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Improper Roof Shedding of Rainwater

Improper Roof Shedding of Rainwater

Improper Roof Shedding of Rainwater

The 4 unit condo building I live in has a large flat roof (100' long by 40' wide). Water drains in one direction to a single gutter on the long side. This gutter then supposed to drain to a 3"x4" downspout on each end. It works properly on an identical building nextdoor.

However, damage has been observed on the corner siding just below one end of the gutter. An insurance adjuster said that the roof is not sloped properly which is causing too much water to shed to one side. The downspout does not have enough capacity, so the water spills over the gutter at the corner causing damage to the siding. That makes sense to me. There is also water damage on the wall inside the house. It's less clear why that is happening, but I suppose it could be similar to what happens when an ice dam causes ice/water to be pushed up under the flashing at the roof edge.

Anyhow, my first thought was that the least costly solution would be to install some type of glue down diverters on the roof to direct the water more evenly and perpendicular to the gutter as intended. But if the slope of the gutter is still off, that probably won't do much to help.

My next thought was even simpler: Put a stop/divider in the gutter, somewhere between the center and the end that is overflowing. It might be difficult determining where best to place that, but it seems that would prevent any excess water from draining to the one side and force it back to the other. Of course if the slope is significant enough it will also cause ponding in a portion of the gutter which is not ideal.

Any idea on how best to troubleshoot/solve this?

Thanks in advance.

RE: Improper Roof Shedding of Rainwater

I wonder if the roof is fine but the gutter is sloped wrong. Add a larger downspout to the end with more flow.

Richard A. Cornelius, P.E.

RE: Improper Roof Shedding of Rainwater

remove the siding, gutter and downspout
replace everything, slope the gutter more, put in a larger downspout or put in one on each end. 100 feet is way too long for a single gutter with just one downspout. I would have expected two gutters, one on each side of the building

RE: Improper Roof Shedding of Rainwater

Add extra downspouts?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Improper Roof Shedding of Rainwater

well, without any sort of details, I have to assume the capacity is backed up at the discharge point which is the downspout. more slope on the gutter might help. it seems like the main problem is just one gutter instead of two. The main roof on my house has about 1200 sq ft and drains to 4 spouts. 300 sq ft per spout. So literally I have about 13 times the capacity

this old house says one 3x4 can handle 1200 sq ft., so according to that you need 4.

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