Roof Drain Leads (to underground storm sewer)

[Scoomer]

I would like to get an opinion from someone on roof drain leads. Here's the situation:

Got a building with a roof. Owner does not want to go with internal plumbing with the roof drains to my one stub to the building. The Owner has decided to go with external downspouts. However, I have to convey that roof water to a storm sewer system (very tight site).

So, I devised an external storm sewer system (HDPE piping and bends) with individual stubs to each downspout. From each underground stub I come up with two 45-degree bends and then bring a riser up to about 4 inches above grade right against the building wall. The downspouts empty directly into the risers and to the storm sewer system.

Question: What pipe material to use for the downspout riser in Minnesota climate? I have used DIP before, but what about HDPE? (I would like to keep the pipe materials the same without having to mess with couplings).

Or, if you have some completely different ideas, that's good too. Thanks!

 

[SlideRuleEra]

Scoomer - Here is a link to service temperatures (see page 4) of typical HDPE pipe from the "PP900 Engineering Manual for Performanance Pipe" (formerly the Drisco and Plexco brands)

http://www.cpchem.com/docs/EMB1C1.pdf

They offer other good techncial info at

http://www.cpchem.com/performance_pipe_tl_technical_library.asp

Best Wishes

 

[LHA]

PVC embrittles in sunlight, I do not know if HDPE does or not...maybe post to a chem. engineering thread? This will be a consideration.

Just curious, why don't you want to use transition couplings? They are not appreciably more expensive than other couplings and the installation is the same. Perhaps an adaptor made of some UV resistant material after the HDPE daylights would solve this problem.

 

[dicksewerrat]

Don't forget to put a heat cable inside the downspout. Or you will just have a frozen and split downspout. If you size the 'riser'from below grade one size larger than the downspout, you can slibe the downspout a foot or so in the riser. No coupling. Put a little oakum and plumbers putty in the space left over.

 

[bltseattle]

HDPE is subject to extreme expansion/contraction due to heating, so make sure you consider a slip connection that can handle the changing lengths of pipe.

 

[Scoomer]

dicksewerrat:

Yep. My intent is to put the downspount down into the pipe and to have an open-air transition between the downspout drain and the rain leader. BTW, if it does freeze and plug, I do have an overflow route. The stubs and leaders are buried to 5 feet.

Iha:

I would have wanted to do a transition coupling between the DIP riser and the HDPE pipe. However, this particular contractor is very frustrating to work with, so I wanted to give them something very straight-forward (i.e. same materials) to work with. I have done a DIP riser to an HDPE stub with other contractors that are used to doing more "urban" type of construction with special roof drainage considerations like this project. The thing I am most concerned with is freeze-thaw cracking of the HDPE riser above the frost line.

 

[dicksewerrat]

The heat tape should minimize the freeze=thaw damage. You could use plain old alum. downspouts into the buried pipe.

 

[zzmarkzz]

ads makes downspout adapters:

 

[waterworker]

HPDE pipe will of course freeze- and I am tempted to say it won't crack , having seen several HDPE pipes frozen solid below minus twenty with no cracks ( thaw and re-use) .

For freezing crack resistance High Density Poly Ethylene is the best choice, in my opinion, having more freeze-crack resistance than Poly Vinyl Chloride pipe. Attached to a structure such as a roof drain, it may or may not violate the fire code- they don't want large plastic pipes on fire. I don't know about your fire code, but it's worth a check.

As you may know HDPE and Iron pipe size Outside Diameters are the same, transitioning from HDPE with a ROBAR or ROMAC coupling ( and internal sleeve in the HDPE) is pretty easy.

More Ultraviolet light reaction inhibitor can be added to HDPE if you specify this when ordering it. In a roof drain application ( there is no pressure-so pipe is way over strength for the function ) , I wouldn't discard Ultra Violet weakening as a factor, but my GUESS is it would be fine. Or Box it in if UV is a concern. As mentioned, HDPE contracts and expands with temperature-nine times more -than PVC, so don't pin it down mechanically but rather use transition fittings that it can slide inside of .