I was looking at a building yesterday and talking about putting in a drain line for the down spouts.
I looked at the FHB article on gutters:
http://www.taunton.com/finehomebuilding/pages/h00046.asp
And found one thread on the subject:
http://forums.taunton.com/tp-breaktime/messages?msg=20630.1
Neither really provided any info on what I need.
The building particulars are: 100′ X 200′ rectangular steel building with a low sloped roof. (Maybe 1/12 or so) The ridge splits the building in half, so you’ve got a 100′ X 100′ slope draining each way.
On each end of the building they installed a HUGE gutter and 5 rectangular downspouts that look to be about 3″ X 4″.
According to the map in the FHB article, the rainfall intensity is roughly 6.5″ per hour.
The building was recently completed. And we need to figure out how big of a drain line we need for those downspouts. There’s a ditch about 100′ away that’s roughly 12′ lower, so we have a good place to drain it.
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The biggest problem here is that this building is at the local historical society. Design and construction of most things is done by a bunch of old guys standing around argueing with each other. (My Dad included) The vast majority of the labor is volunteer.
One of the guys who was there yesterday wants to borrow a trencher and run all 5 downspouts on each end into a single 4″ flexible black field tile.
I don’t think that’s anywhere close to adequate. But I also have no way to prove it.
Seems to me there’s gotta be a formula to estimate how much flow a given pipe size will handle. And there should be a way to estimate the GPM of water that would come off the roof. But I have no idea how to do it.
Can anyone help me out?
The greatest mistake you can make in life is to be continually fearing you will make one. [Elbert Hubbard]
Replies
Worst case, you have a column of water that's 3"x4" at five different points along the roofline. How would a 4" plastic tile handle that? It won't. Call a gutter contractor and ask what they would use, or ask the local building inspector. You have to design for worst case, not best case. Otherwise, you'll have water coming over the front of the gutters or backing up onto the roof.
downspouts that look to be about 3" X 4".
Which is a stock metal building size d.s. I'll bet the gutter is about 5" x 7" with a medium sharp "V" in the front vertical face, too, for the same reason.
According to the map in the FHB article, the rainfall intensity is roughly 6.5" per hour.
Well, if that's a design maximum, that's 100 x 100 x 0.5, or 5000 gph draining into 5 downspouts, or about 1000 gph per d.s. That's about 17 gallons per minute down the d.s. at max.
I don't think that's anywhere close to adequate. But I also have no way to prove it.
Probably not. I'd guess (just guess, mind you) that each side needs around an 8" pipe to the ditch. I'd also guess that first ds to second ds could be 4", then 2nd to 3rd, 6", but you see where that'd get you.
Seems to me there's gotta be a formula to estimate how much flow a given pipe size will handle. And there should be a way to estimate the GPM of water that would come off the roof. But I have no idea how to do it.
There's a plumber's handbook on water volume for pipe that also includes rainfall values. I know the PC used one to size up the central DS for the wacky building last year.
Googling did not turn up anything useful under "rainwater calculations"--will keep thinking about it.
This may not apply to your location but as a guide it may be helpful...
Try..http://www.nyc.gov/html/dob/bldgs_code/bcrs16.pdf
go to Reference Standard 16...Plumbing...page 53 (of 92) has some tables on drainage....
Boss, besides volumn of water carried, you have to consider what else is going to get sucked thru the drain. Got any trees nearby? How about pigeons? Any kids play nearby? It doesn't take much to stop up a 4" tile ( acouple of twigs and some leaves, one pigeon, or a softball). Since you've already got the downspouts in place (which is where the design process should have started), we'll work off that. Five 3x4 pipes gives a cross section of 60 sq inches. 8" round will give you that as mentioned above. It's doubtful, however, that the downspouts will be totally maxed out more than once every 5 years or so. I'd probably go with a 6" dia. pipe, which will carry the runoff 99% of the time and not easily clog. Good Luck.
Scissors cut paper. Rock breaks scissors. Paper wraps rock.
Ron,
This site has info.
http://www.mifab.com/pdf/r-sizing-ca.pdfKK
Five 3x4 pipes gives a cross section of 60 sq inches
BH did not say, but I'm hoping, that's 5 the side of the 200' length of this monster (which gets back to "stock" MBCI every other bay spacing), for a total of TEN DSs.
Which would likley mean an 8" from each side of the building. And, I'll guess that 2 8" together want like 11" of diameter (so that a 10" is too small and a 12" is too big; welcome Alice, to the world of plumbing supply, teehee).Occupational hazard of my occupation not being around (sorry Bubba)
Better to have them too big. There should be some way to keep critters out, too. And maybe a cleanout.
"I cut this piece four times and it's still too short."
"BH did not say, but I'm hoping, that's 5 the side of the 200' length of this monster, for a total of TEN DSs."
Yup - 10 downspouts. Five on the west end, and 5 on the east end. Each set of 5 downspouts will feed into ONE drain tile that runs downhill to the south of the building.
I tried to get a sketch done and posted before I left work, but couldn't.
A young man was sitting in class when the professor asked him if he knew what the Roe vs. Wade decision was. He said, "I think this was the decision George Washington made prior to crossing the Delaware."
"Got any trees nearby? How about pigeons? Any kids play nearby?"
No trees really close. The building has 16' side walls, and doesn't typically have kids around.
I once dated a girl on the track team. It didn't work out.
She kept giving me the runaround.
!00' x 200' x6.5"/hr = 180.56 cuft /min.
I'll let someone else convert that to Gals 'cuz I got CRS. It'll be between 1200 and 1600 GPM.
For what it's worth, I'ld run a 4" from #5 to #4, 6" to #3 and #2, the 8" the rest of the way.
A 10"er will handle 1 3/4 x 8"ers. A 12" pipe will handle 3 1/8 x 8" pipes. Ideally.
With that kind of volume, you will want a flapper grate on the end. See what sizes are available before you buy the pipe.
SamT
Edited 1/31/2005 8:56 pm ET by SamT
one thing i would check and i've seen it alot is that on metal building they have the outer edge of the gutter higher than the inside edge... so when/if you have a blockage or huge rain it will back up over to the inside... i've cut holes with flaps that open in gutters where thats happened to me the first time
pony
it will back up over to the inside... i've cut holes with flaps that open in gutters
Ooh, excellent "catch." I've got a postit note in the book of metal building details to remind me of just that--which is why I did not remember it before, I have a not to remind me . . . Occupational hazard of my occupation not being around (sorry Bubba)
Thought y'all might like to know what I found out. One of the guys from the GC that put up the building stopped by my office. I mentioned the downspout drain question to him, and he had an answer. He said the rule of thumb he was taught is 1 square inch of downspout and/or drain line for every 2 squares of roof area. Since each side of the roof is 100' square, or 10,000 square feet, that would work out to 50 square inches that are required.. They used 5 downspouts that are 3X5, which works out to 75 square inches. So they overkilled that a bit, which is probably good.To get 50 square inches in a drain line, we would need an 8" round pipe. (4 X 4 X Pi= 50.26 square inches)But the guy said he had a couple hundred feet of 6" pipe left over that he'd give us. And he said he give us whatever elbows ans "T"s they had for the stuff. While that only gives us about 28 square inches, I figure that's close enough. This should work out well as a compromise. If they GIVE us the pipe, the old guys who wanted to run it all into a single 4" line won't have anything to complain about. And I don't think there's any way I'm gonna talk 'em into buying 8" pipe. I have to run this by the board of directors, but hopefully it's resolved now.
Q: What are the three types of men?
A: The handsome, the caring and the majority.
Will the trench the drain runs in have to carry any water from the ground? If so, this sounds like a good chance to channel ground water too. Maybe a second, and perforated, drain tile?
No ground water - Just downspouts.This is a steel framed building with no floor. It will be used for a display building. Probably not even concrete - Just gravel.
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