I have a flat roof, ~ 150 m2. The roof is under a ~ 5 degree slope.

Since the roof is approx 15 meters long (by 10 meters wide), I was thinking to make (on the low side) the high point of the gutter in the center of the wall. Then install the drains on each side (125mm dia).

           drain          CENTER         drain
           (low)          (high)         (low)
             |--- 7500 mm --||-- 7500 mm --| 
             |                             |
        (0.5% drop)                   (0.5% drop)

I would like to use zinc gutters (½ round), but I'm a bit concerned how to connect these at the center.

If I buy a long piece of zinc gutter (~ 6-10 meter), would this by itself be flex enough so I can achieve a 0.5% drop/pitch? Or would I need to solder two parts in the center under an angle?


Simple sketch

  • Hello, and welcome to Home Improvement. How much (linearly) is that rise in the middle? And, what do you mean by "the roof is under a ~ 5 degree slope"; is it a flat roof? – Daniel Griscom Sep 9 '19 at 14:30
  • 1
    @DanielGriscom thanks! Yes a flat roof. Not sure how you call this. Its square but the front lays higher than the back (approx 5 degree). So water runs down natural atop the back side. The "diagram" resembles the back of the house. – Roger Sep 9 '19 at 14:45
  • The alternative, of course, is just to make a cut at the center and lap somewhat, sealing between. It'll be almost invisible from below. – isherwood Feb 6 '20 at 16:21

Galvanized steel will easily take a small strain like that. The normal yield strength of steel is not reached until total strain is 0.5% strain. That is ,it would need to be held in place to prevent "springback" at 0.5 strain. However , 0.2 % is permanent strain ( no springback). Or , extreme case ,you bend it slightly ( I doubt that will be necessary). If you want a 6 m ( about 20 ft) length with no seam you will need to use aluminum , not galvanized steel.


From your question, it is not clear on what direction your roof has a 5% slope.

For a 0.5% drop, that is only 6.5cm on each side. (7.5m * tan 0.5%) I think you will have no problem with the gutter flexing only 6.5cm over 7.5 meters.

  • sorry, I was not sure how you call it. Its' just a flat roof, but the front lays a bit higher. Water will (naturally) run down to the back of the house. I don't think I can buy munch longer then 4-6 meter zinc, so I was hoping if I put the center off that piece in the middle, it would naturally flex enough. I added a simple drawing. – Roger Sep 9 '19 at 15:02
  • One nuance I didn’t think of earlier: if you use a straight piece of gutter and pull down both ends, you don’t end up with a constant 0.5% slope; you end up with 0.5% at the ends, 0% in the middle, and a varying amount of slope in between. This may or may not be acceptable to you. – spuck Sep 10 '19 at 2:21
  • thanks for the after thought! Good point. Do you have any idea how you would normally get the angle (at the center)? I tried searching, but all examples are tiny cutters like a only a few meters, and they just have the water run of to one side? Cut it and solder? – Roger Sep 10 '19 at 6:31

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