View Full Version : Draining downspouts

D Felix
04-23-2004, 07:12 PM
Anybody have any formulas for calculating the size of drainage pipe you need for connecting multiple downspouts together?

We are looking at a project right now where I am on the fence whether a 6" pipe would do what needs to be done. I'm pretty sure a 6" SDR 35 would work, but not sure about a 6" corrogated.

I've got a chart for field tile drainage, but it doesn't help much since I can't remember exactly how to use it and we aren't draining a field.:D And the Time Saver Standards is about 10 feet over my head on this stuff too! If anyone can help make sense of that, that would help....

Usually any more than 2 downspouts and I automatically bump up to a 6". Here we are looking at needing to run out 4 on one side of the house, and it will drain 1/2 of a fairly large roof. Basically the same situation on the other side too.

Any help in the form of calculation formulas would be greatly appreciated!


04-23-2004, 10:25 PM
Sorry Dan can't help you with a formula ... basically I am interested so I Posted to get a reminder if anyone replys.

My instinct tells me that I would run it into 2 sections instead of one.

D Felix
04-26-2004, 10:39 AM
I don't think I need 2 sections of 6"..... At least per side.... The pitch of the roof is probably close to a 10/12, maybe as much as 12/12. I would guess total square footage of the house and attached garage at around 2000-2500 square feet.

I would need to hook 4 downspouts per side into whatever drain line we would run. We are probably looking at two runs, one per side. It's the final size of pipe that I'm just not sure about........?


04-27-2004, 01:20 AM
I would also like to hear some input on this subject.

In the process of running downspout drainage on a scape job.
I am working with a typical A roof line and only four down spouts.
Can't see why 4" total run wouldn't work.

opinions welcome.

D Felix
04-27-2004, 09:36 PM
Went back to the site yesterday to take some measurements. Took the transit along to get an accurate measurement of fall.

I estimate the total roof area somewhere in the nieghborhood of 3000 square feet, give or take a few. I would guess that the north side of the house is roughly 1100-1200 square feet, the south side around 1800-1900 square feet.

I'm figuring on two main lines, one on the north side, one on the south. I think a 6" corrugated pipe will be enough to handle whatever rain we have. I checked with a friend of ours who built bridges and roads for 50 years, and he thinks that a 6" should be enough too. Who know how many hundreds of thousands of feet of pipe he has put in the ground over the last 50 years.... He knows his stuff.

I also did some quick figuring this morning, figuring the heaviest rain we should see is 2 inches/hour. And that's a lot of rain! Here's the gist of it:

1800 square feet of roof * 2 inches of rain/hour= roughly 300 cubic feet of rain/hour

300 cubic feet * 7.48 gallons per cubic foot= 2244 gallons/hour

2244 gallons/60 minutes= 37.4 gallons/minute

Figuring the same way for the other side of the house gives 24.9 gallons per minute.

This is with 6 feet of fall on the south side over 200 feet of run, and 5 feet of fall over 150 feet of run on the north side. Plus 10 feet of fall through the downspouts!

Based on this calculation, and what Jack told me, I do think a 6" pipe will be adequate. A 2"/hour sustained rain is virtually unheard of around here. Even a sustained 1"/hour rain is rare. We will have bursts of rain that would equal 2"/hour every now and then, though, but they rarely last for 10 minutes before letting up.

The fire service usually figures a 2 1/2" outlet on a truck will put out 250 gpm, and a 5" outlet will do 500 gpm. That's under 100+ psi, so I don't know why a 6" pipe wouldn't handle 30 gpm on a short term basis.... Granted, I'm comparing apples to oranges somewhat, but it makes sense to me.:D

The drains will outlet at a ditch, so Jack suggested making the last little bit of pipe solid PVC, and to put an animal grate over the end. I probably would have about it at some point but hadn't yet.... Our last 10' will be SDR 35.

Anyway, I've rambled on long enough about what I've figured out thus far. I would still like to know if there is a formula that I can understand and use to figure out for sure whether this will work....

How much roof space are you looking at? If it's a summer cottage that's 20x20, you will probably be fine. On a typical residential structure though, after two are hooked together, I REALLY hesitate to run more than that through a 4" line. If you find that cost on a 6" is prohibitive, use another section of 4" and run 2 downspouts into each 4". It might work out cheaper that way, I dunno.... I think you are more likely to get seasonal heavy rains due to hurricanes than we are. Without seeing the place, I would suggest either two 4" main lines or one 6" main.


04-27-2004, 09:56 PM
Sounds like you've thought this out Dan ... You are going to wrap it in clear stone and fabric right?

D Felix
04-27-2004, 10:00 PM
No need to. It's all solid walled pipe, not perforated. We will wrap all of the connections with tile tape, though that's probably overkill...


04-27-2004, 10:02 PM
My mistake I misread your last post....

04-27-2004, 10:18 PM
Dan excuse my ignorance but do you live in an area of freeze / thaw conditions?

D Felix
04-27-2004, 10:22 PM
Yes, we do.

I do have concerns about tiling out downspouts, but don't see any alternative.

Most of the time, though, the ground doesn't freeze more than 12" below surface, especially under a well established lawn.

I hope it never becomes an issue, and have never heard of it being an issue around here. I'm sure someone, somewhere, has had problems, but they are few and far between...

04-27-2004, 10:26 PM
Yes ... I have had problems in the past ... but our ground freezes to 3-4 ft. What i usually do now is hook up a quick connect system that allows the customer to unhook it in the spring and fall.

D Felix
04-27-2004, 10:29 PM
That's a possibility with the way we do it. Should the pipes every freeze shut, I don't think it would take too much to disconnect the pipe from the downspout.

Chances are, if the pipe is frozen shut, 75% of the time, the downspout and gutter will be too...

04-28-2004, 01:31 PM
I am glad to see this topic come up. I too have always been told to limit 2 downspouts per 4" line, and I have always been able to just run another 4" line for additional downspouts. I have meant to do a few calculations on this and this prompted me.

Based on my elementary fire department math, I came up with these circle areas:

2.5"-4.9 square inches
4"- 12.56 square inches
5"- 19.6 square inches
6"- 28.26 square inches

So, at least to me, this confirms what most have thought, 6" line should handle 4 downspouts well enough, since friction loss in 6" smooth pipe with low pressure should be nearly non-existent. The fire service formula quoted earlier sounded off to me, I was always told you could get about 4 times as much water from a 5" line as a 2.5", and this is with quite a bit of pressure.

D Felix
04-30-2004, 10:57 AM
You can get more water out of a 5" line than with 2- 2 1/2's. I guess what I was thinking at the time was when I was in probie training, we were told to count the number of outlets and to multiply the figures I gave per outlet size, and that should equal what the pump was rated for.... Sorry about the confusion.:)