Installing roof drains in a concrete roof


#1

Hello,

I’m installing some roof drains in a concrete roof and need some advice. I just had a roofer install 6 drains for me and I need at least 10 more. The building is 40,000sq ft. with a flat roof. Some drains were present when I bought the building and some are new. The roofer that installed the 6 drains did it like this:

  1. spud back the rocks.
  2. Cut a hole.
  3. put put the drain on the concrete.
  4. flashed with some kind of tar.
  5. flashed with lead putting black tar under the lead.
  6. covered with lead with more black tar-like stuff.
  7. put tar over the lead.
  8. Put a fiberglass mesh on the tar and more tar over that.

The job looks OK, but the new drains he installed sit about 1" higher than the roof. The result is a roof drain that sits in a puddle about 1" deep and does not drain all the water.

I’m considering installing more drains myself and maybe fixing the roofers drains. Here is what I’m thinking.

  1. spud the rocks to the felt.
  2. Cut a hole the size of the drain.
  3. cut a recess in the concrete for the drain to countersink into. The roof deck is 3" concrete and my recess would be about 1/2" - 1".
  4. Set the drain in the hole (that is now below the roof surface).
  5. flash with lead.
  6. cover with tar.
  7. flash with tar and fiberglass mesh.

My hope is that my roof drains will sit level to, or just below the roof. I think this will help drain more water and keep the roof dry.

Any comments on countersinking the drains a bit? Is there any reason not to do it?

Also, what materials should I use to flash. I know I can get lead sheets but do i need to use fiberglass too?

This is the drain I plan to install:
bigrocksupply.com/marathon-b … i-864.html

Thanks in advance for your help with this!


#2

There are several different “concrete roofs” and the devil is in the details. You didn’t specify the precise type of concrete roof you have. But no matter what system you have, if the invert of the drain is sticking an inch above the surface of your roof, it’s probably installed wrong. Please see:

zurn.com/operations/specdrai … D13-16.pdf

The manufacturer of your drains has installation instructions at:

marathondrains.com/Drains/Ne … st_pak.pdf

and after looking at them, it appears that this is a new construction (or re-roof) drain. It’s not a retrofit drain. If you look at a retrofit drain, it sits lower than a new construction drain. You may want to consider using retrofit drains, but again, I don’t know exactly what roof system you have.


#3

sounds like you should have done the first 6 drains.
do it like you say and make shur not to apply layers of tar and mesh as to not build them up to high.
i prefer to have just one layer of lead and modified
clanped in clamping ring.

gweedo.


#4

What kind of roofing system is it?