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02-08-2011, 12:54 PM   #1
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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?

02-08-2011, 03:46 PM   #2
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Do you have excel, I could send you a program that I built.

Mark

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 02-08-2011, 03:53 PM #3 Member   Join Date: Dec 2010 Location: Central Ohio Posts: 227 Rewards Points: 156 Post your photos and sketches--it would really help. I believe the only angle you will need to know is 90°.

 02-08-2011, 04:43 PM #4 Experienced     Join Date: Dec 2010 Location: Southern Michigan Posts: 2,822 Rewards Points: 2,000 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 __________________ When its all said and done there is usually more said than done Last edited by Jackofall1; 02-08-2011 at 09:12 PM.
 02-10-2011, 12:58 PM #5 Member   Join Date: Jun 2009 Posts: 50 Rewards Points: 41 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.
 02-10-2011, 01:24 PM #6 Member   Join Date: Jul 2010 Posts: 2,146 Rewards Points: 152 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.
 02-10-2011, 01:39 PM #7 Member   Join Date: Jun 2009 Posts: 50 Rewards Points: 41 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!
 02-10-2011, 01:46 PM #8 Experienced     Join Date: Dec 2010 Location: Southern Michigan Posts: 2,822 Rewards Points: 2,000 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 Mark __________________ When its all said and done there is usually more said than done Last edited by Jackofall1; 02-10-2011 at 01:57 PM.
 02-10-2011, 03:49 PM #9 Member   Join Date: Jul 2010 Posts: 2,146 Rewards Points: 152 As my dad would have told me, even a blind squirrel finds an acorn once in a while! Glad to help.
02-12-2011, 06:13 AM   #10
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Quote:
 Originally Posted by DexterII 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

02-13-2011, 11:12 AM   #11
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Quote:
 Originally Posted by tomseeley 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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Joe Carola

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