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Problem

We have a screened and carpeted porch with gutters along two sides. In very heavy bursts of rain, the water gushes in through the screen of the porch and floods the carpet. (We have a crawlspace below with storage, which we don't want to get wet.) Sometimes the carpet has been not just wet, but pooled with water.

Context

The roof is metal (over asphalt shingles). We've been told this causes it to shed rain faster, and therefore we should have bigger gutters. We replaced our 5" gutters with 6" ones, and our 3" downspouts with 4" ones. (I think this helped, but we still occasionally have the above problem.)

The gutters on the sides of the porch are connected at 90-degree angles to gutters going along the main house. So the volume of water in those gutters interacts with the volume of water in the porch gutters.

Looking along underside of gutter

Side view of gutter and porch

I know the downspouts are not blocked, because we have cleanouts near the bottom of them, where I have observed water flowing pretty well during these gushers. I also have downspout guards at the top of the downspouts. This last time the gushing happened, we had just checked the gutter for leaves the week before. After the gusher, I checked and found a small clump of leaf matter next to the upper downspout guard, probably causing some partial slowdown of water flow. I guess the heavy rain brought down a lot of leaves.

Potential causes

Observing the problem is hard, because it only occasionally gets this bad, among other reasons. But when I do see it, the water seems to overflow the outer edge of gutters, run down the outer edge, and along the underside of the gutter toward the screened porch. What is undeniable is that the water is coming in at high volume, with a strong horizontal component; we see it happening, and can see solid wet areas of the screen high up on the screen "wall" afterwards. It's not from splashing off the deck. (Splashing off the deck could make the carpet damp, but this is a lot worse.)

From my observation of this problem over time, I'm pretty sure that while we definitely have problems when the gutters are clogged with leaves, it happens sometimes even when no leaves are in the gutters (including summer time when leaves are not falling). Therefore the problem may be just that even the large gutters and downspouts are inadequate to handle the fast flow of water off the metal roof in a heavy rain. In that case, the gutters will overflow sometimes even if we keep it completely free of leaves all the time.

On the internet I find lots of information about stopping water from running down between gutters and the fascia. That is not what this question is about, because I'm pretty sure that's not what's happening here. But in the interest of staving off the "XY problem," here is why I think so: The gutters are tight to the fascia, so any leakage in between them ought to be drips going straight down, not torrents running sideways.

Solutions

Some will probably suggest gutter leaf guards of some variety, to prevent possible clogging of the gutter and thereby prevent overflow. We actually have installed a mesh-type leaf guard on some of our gutters that are higher off the ground, but not these ones that are fairly easy to clean out with a stepladder from the deck. It's possible that gutter leaf guards would help, I'm not sure. Reverse curve gutter guards are said to have problems with too much water running off the guard and not into the gutter, especially in a heavy rain. But I don't think mesh type guards have that problem. And I may actually try that.

But since the problem seems to happen even without leaves and clogging, what I really would like is a way to make sure that when the gutters do overflow, the water falls straight down rather than back inwards, which would be a much better outcome. For that, I'm thinking some kind of flange that hangs straight down from the bottom of the gutter:

enter image description here (cross section)

So my question is, would this kind of flange stop the water from flowing inward onto the porch? And if so, how would I go about making one? Does it exist as an available product? Is there a source for narrow, straight strips of aluminum? What would I use to adhere it to the bottom of the gutter?

Update:

Here are a couple of photos from above the gutter, as requested.

Gutter from higher above Gutter from above, low angle

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  • 2
    Anyway to split the gutters from main house to another downspout and reduce the amount of water? Are you sure the wind is not blowing rain or gutter water in? Water usually likes to fall straight down.
    – crip659
    Sep 11, 2021 at 18:27
  • Please post a picture from above, showing how the old shingles and roof panels align with the gutter in terms of height and reach.
    – P2000
    Sep 11, 2021 at 20:57
  • @crip659 water adheres to the surface it runs off, and the shape of the gutter provides it with a horizontal velocity component, like when water/paint drips back over a bucket's edge. This is the reason for instance why rafter ends are cut plumb rather than square with the roof line
    – P2000
    Sep 11, 2021 at 21:11
  • I'd try to get up there when the rain is coming down, and find out whether it's simply filling the gutter and overflowing, or whether it's shooting over the top. Also you say you have downspout guards. The ones you link are better than the ones I have, but while they keep leaves out of the downpipe, they're also easily restricted by the same leaves, and leaves often appear with heavy rain. So they may be making things worse in the short term by reducing the maximum flow just when you need it the most
    – Chris H
    Sep 15, 2021 at 15:49
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    Does this answer your question? How can I prevent water from getting behind the gutter?
    – isherwood
    Sep 15, 2021 at 16:41

3 Answers 3

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would this kind of flange stop the water from flowing inward onto the porch?

It certainly would.

You can probably find flashing already formed into the Z-shape you need in the Gutter and Downspout section of a big box home supply store. If not, you'll need to find something close and bend it a little. Note that it is easier to relieve or partially straighten a crease than to create a new one.

Attach your flange like this:

enter image description here

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  • Thank you. The home improvement stores do have a variety of "Z flashing" available. I'll take a look at it.
    – LarsH
    Sep 20, 2021 at 12:45
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Please post a picture from above, showing how the old shingles and roof panels align with the gutter in terms of height and reach.

Since the metal roof panels possibly sit a bit higher than normal and the panels likely reach past the drip edge of the old shingles, there is possibly too little clearance for water rushing down the roof at high speed to be caught by the gutter, causing some water to "jump" over the gutter.

The solution would be to offset the gutter from the rim, or to raise the far lip (tilt gutter up) or to shorten the shingles/panels.

You can confirm the cause by running a hose onto the roof and observing how/whether the water is jumping the gutter before the gutter & downspout volume & friction become factors in restricting flow.

Additionally you can check the gutter's leveling by blocking the downspout and filling the gutter with water to see where it overflows first.

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  • Thanks for your response. I posted a couple of photos taken from above the gutter (see question). It doesn't look to me like there's too little clearance -- what do you think? I haven't tried the hose test yet.
    – LarsH
    Sep 14, 2021 at 21:47
  • Hi @LarsH, thanks for the pics. Well, it looks marginal, so maybe yes or no. Extending the slope seems to get you close to the gutter edge. The hose test, or waiting for rain, would help, if you're inclined to tackle the root cause. (I had a similar problem)
    – P2000
    Sep 14, 2021 at 22:44
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My bet is that this is your problem:

The gutters on the sides of the porch are connected at 90-degree angles to gutters going along the main house. So the volume of water in those gutters interacts with the volume of water in the porch gutters.

You've simply got too much water for the gutter to handle, even though you've got a larger gutter and bigger downspout.

Try adding an additional downspout to handle some of the water.

This is actually something I'm looking at for the front of my house where the main roof drains into a gutter which then empties into the gutter for the porch roof. During a good rain storm, I can watch the rain flow over the edge of the porch gutter. I'm going to install a new downspout direct from the main roof gutter to the ground, bypassing the porch roof's gutter entirely.

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  • In that case OP best also to investigate all details (roof, gutter, spout, pipe, drain) during heavy rain when symptoms are apparent. Simply disconnect spout at bottom, letting water drain freely (e.g. guide with plastic onto lawn away from house), and see if backup is mitigated.
    – P2000
    Sep 15, 2021 at 16:20
  • I'm not sure, @P2000, if I follow the "Simply disconnect spout at bottom" comment. If the downspout is the weak link (i.e. not enough volume to carry all the water coming down), how is disconnecting the bottom going to help? Unfortunately, the only way to really test the theory is to open a hole in the gutter to let water out at the top. One would, logically then, attach a downspout to send the water where you want it and not just let it run everywhere...
    – FreeMan
    Sep 15, 2021 at 16:34
  • Yes, @FreeMan that's what I'm really after: not letting the water fall and splatter all over the deck etc... I think if the bottom is open it's likely as good as opening anywhere along the vertical. Top or bottom, usually one screw. OP will see whether water is obstructed in the gutters and upper pipe bends, or whether it's backed up from below. No? Hard to trouble shoot and anticipate over the internet. Best for OP to be there during rain & try things out.
    – P2000
    Sep 15, 2021 at 17:16
  • Thanks for this suggestion. You may well be right that we need an additional downspout. I would still like the water to fall straight down when it does overflow the gutters, but not overflowing in the first place would be even better.
    – LarsH
    Sep 20, 2021 at 13:32
  • One of my difficulties in investigating the details during these flood conditions (pouring into the screened porch) is that they don't seem to last more than a minute or two. Still, there are longer periods of heavy rain where I can observe something.
    – LarsH
    Sep 20, 2021 at 13:34

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