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I am attempting to build a waterfall out of a gutter system. I currently have an awning that is 72" by 48" with 3" high (tentatively) buffers on each side to keep the water from flowing off. The awning is tilted 95 degrees downward from the side of a building. Connected to the awning I have a gutter with the cross-sectional dimensions below and a length of approximately 72" plus or minus 5". I plan on drilling holes in the bottom of the gutter along a linear path so that a waterfall will be created, but I am not sure the amount of surface area I need to create with the holes in order to balance the amount of water/rate that is flowing in and keep the gutter from overflowing. Any help in this would be greatly appreciated.

Gutter cross-section

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  • I have an alternative idea that may give you the desired effect. Could you simply flood the gutter so it runs over the top? Fill it to the brim and then adjust your incoming flow rate to get the water fall volume or appearance that you want.
    – Freiheit
    May 29, 2015 at 14:30
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    "Rain chains", as replacement for downspouts, are another way of getting waterfall-like effects. There are also baffles that replace gutters, which has the advantage of distributing at least part of the spray outward from the house. For all of these, you're giving up some of the ability to move rainwater away from the foundation, and/or to feed a rain barrel... and the next buyer may not like your solution, so consider cost to undo it if you expect to move any time soon.
    – keshlam
    May 29, 2015 at 15:53
  • Are you trying to create a decorative waterfall or a waterfall to move water away from the edge of the awning?
    – Freiheit
    May 29, 2015 at 17:52

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You have a lot of flexibility, as if there is not enough water flowing out the depth in the gutter will increase, increasing the pressure and flow rate. Here is an online calculator which says a 1/8" hole at 0.036 psig will flow 3.7 gallons per hour. I picked the pressure to be roughly one inch deep water. The flow rate increases with the square root of the pressure.

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  • What would the pressure be if the gutter in my schematic were filled completely to the top with water?
    – Claire B.
    Feb 23, 2015 at 19:11
  • 4 x 0.036 = 0.144 psig
    – Jiminion
    Feb 23, 2015 at 19:14
  • Note that if the gutter gets truly full, you may challenge the strength of the supports, though one would hope they would be up to it.
    – Ross Millikan
    Feb 23, 2015 at 19:17
  • I think that the main issue I'm worried about is that I need the volumetric flow coming out of the bottom of the gutter to be less than the volumetric flow of a very intense thunderstorm rainfall, and I'm also not sure how the running water down the side of the awning combined with the falling rain straight into the gutter will contribute to the overall volumetric flow.
    – Claire B.
    Feb 23, 2015 at 19:49
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    It looks to me like the water from the awning will dominate over the water falling straight into the gutter, just based on the areas. The awning is eight times wider. To get a flow rate, take your favorite maximum rainfall rate for the area. If it is one inch per hour, you have $48*72/231 \approx 15$ gallons per hour falling on your awning. The problem may be that if you size for that, in any normal rain you don't like the waterfall you get.
    – Ross Millikan
    Feb 23, 2015 at 19:57

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