Updated - Downspout in ground drain pipe size and material question

[chrisntam]

Working at my mom's house, I'll be digging a trench for downspout drainage, connecting two downspouts which are about 30 feet apart.  Total length of the in ground drain line will be approximately 75 feet from the furthest downspout to the open exit to an alley.  The run will be basically straight, except for the 90s to connect the downs to the horizonal in ground pipe and a wye to connect the two runs into one.

Roof area to be drained is approx. 1200 square feet with a 5/12 pitch.

According to the first attached chart, supposing a heavy rainfall of 3 inches per hour, the amount of water will be 1.87 gallons per hour for each square foot.  1.87 (gallons per sf) X 1200 sf = 2,244 gallons per hour.

According to the 2nd attached chart, a 3 inch pipe will carry up to 8400 gallons per hour, fed by gravity or low pressure.

1) Am I correct in my assumption that a 3 inch in ground drain pipe is more than ample?  I'd much rather use 3 inch drain line than 4 inch.

2) I plan on using 3 inch PVC, not corrugated, flexible roll pipe.  I'd also prefer to use the thin wall plastic pipe, not the schedule 40 thick (and heavy) wall pipe.

3) I don't plan to glue the pipes/connectors together.

The pipes will be in ground, only walked on, no heavy equipment or vehicles.

Are my assumptions and plan good to go?  I understand the fall should be about 1/8" to 1/4" per foot.  Wow, 75 feet @ 1/8" per foot is a 9 inch drop.

Thanks for your insight, it is appreciated.

chris.

[mike4244]

Working at my mom's house, I'll be digging a trench for downspout drainage, connecting two downspouts which are about 30 feet apart.  Total length of the in ground drain line will be approximately 75 feet from the furthest downspout to the open exit to an alley.  The run will be basically straight, except for the 90s to connect the downs to the horizonal in ground pipe and a wye to connect the two runs into one.

Roof area to be drained is approx. 1200 square feet with a 5/12 pitch.

According to the first attached chart, supposing a heavy rainfall of 3 inches per hour, the amount of water will be 1.87 gallons per hour for each square foot.  1.87 (gallons per sf) X 1200 sf = 2,244 gallons per hour.

According to the 2nd attached chart, a 3 inch pipe will carry up to 8400 gallons per hour, fed by gravity or low pressure.

1) Am I correct in my assumption that a 3 inch in ground drain pipe is more than ample?  I'd much rather use 3 inch drain line than 4 inch.

2) I plan on using 3 inch PVC, not corrugated, flexible roll pipe.  I'd also prefer to use the thin wall plastic pipe, not the schedule 40 thick (and heavy) wall pipe.

3) I don't plan to glue the pipes/connectors together.

The pipes will be in ground, only walked on, no heavy equipment or vehicles.

Are my assumptions and plan good to go?  I understand the fall should be about 1/8" to 1/4" per foot.  Wow, 75 feet @ 1/8" per foot is a 9 inch drop.

Thanks for your insight, it is appreciated.

chris.

I would change the piping to schedule 40 PVC at the least. You don't say where your from, makes a difference. If your ground is subject to frost heaving I would go with schedule 80 PVC. I would use the solid pipe for 10'-0 and drain pipe of your choice for the remaining 65'-0 or so feet.
I would use schedule 80 PVC for the 15'-0 from each down spout to the Y. Thin wall PVC may be cheaper and lighter but schedule 80 PVC is installed once. Were talking about a total of 40'-0 of Schedule 80. No need to do the remaining run with solid piping. 
Place a screen in front of the runs termination to keep critters and debris out. 1/4" hardware cloth will do the trick.
mike

[chrisntam]

Thanks for the reply, I'm in Dallas.  Frost heave is not an issue.  I think Schedule 40 is thick enough, didn't really want to install that (heavy).

When you say "drain pipe" do you mean perforated?   Or just a thinner walled pipe?  If I used Sch 40 solid pipe, I would just use that for the entire run.

I've attached a diagram showing what I'll be doing.

Good idea on the hardware cloth.

Thanks again,

chris.

[mike4244]

Thanks for the reply, I'm in Dallas.  Frost heave is not an issue.  I think Schedule 40 is thick enough, didn't really want to install that (heavy).

When you say "drain pipe" do you mean perforated?   Or just a thinner walled pipe?  If I used Sch 40 solid pipe, I would just use that for the entire run.

I've attached a diagram showing what I'll be doing.

Good idea on the hardware cloth.

Thanks again,

chris.

Chris, schedule 40 will be fine, in Southern NJ we get freezing  soil temperatures and did not know you were from Cowboy land. 
Your diagram explains a lot. Go with what you drew. Perforated pipe would be a mistake seeing how the piping is parallel with the house. 
mike

[jamesglenn]

Schedule 40 is plenty strong enough for this application. Even the thin walled drainage pipe will hold if there is no heavy equipment, but it needs to be at least 12" deep to disperse some of the pressure.

I would suggest making it a straight run from the back downspout all the way to the alley and eliminate the one bend by the driveway and alley. It would make the flow easier and eliminate a couple elbows.

[Splinter Puller]

If the downspout is not sealed tight to the underground then the size is less important as it will handle a "normal" case. Anything extremely heavy will spill out or over top the gutters anyway.

[chrisntam]

There’s a tree stump that I have to navigate around near the alley. The stump has been ground since the picture was taken but there are still big roots in the way.

[TDKPE]

Some sources recommend Schedule 35 SDR for outside drain pipes instead of Sch. 40.  More flexible and less likely to crack from ground movement.

I know my house has Sch. 35 drains for the gutters, and it's slightly bigger in diameter than the large dimension of the downspout, which would make it 4" pipe with a transition piece from rectangular to round.

[chrisntam]

Dallas has ground movement, settlement when the clay soil is dry, heaving when wet.  Perhaps a bit of flexibility would be good.  A few years ago we had our pool redone, they put in 4" sdr pipe at our house, it is under travertine tile deck.

Does the size (3") look ok?

[TDKPE]

Looks ok on paper, but I recall back when I was doing reinforced concrete pipe (RCP) drain work, the engineer telling me that it’s customary to use only half the capacity of the pipe.  Or size the pipe for twice the flow, in other words.  Silting, you know.  

Personally, I’d use the larger size.  But my house in NJ had 3” pipe, built in ‘61, and it ran all the way out to the street and actually came through a hole in the curb, one from each end of the house.  Mine was the only house on the street that actually flowed water through those pipes, though, so there’s that.  

   

Hmmmm, looks like the decorative tree has gotten a bit out of hand in 20 years.