New FlatRoof Questions

I have dueling architect, roof consultant, roofer and contractor (that should be a clue - too many people) and I need some independent input,…HELP!

This email is so long I’ll do two because there are two main issues --one is drainage and one is the composition of the roof. Here we’ll do drainage

I’m in middle america and have a 4000 sq foot modern house with a couple of levels, with a 33 year old flat roof that is leaking, old etc. Everyone is in agreement it needs to be replaced. Here are the alternate proposals about drainage

Drainage. The current drainage design for the roof is not very good and there are only a few small “scuppers” here and there. We had a flood 9as did a prior owner) when the drainage system backed up so we just cut some holes in the sides so excess water could flow out. Since that time we have had no flooding and the draingae is great BUT in a big rain we literally have a waterfall flowing off the roof and it is creating some water damage to things below…

Architect and her roofing consultant have said we need to re-slope the roof and have everything go to the middle of the roof and into drains in the middle of the roof. The drains will hook into some PVC pipes that go UNDER the roof and then off to the side of the house

Roofer and contractor say that putting drains in the middle of the roof is crazy and that it will lead to water pooling on the roof which is bad for a flat roof + drains get clogged etc and there is no back-up because we would block up the holes we cut in the sides + my wife and I are concerned as to what happens if any of the PVC pipe connections or PVC pipes going under the roof fail - we’ll just have an enormous amount of water flowing into our house. So roofer and contractor propose that we do a more modest re-sloping of the roof and install large water receptacles on the sides of the house (we can put them in a place where it looks okay) to collect the water and then they would have (enclosed) guttering or PVC pipes attached to it and it would take the water to the side of the house – kid of the system we have now but just enlcosing it so we don’t have the water fall effect.

My wife and I think the roofer and contractor have a better idea and we fear the drains in the middle of the roof + the failure of the under-roof drains. But we definitely have little knowledge of these issues and would like some input.


Holy moley…i should have read this first!

Hell no, don’t put drains in the middle of your roof.

Why take rainwater to the middle of your roof only to bring it through your house and then back outside of your house? If the architect is worried more about aesthetics and SHE does not want to see gutters at the permiter of the house (and you don’t mind) then tell her no.

It will be costly to retrofit drains, hire a plumber, re-do sheet rock, chase PVC pipes through beams, add furr downs to hide pipes…the list goes on.

If you do use gutters, a nice 9" box gutter and 4" x 4" downspouts would work well to handle the water coming to the edge of the roof.

I am assuming that you will be using a tapered insulation system to provide positive roof slope / drainage, correct? If so, have it designed by a pro that knows how to do it. They’re expensive and you have one chance to do it correctly.

Thanks for the input!

The architect did not mention that his was an aesthetic move but that makes sense. Since we will put the gutters at the back of the house we don’t care…we just want to have a good system.

Is anyone else out there willing to weigh in on the issue? I am clueless on this (although I am learning a lot here) and I have two experienced respected professionals telling me exactly opposite things and both are pretty vocal about it.

I would very much appreciate any thoughts anyone has on this.

Thank you, Blaster

The reason he said this is because an interior drain is the best way to drain a flat roof. Not a scupper. The heat from the building will keep it from freezing and backing up. the scuppers should stay as a backup if the drain were ever to clog. Your architect is right.

I agree with RooferJ that scuppers are not the best setup (they tend to be leaky), but changing your roof to a ‘drain to interior’ type would be expensive.

The roofer just wants to go with the drain to exterior because it’s cheaper/easier.

This is going to sound awful, but in my experience architects are almost always wrong when they are in conflict with roofers. They simply lack the experience with roofing and have a ton of ego. They just don’t get the idea that they can’t all be the next Frank Lloyd Wright and tend to be pig-headed about their ideas.

If your building is sloped to the outside and has scuppers, sloping to the middle is easily done with tapered insulation, however adding drains will be more complicated. The entire thing will be expensive.

I recommend going with the roofer and contractor. Their plan makes far more sense.

Both are acceptable solutions and are common practice in a commercial application. However, I’ve never ran into a residential center drain app… It would be acceptable though if installed correctly, and will be costly. Not to mention inconvenient. I would suggest (to keep costs down) to simply increase the scupper and drain size to properly accept the heavy flow… With all scupper drain systems there should be an overflow outlet directly above the scupper outlet to allow in the case of excessive rainfall. This is simply a hole similar in size, a small distance (usually 6" to 8", determined by roof size, same as the scupper) directly above the scupper hole… If there are already alternate drain areas and you don’t mind the way a finished application would look, then by all means use them… I would suggest the new roof surface be either fully adhered or mechanically fastened EPDM or TPO system because they perform the best in slow moving or ponding conditions. Pics of existing would definitely help with advice. Also, any manufacturer will advise and guide you and/or your roofer/architect through the proper installation.

Both and internal roof drains are an acceptable methods to drain a low-slope roof. During winter months, internal drainage will typically drain better because the drains don’t freeze; however both will perform well if properly designed.

If you go with internal roof drains, you still should have over-flow scuppers or overflow drains.

If you stay with the through-edge scuppers, just add sufficiently sized sheet metal leader heads (also known as conductor heads) and downspouts. This will eliminate the waterfalls. You should also have a overflow if the leader head gets clogged.

There are pros and cons with both solutions. The things you need to think about to help you decide:

  1. Cost
  2. Cosmetics
  3. Where are the drains lines going to go

No matter what, make sure you have proper slope to the drains/scuppers.

Find a way to add some pitch to your roof. Period. All flat roofs leak at some time. Problem is that they really tend to leak a lot when they do because of ponding. Commercial roofs leak all the time but they usually have ceiling tiles underneath that are easily changed. Houses normally have sheetrock.

There comes a point where you can’t patch repair flat roofs anymore. Through the roof drains work better in commercial apps because they usually have steel web trusses and just the sheer size of the roofs. You can add pitch just as cheap as adding internal roof drains.

If at all possible add some pitch now.

A lot of times adding interior drains on an existing flat roof in the low areas is not as much money as people think. I use a plumber that does this all the time, and its a lot cheaper than going tappered insulation. I also use him to enlarge drains. I dont like to reley on a U-flow or insert.

I would go for an internal drainage system.They require the installation of aluminum or copper inserts into areas of the roof where water pooling commonly occurs. Plumbing systems are attached to the drain inserts to guide the water outside the building through a sidewall. Pipes run down an inside wall and are hung underneath the roof decking through the rafters. To be effective, this system should be installed by a licensed plumber.