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