×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Roof Drain Leads (to underground storm sewer)

Roof Drain Leads (to underground storm sewer)

Roof Drain Leads (to underground storm sewer)

(OP)
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!

RE: Roof Drain Leads (to underground storm sewer)

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.

RE: Roof Drain Leads (to underground storm sewer)

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.

RE: Roof Drain Leads (to underground storm sewer)

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.

RE: Roof Drain Leads (to underground storm sewer)

(OP)
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.

RE: Roof Drain Leads (to underground storm sewer)

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

RE: Roof Drain Leads (to underground storm sewer)

ads makes downspout adapters:

RE: Roof Drain Leads (to underground storm sewer)

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 .

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources