Negative Slope on flat roof

[Zane Remenda]

Hi all,

I frequently inspect flat roofs. Very often there is a negative slope, thus creating stagnant water which reduces the lifespan of the membrane.

For a tar and gravel (bitumen) roof, is anyone aware of anyway that the slope can be repaired? What about a negative slope on a "roll roofing"?

Thanks

Zane

[Billy Stephens]

Hi all,

I frequently inspect flat roofs. Very often there is a negative slope, thus creating stagnant water which reduces the lifespan of the membrane.

For a tar and gravel (bitumen) roof, is anyone aware of anyway that the slope can be repaired? What about a negative slope on a "roll roofing"?

Thanks

Zane
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One Way When it Rains it Pours – Roof Drains Explained (http://www.theplumbinginfo.com/plumbing-news/plumbing-information/rains-pours-roof-drains-explained/)

[Jerry Peck]

Reverse slopes can be corrected, but it is a bit expensive as they would need to cut the roofing back, install tapered roof insulation, then re-roof.

That area looks like a perfect spot for a roof drain.

[Kristi Silber]

That's a very saggy roof! How old is the building? I'd be concerned about unseen leaks that have compromised the roof structure. When full of water there would be enormous weight on the center. Is that a roof drain I see in the foreground, the rusty pipe sticking up a little? That would definitely be the way to go. Otherwise if you raise the middle you're looking at reroofing the whole thing, since it's tar and gravel.

Roll roofing shouldn't ever have a negative slope, it's not designed for flat roofs. But to increase the slope the repair Jerry suggested is suitable.

[Raymond Wand]

Its not all that uncommon, difficult to always get a positive slope, and could be difficult to correct. Likely easier to put another drain in.

[John Kogel]

I am surprised we don't see more pond pumps on roofs like that. The pump drains the puddle thru a drain hose and then shuts off. They are apparently common in jolly old England, where flat roofs are a curse.