Help! roof truss length

[FrankInWIS]

I half the 1 1/2 plan from John.  My son is about to go up to our land and start building the trusses.  We sat down to figure material, and we're confused.  The plan, and my son's class in school said for a 12 / 12 roof for each foot of height there is 16.97 of material.   Well, it's 9' 3" 3/4 inch in height and that times 16.97" would give us a a length of of about 13.15 feet.  That plus the extension would be about 14 and 1/2 feet.  

The drawing says 1/2" = 1 ft.   When I measure the tuss on the plan I come up with 8 1/4 "  so that would be 16 1/2 feet.  

How are we goofing this up?  Help!

[John_C]

I don't have the plans and your description is a bit confusing.  I presume you are trying to calculate the top chord of the truss for a 20' wide building.  Correct?

According to my math with a 12/12 pitch..... 20' wide building ..... 1'8" horizontal overhang ... the top chord would be 16.49'    

Where does the 9' 3-3/4"  come from???  Are measuring inside a 4" stud wall, instead of from the outside?

If all else fails borrow a trick from the boat builders and draw (loft) the thing FULL SIZE on the floor.  

[FrankInWI]

Let my Son Explain.... I goofed up the explanation.  Here's Ben!

          We can't find our Rafter Line Length i don't know lol I know what you need to clculate it but i forgot how can anyone calculate this for us please?        

        Span: 20'
        slope: 12 to 12
        Table#: 16.97"
         O.H: 1'

Please help, i hope to leave in a few days and start building!
       

[John_C]

It's still a bit confusing.  Above you said you were laying out site built trusses and now you said rafters?

Anyway.  The length is 16.97" per foot of run.  

1/2 the span is 10ft. + 1ft. overhang is 11ft.

11 x 16.97  =   186.67"  or   15.56 feet   or just a tad over 15' 6-5/8"   IF this is a rafter you would need to take into consideration 1/2 the width of the ridge beam.

This is a easier to layout full size than to explain.

[FrankInWIS]

Thanks!  He had the right formula but we must have been doing some poor converting.  Now he can go to work.

[John Raabe]

The easy way to do this is to snap it out full size on the deck.

Old carpenters never used math when a template could be made.  

[FrankInWI]

John, just you saying "snap it out" enlightened me SO much.  I realize I could do it with a caulk line and that is so much easier and I can use that for other things too.  Thanks for the tip, you made Ben and I better junior carpenters.