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-   -   Calculating roof square footage (http://www.diychatroom.com/f9/calculating-roof-square-footage-94905/)

tomseeley 02-08-2011 12:54 PM

Calculating roof square footage
 
I'm trying to calculate the total sq ft area of my roof. We plan to install a rainwater harvesting system, and I know the mfrs can do this for me, when designing their system, but I'm trying to get an idea before I start shopping what amount of area I have that will capture rainwater.

I can measure the horizontal dimensions. I've drawn sketches and taken pics. But I've got a lot of segments that are (I think the term is) hip roof, and one that (I think the term is) gable roof, and I don't know how to figure the pitch or angle, so I don't know how to calculate the lengths of the tilted sides of each surface.

I've got a saw protractor I can use to set saw angles from 0 to 60 degrees, and I can stand on the ground and hold it up and sorta eyeball the angles, but when I do that, and then I do the math, it just doesn't give me logical answers! I can do one side quite accurately, but the other sides don't come out right at all compared with the one I'm almost positive I got right, so I don't think I'm figuring the angle of the tilt or slope and therefore I'm not using the right tilted side lengths in my math.

Does this make any sense at all?

What can I do to get a better idea of how big the roof is?

Jackofall1 02-08-2011 03:46 PM

Do you have excel, I could send you a program that I built.

Mark

mem 02-08-2011 03:53 PM

Post your photos and sketches--it would really help. I believe the only angle you will need to know is 90.

Jackofall1 02-08-2011 04:43 PM

You can measure the angle, by eye as you stated this will be close enough, measure the width, divide the width by 2, multiply half the width by the trig function "sec" in attached table, this will give you the angular length multiplied by the length will give you the area for that roof section.

I would consider an gable as intersecting with the roof at 90*

http://www.eugeneleeslover.com/USNAV...FUNCTIONS.html

Mark

tomseeley 02-10-2011 12:58 PM

JackofAll1

Thx for the help. Pls send your excel file to me at tomseeley @ hot mail dot com. I have MS Office 2010 so any version of Excel should work.

DexterII 02-10-2011 01:24 PM

Maybe I am reading something wrong, but I believe that you are incorporating too many non-essentials in the equation, and that you can do everything that you want on the ground, with no regard to roof slope, using only a tape measure. The amount of rain that will fall on your roof is the exact amount that would fall on the earth if your home were not there, so, again, unless I am missing something, all you need is the square footage of your home, and since you can only guess at the amount of a typical rainfall anyway, you could take that amount, and divide it by the number of downspouts that you have, in order to obtain an approximate volume per spout; the roof area, factoring in the pitch, is irrelevant. You probably already know the calculations, but there are 231 cubic inches in a gallons, so take your square footage, multiply it by 144, and divide by 231 to obtain the number of gallons per inch of rainfall.

tomseeley 02-10-2011 01:39 PM

well, duh..........

That makes such perfect sense, I'm embarrassed that I didn't think of it! You're right; I was definitely making it more complicated than I needed to!

Jackofall1 02-10-2011 01:46 PM

Ok just sent that to you with a brief explanation on its use.

Dexter, you hit that one right on the nail head, sometimes you just can't see the forest through all them darn trees:thumbsup:

Mark

DexterII 02-10-2011 03:49 PM

As my dad would have told me, even a blind squirrel finds an acorn once in a while! Glad to help.

seeyou 02-12-2011 06:13 AM

Quote:

Originally Posted by DexterII (Post 588323)
Maybe I am reading something wrong, but I believe that you are incorporating too many non-essentials in the equation, and that you can do everything that you want on the ground, with no regard to roof slope, using only a tape measure. The amount of rain that will fall on your roof is the exact amount that would fall on the earth if your home were not there, so, again, unless I am missing something, all you need is the square footage of your home, and since you can only guess at the amount of a typical rainfall anyway, you could take that amount, and divide it by the number of downspouts that you have, in order to obtain an approximate volume per spout; the roof area, factoring in the pitch, is irrelevant. You probably already know the calculations, but there are 231 cubic inches in a gallons, so take your square footage, multiply it by 144, and divide by 231 to obtain the number of gallons per inch of rainfall.

For this question, your answer probably serves the purpose well enough, but for properly sizing a gutter system, more factors come into play.

The statement I highlighted is true only if the rain is falling completely vertically which seldom is the case. One side of a gable will usually receive more water than the other due to rainfall angle, so the sizing needs to be related to roof plane area/slope rather than building footprint. A 12/12 roof next to a 6/12 roof with the same footprint will catch more water.

Here's a sizing guide:
http://www.guttersupply.com/file_are...ing%282%29.pdf

Joe Carola 02-13-2011 11:12 AM

Quote:

Originally Posted by tomseeley (Post 587071)

What can I do to get a better idea of how big the roof is?

Go in the attic and measure the length of the rafters from outside plate t the ridge. Add for the overhang. You can't just guess at the length.


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