4″ “Round” leader pipe Pi R sq would be 2 2 = 4 3.14159265 = 12.56 SQ inches as opposed to the 44 =16 sq inches..

          Simple math But now consider the connection to the leader PIPE . most lead pipes are round CAST IRON and plastic, so the trasformation of ROUND to ROUND would look neater using round pipe as opposed to Square to round configurations.

          So suppose you opted for a 5″ round leader would give you a drainage piping of 19.62 Sq inches

          which is more capacity then the 4 4 square pipe. All you need to do is size your roof figure your pitch convert the Square feet into fixture units size the gutter and figure the size of the actual down spout needed.

          ANYONE can figure the size of this piping.

          SQUARE = L W

          Round = Pi times the radius squared Radius is half the dia times itself so a 4″ round would be 22 3.14 TA DUM NOW you decide WHAT you like better Round Squaire OR RECTANGLE again L W.

          DARN it Man we can put a BLOKE on the moon WHY cant the home owner HAVE a say how a bloody down spout should be in what shape YOU want.
          YOU have options sir dont let the bloody contractors give you the old heave ho GOOD LUCK MATE

        • #286756
          Avatar photoGuest


            I know how to calculate area so you wasted your time in that respect. What I was looking for was a more in depth answer which addressed the issues of flow pattern through rectangular versus round sections. Even I can see that a linear flow pattern will be formed in the square section whilst a spiral flow pattern will develop in the round. I need to know if this flow pattern in the round section will reduce the total flow through the pipe? You will be aware that the square section pipe is noisy whereas the round is not. A definite advantage in my opinion. I have resolved to select the round section regardless, and if my gutters overflow (which I doubt) then I will simply add another downpipe!

          • #286757
            Avatar photoSylvanLMP

              Would you like to go into flow like Hazen Williams formulas with flow restrictions and the reason we do use round pipes to get a better scouring action with much less resistance in the corners as you would have with a flat piping arrangement.

              Ever hear the WIND effect of a rectangle or square duct as opposed to a round duct?

              I am sure if we both hit the books we could find lots of great formula like a rolling offsets etc.

              For what you requested I would think round would be more suitable BUT hey the old Churches have some fantastic rectangle heavy copper leader lines (that match the Gutters) lasting over 300 years. I guess YOU have to decide what is to your liking good luck.

            • #286758
              Avatar photohj

                Actually the opening into the drain will have a greater affect on the flow. Due to the Corealis effect, water draining into an opening will develop a circular pattern. It will also create a suction that will pull air into the flow. That air will take up space and reduce the capacity of the down pipes. In order to reduce or eliminate the restriction, non-eddy drains introduce a plate across the opening to break up the circular flow and force the water to drain straight down. Placing a baffle across your openings will also increase the effective size of the downpipes.

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