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# extension calculation.

## extension calculation.

#1
01-22-06, 07:13 AM
Member
Join Date: Jan 2006
Posts: 4
extension calculation.

Hi, i've just had planning permission to build an extension. The roof of the ext is going in the opposite direction to the existing roof and is going to link at the ridge with 2 valleys.
I would like to know what the degree of roof pitch is going to be on the new extension from using the span between walls.
Does someone know any formula for this to be calculated.
Many thanks.

#2
01-22-06, 09:53 AM
ollie
Visiting Guest
Posts: n/a
Any of these book will show you how to lay out your roof.
"Measuring, Marking & Layout" A builders guide. John Carroll
"Precision Framing" Mike Guertin and Rick Arnold
"The Very Efficient Carpenter" Larry Haun
Try Amazon. this sight or your local library.
HTH
Ollie

#3
01-22-06, 10:40 AM
Member
Join Date: Jan 2006
Posts: 4
Thanks for the reply ollie. However i'm not looking for a book as i'm having a pro roofer do the work for me. it's just out of interest that I would like to know my previous question. I'm sure theres somewhere on the net to give me the calculations as I found a calculation to work out pitch degree.
Cheers.

#4
01-22-06, 12:11 PM
Group Moderator
Join Date: Dec 2004
Posts: 27,105
That's simple trigonometry. If you know 2 sides of any right triangle, you can find the length of the third side and the other 2 interior angles. Half of your new roof will be the right triangle. One leg of the triangle will be the distance from the top plate to the peak (minus the depth of the rafter), and the other leg of the triangle will be half the distance from fascia to fascia. This won't be exact, but it will get you in the ballpark.

You can find trig calculators using google by entering: +trigonometry +calculator

Here's one I came up with: http://www.carbidedepot.com/formulas-trigright.asp

#5
01-22-06, 01:58 PM
Member
Join Date: Jan 2006
Posts: 4
Thanks for the reply xsleeper. I don't know if i'm being thick but I cannot grasp what you are saying. Could you give me an example from the following.
From one wall to the other, there is a space of 30' and from floor level to the ridge there is 15'.
Cheers.

#6
01-22-06, 09:06 PM
Group Moderator
Join Date: Dec 2004
Posts: 27,105
forget the floor, you're trying to calculate the roof. Picture the roof itself minus the walls. If it's 15' from the floor to the peak, subtract the wall height. If you have 8' walls, then you have roughly 7' of roof height. If the room will be 30' wide, you need to know how wide your soffit will be. Some houses have 2ft soffits, which would make the total distance from fascia to fascia 34'. Divide that by 2 = 17'. So you would have a rise of 7' and a run of 17'. Enter those numbers into the calculator to determine both the hypoteneuse and all the interior angles.