Rain Water Deflection from Adjacent Buildings

[civilnick]

A 50-story building is proposed next to a 6-story building. The owner of the 6-story building hired an engineer who determined that rainwater could deflect (presuming it is coming down at some angle) off the side of the adjacent 50-story building onto their roof. They want to know how the owner of the 50-story building plans to mitigate this issue.

Is this really an issue? Is there any precedent for their claim? When I calculate roof runoff for designing building drainage systems, I assume the footprint of the building as the area for my calculation. I never consider runoff from taller adjoining buildings that may deflect onto my roof.

Does anyone out there have any experience with this?? Any advice would be appreciated....

 

[PEDARRIN2]

I would wonder how close they are together. If a wall of the 50 story building is against the 6 story building, some of the wall area would have to be considered. But not the total vertical area.

Typically, I only use the bottom 30 feet of the wall area.

I would think the 50 story building would also intercept a lot of the rainfall falling the other direction.

Maybe they offset each other.

 

[chicopee]

Gutters may be one solution.

 

[gibfrog]

If the buildings are adjacent one to another with a common wall, then this is a very significant issue. Depending on the direction, you will increase the peak load on your neighbor's roof drainage system and you need to analyze the worst case scenario. CHECK YOUR LOCAL/STATE BUILDING CODE.

Here is the relevant section of the 2007/09 Florida Building Code
"1106.4 Vertical walls. In sizing roof drains and storm drainage piping, one-half of the area of any vertical wall that diverts rainwater to the roof shall be added to the projected roof area for inclusion in calculating the required size of vertical conductors, leaders and horizontal storm drainage piping."


Clifford H Laubstein
FL PE 58662

 

[PEDARRIN2]

If the buildings are adjacent, you probably would not have to account for the entire vertical distance - given that any rain that hits the upper portions of the wall would not reach the lower wall by running down the side of the building.

You would likely be safe to account for the lower 50-60 feet of the other building in your calculations.