Downspout Calculation

[captainbunsaver]

Hi All;
I thought I would be smart and get Revit to calculate the required number of downspouts for the given roof areas of a project.
Well, I used Arch Graphic Stds of course (page 398 in mine), and came up with the following: (let's assume Zone 7 and 6"x6" downspouts)

(Area (of roof)/175) X 1/64 = Number of downspouts required

I must be missing something, because this gives me a small number.

Any takers?

TC

[DoTheBIM]

It might help to quantify what you call small with what small is and the area of your roof. Other than that I doubt I can help as I don't know much about calculating downspout. But if I had to guess... it would be something like roof area / 175 (<--a chart lookup based on location I assume) = sq in. of down spout needed. Then a 6x6 would have 36 sq in to use... so you'd take your result and divide that by 36 which gives number of downspouts which then needs rounded up to the nearest whole one. So take a 14490 sq ft roof plane for example.

14490 / 175 = 82.8 -> area of downspout needed
82.8 / 36 = 2.3 -> number of downspout needed
round up 2.3 = 3 -> total needed.

Not sure where the 1/64 is coming from.

[captainbunsaver]

Oops, my bad... I was looking at two different sets of possible calculations and picked up a wrong number.
The 1/64 should have been 1/36. Good catch!

I guess that it seems small when a 30,000sf roof calculates to need only 5 downspouts of 6"x6".

30,000/175 = 171.42857

171.42857/= 4.7619

Round up to 5

[DoTheBIM]

6x6 seems rather large and can move a lot of water, but then I'm in residential and 30000 sq ft of roof area on one plane seems like a lot to me ;) To put things in another perspective... Using residential downspout (4x3 i think) you'd need 15. I think your calculation is fine if those are the specs you need.

[patricks]

Yes I do gutter and downspout calcs on almost every project we do. I guess that sounds about right for a flat or near-flat roof. Don't forget the multiplier for roofs of higher slope, since the water moves faster.

Also, doing it the graphic standards way, I believe you're supposed to multiply the roof's [b]2D flat plan area[b] by the slope multiplier, not the actual roof area.

So a 12:12 roof is going to have 1.414 times more actual area than the plan area, and the plan area is what you use when doing downspout calcs... I think. I don't have Graphic Standards in front of me at the moment.

[DoTheBIM]

I understand why someone would feel the need to adjust for slope, but the same amount of rain falls across a horizontal cross section not a sloped one. The slope in my mind just spreads out the rain fall over a larger area and doesn't really accumulate any more than if the the roof was flat. But this site here (http://www.copper.org/applications/architecture/arch_dhb/gutters_downspouts/intro.html) explains that it should be adjusted to true area if your using plan area due to "possible" rain falling perpendicular to the slope of the roof. Better safe than sorry I guess.

[patricks]

I understand why someone would feel the need to adjust for slope, but the same amount of rain falls across a horizontal cross section not a sloped one. The slope in my mind just spreads out the rain fall over a larger area and doesn't really accumulate any more than if the the roof was flat. But this site here (http://www.copper.org/applications/architecture/arch_dhb/gutters_downspouts/intro.html) explains that it should be adjusted to true area if your using plan area due to "possible" rain falling perpendicular to the slope of the roof. Better safe than sorry I guess.

Well actually I think the slope multiplier comes into play for sizing the gutters, because with more slope the water is falling faster, and so you need a wider gutter to keep the water from overshooting the edge of the gutter.

[kare2trot233977]

Is there a way to break the area of a roof down by plane in Revit? For example, if you need 10 downspouts and have a simple gable, it is easy enough to put 5 on each side. But if you have a hip or other complicated roof structure, it would be nice if Revit could calculate roof area by plane to determine the number of downspouts for each side of the roof.