HELP How to use Farming Square

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Blog entry by jim1953 posted 12-13-2009 10:38 PM 2439 reads 0 times favorited 23 comments Add to Favorites Watch

Iam Building a New Shop And Want to Build My Own Roof Rafters the out side is 28ft by 28ft I want a 4/12 Pitch with 1ft over hang Iam going to use 2by6 how long do I need I need HELP with the square THAHKS Jim

-- Jim, Kentucky

23 comments so far

View NBeener's profile


4816 posts in 3682 days

#1 posted 12-13-2009 11:08 PM

View smartlikestick's profile


54 posts in 4087 days

#2 posted 12-13-2009 11:30 PM

Hey Jim

To calculate the length of the trusses, you can just use Pythagorean Theorm. To calculate the horizontal length of the truss, you would take the width of the overall structure – 28’-0”, divide it by 2 and add the 1 foot overhang totalling 15’-0”. Based on the 4-12 pitch, the height at the peak of the truss would be 5’-0”. Therfore, using the pythagorean theorem, the length of the diagonal member (measured along the outside face) would be ~15’-9 3/4”.
I would caution that the building code where I am from requires engineered stamped drawings on roof trusses, as they are a main structural member and these drawings will be required at the time of inspection. In addition to the top and bottom chord, you will require intermediate members to distribute the load through the truss to clear the span and the connection points also need to be reviewed as they will carry considerable load. In my experience building our last garage, the cost of pre-manufactured (and engineered) trusses were barely more then the cost of the lumber. With the difficulty to construct these to ensure a nice flat looking roof, I would consider this a better option. Just my opinion…

-- -- Mike Beauvais

View greg's profile


88 posts in 3705 days

#3 posted 12-14-2009 12:38 AM

you could get you a 6 inch or 12 inch speed square which comes with a book on how to with illustrations and the lengths and or formulas. you probably should check on building codes but where i live we build what we want with no questions asked.

View F Dudak's profile

F Dudak

342 posts in 4319 days

#4 posted 12-14-2009 01:17 AM


Follow the first link that Neil gave you. There is some explanation there that should be good for the novice. Taking on a roof is no easy task so you best be prepared to falter. Engineered trusses may be your best bet if you have no plans for bearing walls in the structure. Getting help from some one who has done this kind of work before would be my advice as well. If you are looking to save some money doing it by yourself, it may be wise to pay some one looking to make a few extra bucks on the side that is working for a framer.

-- Fred.... Poconos, PA ---- Chairwright in the making ----

View CaptainSkully's profile


1611 posts in 4067 days

#5 posted 12-14-2009 02:53 AM

I use the thumb screws to lock in the angle on the framer’s square, once I’ve got it laid out against the board. It helps with accuracy and repeatability. Trusses are one of the most fun and frustrating parts of carpentry. They’re right up there with stringers. If you leave the rafter tails long, you can always trim them to length later at the proper plumb soffet angle, as needed. It’s easier on the layout, harder on the cuts. Good luck!

-- You can't control the wind, but you can trim your sails

View jim1953's profile


2738 posts in 4350 days

#6 posted 12-14-2009 03:45 AM

The truss are 1140.00 dollors The Lumber to Make Roof Rafters is 225.00 dollors I can save 915.00 dollors Ive got more time then moneyThere is no codes here my House has man made Rafters
Thanks For All The Help I Appreciate

-- Jim, Kentucky

View GaryK's profile


10262 posts in 4497 days

#7 posted 12-14-2009 03:53 AM

Here is an old book that covers just about every use of a framing square. It’s about 23MB but worth it.

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

View FredG's profile


139 posts in 4205 days

#8 posted 12-14-2009 04:17 AM

View TopamaxSurvivor's profile


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#9 posted 12-14-2009 04:58 AM

Assuming you need to span 14 feet for the 28 foot building, the no math framing square method is to lay the 4” mark on one side of the square on the edge of your board. Now lay the 12” mark on the same edge. Your square should be froming a triangel above your board. Make a mark, move this setup 14 times and there yoiu have the lenght of the rafter, add another for the overhang. If you want to use a math method, look above ;-))

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View hunter71's profile


3439 posts in 3695 days

#10 posted 12-14-2009 01:59 PM

Guess you don’t need my help, just encouragement. Lots of good advice already given. Lets see pictures when it’s done.

-- A childs smile is payment enough.

View nailbanger2's profile


1041 posts in 3652 days

#11 posted 12-14-2009 02:30 PM

Hi, Jim, I’ve got a question for you. How much extra would it be if you used 2×8 s ? 2×6s just seem a little small for that span. You can probably find a lumber load site online.

-- Wish I were Norm's Nephew

View nailbanger2's profile


1041 posts in 3652 days

#12 posted 12-14-2009 02:34 PM

#13 posted 12-14-2009 02:39 PM

home depot sells a calculator for framing its the best tool i have in my pouches it will figure out your lengths and your cuts

-- i wonder if obama stood in a wind storm with them big ears of his would he start spinning like a drill bit

#14 posted 12-14-2009 02:57 PM

jim the length is 15ft 9 3/4 in for your rafters that is from peak cut to tail cut or long point to short and the rise is 5 ft .you might want to think of going with 2×8 that is quite a span for 2×6 rafters and i would recomend puting them on 16 in centers

-- i wonder if obama stood in a wind storm with them big ears of his would he start spinning like a drill bit

View nailbanger2's profile


1041 posts in 3652 days

#15 posted 12-14-2009 06:49 PM

Hey tunk valley, I didn’t even think of the centers, just assumed they would be 16 o.c., as all non-engineered rafters are set at that according to our local code. Good point that may have been overlooked.

-- Wish I were Norm's Nephew

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