[FuryRoad]

Please tell me if I'm doing this right. First I decide what ratio/pitch room I want for my area, I think. So say I picked 3/12.

My shed span is 6 feet, so the run is 3 (6/2).


I then did 3^2 + 12^2 and got 153". The square root of that is sqrt(153)=12.37"

As I understand it that is called the "bridge length." From there I multiply that times the total run or 3*12.37 = 37.10".

Is that right?

 

[Californiadecks]

With a construction calculator. There's an app for that!

 

[TimPa]

not sure of the terms you are using.

lets say you have a ridge roof (has a peak in the center) and the total span is 6 feet. and you chose a 3/12 pitch. this means that the roof will rise 3 feet for every 12 feet of run.

since a single plane of the roof is only 3' of run (12'/4), the rise, or peak will be 9 inches (3'/4) above the top sill plate.

using pyth theo, (inches) side one squared + side two squared = square root of hypotenuse (rafter length). or 36^2 + 9^2 = 37.1".

just remember that this dimension would be a line, measured on the bottom of the rafter as it sits on the top sill plate and up against the ridge board. then I usually draw out (subtract) half the thickness of the ridge board before you make the cut. angle calculations are another story.

 

[danpik]

You just need to figure in the thickness of the ridge board

 

[Bud9051]

Designing your rafters is a perfect example of a picture (drawing) is worth a thousand calculations.
First, will you have an overhang on each side and how much?
Will your overhang be TCO or BCO? Top chord overhang or Bottom chord overhang?
Will you be cutting a birds mouth for the rafter to seat onto the top plate?
How will you finish the overhang for a fascia board if used?
Will you be using a 2x6 or 2x4?
How thick will the ridge board be, 1.5" or ¾"?

Decide on what you are going to use and draw a sketch of it. Note, the top edge of the rafter may be a different length than the bottom. Then you can start calculating and you will need to adjust your final number for whatever is planned on each end. Note, a 1.5" ridge board will subtract a bit more than ¾" because of the pitch.

Bud

 

[joed]

This page explains it well with pictures. Just be sure you are working with all the same dimensions, feet or inches or meters, etc.

http://www.carpentry-tips-and-tricks.com/Rafter-lengths.html

 

[Mingledtrash]

Please tell me if I'm doing this right. First I decide what ratio/pitch room I want for my area, I think. So say I picked 3/12.

My shed span is 6 feet, so the run is 3 (6/2).


I then did 3^2 + 12^2 and got 153". The square root of that is sqrt(153)=12.37"

As I understand it that is called the "bridge length." From there I multiply that times the total run or 3*12.37 = 37.10".

Is that right?

its right but its not how i do it. this is how i do it using a ratio and proportions method(after all 3/12 roof is a ratio).

first thing with a proportion is that both ratios have to be the same unit of measure so because you are using inches in the the 3/12 roof pitch ratio you must use inches for all the calculations so 3' is changed to 36"

so you would have 3/12 = x/36 and solve for x.
3*36=12x so 108=12x then divide both sides 108/12=12x/12 so 9=x now you know your rise is 9" and your run is 36" then apply pythag 9^2 +36^=rafter^2

so 9^2=81 and 36^2=1296 so 1296+81=1377 and sqrt(1377)=37.10795

so rafter length should be about 37 1/8"

 

[joecaption]

http://www.blocklayer.com/roof/raftereng.aspx