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Drain Pipe Capacity, Estimation

36K views 14 replies 9 participants last post by  gene gls 
#1 ·
I need to do a drainage project for a customer.

The back of the house has the lawn running gradually towards the house.

There are three down-spouts. I'm wanting to tie them all together and get the water to a better location in order for them to help keep the basement dry.

I'm hoping a 4 inch pipe will be sufficient, but I need to calculate the amount of water that the roof will shed and if the 4 inch will be big enough. Right now I'm thinking it's borderline.

Does anyone know if there is a site or reference where I can determine the amount of water coming off a roof during a rain storm.

Thanks.
 
#4 ·
Well...
You don't have to know the amount of water coming off the roof during a rainstorm per se...

All you have to do is provide ample enough drainage to compensate for all down spouts at full flow,
because once the down spouts are flowing as hard as they can, if it's raining that hard the rest of the water
simply falls over the side of the gutters as it can't get through the down spout to begin with and you might think
I'm being funny but I'm not, I see it all the time, once a down spout is flowing at capacity, that's all she can do.

Hope that made sense, but you do have to provide enough for the down spouts at full flow.
 
#5 ·
Just got off a home building project, flat roof, 8,000 sq. ft. and all the water diverted toward the center of the roof then into 4" drains in the roof which then tied together in the building and then out to a drywell. Never backed up, never a puddle on the roof. We've had some torrential rains over the past few nights and more tonight and over the past year, there has never been a problem.
 
#6 ·
Thanks guys.

I'm thinking 4" should be sufficient. I don't think that side of the home has of 2k square feet of roof at the most.

And Topsites, I do agree. Once every two years or so we get storms where the the existing gutter system on 90% of the homes in the area can't handle the flow of water from a torrential storm.

I'm also thinking about adding a drain tile in the same area to help divert any surface water running towards the house. Though the grade isn't very steep at all towards the house, I still want to catch all the water I can.
 
#8 ·
Do they have a basement? Crawlspace? If so, are they damp/wet/flooded, now?

These would be my concerns. But, as others have suggested....4" to 6" would definitely be fine.

Good luck.
Damp walls on that side of the basement. They do also have a good footer drain and sump pump.

A coat of sealant is also in the works to apply to the inside of the basement walls to help with the moisture.

They want to do everything right the first time and hopefully not to have to deal with it again. The downspout issue is more that when the water drains from the current down-spouts drain, the water wants to go back towards the house.
 
#9 ·
I too am with Topsites. No need to size your pipe any bigger than the downspouts. How big are downspouts? 2' X 3"? That's 6 square inches. A 3" pipe has over 7 sq. in. BUT you sometimes have trouble finding fittings for 3" pipe so watch out.
A 6' pipe is 28 sq. inches, so it'll handle four 3" pipes.

If there are puddles in the yard you might include catch basins in the system. Just don't put them below the downspouts, or when it rains the water will shoot up out of them like a geyser.
 
#12 ·
A coat of sealant is also in the works to apply to the inside of the basement walls to help with the moisture.
This won't help. If they want to do it right, AND...if your piping doesn't solve the problem.....they need to excavate and do the waterproofing job from the exterior. Adding sealant to the inside of the walls is a band-aid.

I'm not trying to say your heart's not in the right place. I'm just giving you my input.

Good luck.
 
#13 ·
WG, if you can't get one of these calculators, send me some info, and I'll see what mine says.
roof sq. ft draining into downspout
soil type unless you're daylighting ends

Example:
3000 sq. ft. roof area
loam soil
2"/hr rain fall
need 3" pipe - 28 gpm

3 1/4"/hr rain fall = 4" pipe size
 
#14 ·
Thanks for all the replies guys. Thumbs Up

Personally, I feel an out-side excavation is over-kill on this project. Part of the issue is the grade in the problem corner of the basement. As it sits, it slopes back to the house, so even after water diversion, a couple of shallow window wells and a simple grade, it should be quit OK.

As for the square footage, I'll get that measurement tomorrow as I will be in the area. 4" should be sufficient but, we will see. I've found a couple of generic estimate equations on line. As soon as I get my numbers I'll post back here to double check my calculations.

I'd like to stick with 4" if I can. One section will be going under an area where the outside edge of a patio might potentially go. That and I want to install a small catch basin also to help keep the runoff down some to help keep the neighbor happy. He has stated that the runoff won't bother him at all.

Keep in mind though, that I'm setting this up so that if the current neighbor ever moves and a new owner takes over, then I can run to day-light to the street. I'm trying to avoid that right now if I can only because of a few trees in the way that I might damage root wise if I were to cut a trench.
 
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