Minimum Gutter size

**Are there requirments or codes for a minimum size a gutter can be?
I see new to old homes with large areas of roof sloping into a gutter that may be from 1Ft. to 5 Ft…

The one pictured is obviously an issue as it is installed by a window & the brick is stained. All of the garage roof discharges into this small gutter.

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it’s hard to tell how the roof on the left structure intersects with the building on the right, but it does look like a poorly designed detail with potential for alot of failure including water penetration and ice issues.

There should be a downspout for each gutter length of 20’-50’. Can they add more downspouts?

What about requirements for minimum size of gutter?

In the photo posted, I don’t think the issue is the size of the gutter, but the size/quantity of downspout draining the gutter. The one standard size spout is probably inadequate for the area of roof (which we can’t see) being drained. No matter how large the gutter is, it will overflow if the downspout is inadequate. One standard downspout should be good for up to 500 square feet.

I don’t know if that one even classifies as a gutter, more like a bigmouth downspout. It’s going to have a hard time handling the runoff before it overflows. That garage was an poorly designed add on. The roof system should have been tied into the main roof instead of running under the overhang. IMHO

Besides a bad roof insection design as was mentioned, I would be curious to see how in hell they provided a gutter system for a roof that terminates against a wall of the house.

Something screwed up here and we can’t quite see the whole picture.

Where does the gutter go toward the back and where does the gutter of the higher roof discharge?


Ah well.

Marcel :slight_smile: :slight_smile:

While that “thing” appears to be made out of gutter material, it really is functioning as a “scupper”, which should be considerably bigger (wider and deeper) to collect the water and direct it to a downspout. If it’s a 2-car garage, say 25 x 25 feet, the downspout is only carrying just over 300 square feet of roof water, so one downspout should be adequate for this half of the roof.

I also wonder about the apparent lack of a cricket at the intersection of the roof and wall, and I agree with others who said that a different roof design should have been attempted. The water should have been kept far away from the wall, not splashing up against it.

There is no way that house won’t have water intrusion.
Now if that is a bathroom window , I bet they could open up and fill the tub.

Use this link and then go down to the pdf file to calculate sizes required.


Be a good time to try out the old B-Cam

Rule of thumb while selling roofing, siding, guttering used to be … one square inch of downspout per square of roof, (which is 100 square feet of roof surface). Translation : one single 2" x 3" downspout is good for up to 600 square feet of roof surface draining that direction. A single 3" x 4" downspout is good for up to 1200 square feet or roof, etc. This is in keeping with several other formulas presented here, but easier to apply I think.

In residential application there is no requirement that I know of.

There is a huge part of the commercial application in commercial plumbing licencing (so the building won’t collapse). Try looking there.

I rant about gutter installation all the time.

Thanks for the info John, however the downspout capacity is not the root of the problem in many cases.

1200 sf roof needs one downspout, but 2 feet of gutter can not handle 1200 sf. Also every 90 degree fitting slows water flow and results in over/back flow. This is what I see all the time.
When there is water in the basement/crawlspace, I always point attention towards the roof (then downspout diversion and site grading).

Gutters are the primary cause of structural damage around here due to expansive soils.

This picture is a “mild” case from yesterdays inspection (it gets much worse).

I think you have pointed to the real problem. It’s not: “what should be the minimum gutter size”, because 5" K-style gutter or 6" shop-built, either one will handle a lot of runoff, given a good opportunity by slope and adequately located downspouts.
The real question that needs answered is “what should be the minimum gutter length.” The facts suggest that when you have hundreds of square feet of roof converging into a gutter length of only a few feet, overflow problems will occur repeatedly. Sooner or later that will showup inside. Dave, before long, I’ll bet the center of your picture will be a good place for you to start pointing you B-Cam.
The way roof shapes are built today with so many roof slope faces, attention to this detail of gutter function often takes a back seat to the appearances of downspout placement. If you asked the owner of the house in the picture if they would prefer to have the additional recommended downspout located on the back side of the large column or right down the middle between the 4 windows in the center of your pic, which do you think they would choose?? :slight_smile: