I have a metal pergola with slated roof. When it rains moderately the water from the slats falls directly down instead of draining into the gutter. looking for some solution to fix this so all water goes into the drain irrespective of the rate of flow.

In the picture I have tried to highlight the water sticking to the gutter because of tension and falling down (red) instead of going into the gutter (green)

enter image description here

I am wondering if something like a water repellent spray would fix this. The idea being I spray it on the edges and the water will not stick to the gutter and flow directly into the gutter?

2 Answers 2


I think a drip edge will work (see red part of photo). A short piece of aluminum bent towards the opening of the gutter should cause any water to drip off of it instead of dripping the other direction. You can silicone a piece in place to experiment with the length and angle but it probably won't need to be very long. Once you get it right I would rivet them in place with aluminum rivets. Drip edge in red Another option is to fasten it to the gutter, see second picture with detail in green. One or two or three pieces fastened to the gutter would be less work than a drip edge on every single slat. Search for aluminum trim coil, comes in white, it's easy to work with (wear gloves, sharp edges) and you can bend it by clamping it between a few pieces of wood. You can also have a siding and gutter company do it for you. lip for gutter

  • That would work, giving an angle for the water to fall into. But my only concern is, there are 42 roof slats and they drain on both sides. So this will quickly become a big job. Plus I am concerned if I rivet stuff into it, it might void the manufacturer warranty too. Do you think something like rainX on the bottom edges would work? if the surface is water repellant, the water might not adhere to it at all and fall in drain as expected?
    – Mukul Goel
    Commented May 19, 2022 at 16:16
  • You can try the Rain-X and if it doesn't work you can use it on your car. If there's a manufacturers warranty you may want to reach out to them and explain the problem, they may have a fix for it. Another option is to put a "lip" on the gutter, Commented May 19, 2022 at 16:30
  • Yeah il try Rain-X, no harm in trying it out. Can you send me a picture or maybe a link to the product of what you mean by "lip" on the gutter so I can explore it more?
    – Mukul Goel
    Commented May 19, 2022 at 17:26
  • So I found these, screwfix.ie/p/… I am thinking if I cut this to size and make a shape like this ibb.co/5nGt6Jd and the stick it to the bottom of the slat with an aluminium adhesive, it should work? Can you please advise if I am on the right track? , thats what you suggested just sticking it instead of rivets really ?
    – Mukul Goel
    Commented May 19, 2022 at 17:46

"Water repellent spray" (I assume that's something like RainX™) will not work to make the water magically jump the gap between the decking and the gutter, it'll just make the water flow more quickly through the gap and onto the ground. You'll actually need to block the gap with some sort of solid filler.

Usually caulk (either acrylic, siliconized acrylic, or silicone - pick something rated for outdoor use) would be squeezed into the gap from a caulk gun. Once cured (though some are designed to be applied to wet surfaces) it will be waterproof and the deck runoff will go over the top of it and into the gutter. Of course, water being water, the next rain storm will identify any and every little gap where you missed a spot by dribbling through. You can apply another little dab on those spots to fix them up.

Since this is such a large gap, you'll want to use something other than caulk to fill the majority of the gap, then do a sealing layer on top with the caulk to make it waterproof. The usual thing to use is "foam backer rod" like this:

coil of foam backer rod
Image courtesy of lowes.com. No endorsement intended or implied.

The foam is stuffed into the gap from below then caulked over from above. This helps hold the caulk in place and prevents it from just falling to the ground before it's had a chance to set up. Of course, since this seems to be visible from below, the backer rod won't be particularly pretty. If the appearance is unacceptable, you may have to caulk over the bottom of it (white caulk) just to hide it, live with the grey rod visible, or find a more visually appealing material to hold the caulk in place in the first place.

It's hard to tell exactly what we're looking at from this pic from underneath, but that's quite a large gap between the decking and the gutter. Is the gutter supposed to be catching runoff from the deck, or is that a roof gutter that you'd like to have serve the additional purpose of catching the deck runoff?

  • If it's dedicated to the deck, you'd probably be better off reinstalling the guttering tight to the deck. This will minimize the gap between the two and it will make caulking much easier.
  • If it's a roof gutter that you'd like to also catch deck water, it's entirely possible that the gutter won't be big enough to handle the water from the roof and the deck and will overflow. It may only do this in the heaviest rain storms, though, so that might be livable.
  • Think there is some misunderstanding. there is no gap between the slat(pergola roof with gutter) and drain gutter. The slat overlaps the drain gutter. Look at the picture in Platinum's answer. The rain water is collected into the roof slat (horizontal in his picture) and then should flow into the drain gutter (bucket in picture) . There is an overlap there and if the water doesnt stick(adhere to surface) and just free flows it falls in the gutter . But on some slats its sticking because of surface tension and falling outside the gutter as shown in my picture. I hope this clarifies situation
    – Mukul Goel
    Commented May 19, 2022 at 16:13
  • 1
    Then you need a proper drip edge, @MukulGoel, as is shown in his answer. A RainX-type product is still very unlikely to help and if it does, it will only last a short time before you'll have to apply it again.
    – FreeMan
    Commented May 19, 2022 at 17:15
  • Thanks. I will explore the edge angle more and if I can find something off the shelf and something I can stick to it instead of rivets that would be ideal.
    – Mukul Goel
    Commented May 19, 2022 at 17:27

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