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Author Topic: I need some help with scissor truss layout  (Read 21534 times)

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Offline Don K

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I need some help with scissor truss layout
« on: March 29, 2008, 07:13:25 pm »
I am sawing out all the lumber I need to build a 24' X 36" woodworking shop. I want to build scissor trusses so that I can have some extra clearance on the ceiling. I would like to use a 6/12 roof with a 3/12 ceiling. I have found plenty of sites that have the cut dimensions for conventional trusses but I can't find anything for scissor trusses. What I have found is drawings showing what a scissor truss looks like, but I know that as I used some in my house. I just can't find a measurement layout. Can anyone help me? My span is 24' even with a 16 inch overhang.

Don
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Offline Don P

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Re: I need some help with scissor truss layout
« Reply #1 on: March 29, 2008, 08:55:59 pm »
I love the written word  :D
So are the walls 256" from outside to outside then a 16" overhang making the total truss dimension 288"?

Offline Don K

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Re: I need some help with scissor truss layout
« Reply #2 on: March 29, 2008, 09:04:47 pm »
Now you are asking me to do math. :D I come up with 288 inches wall to wall plus 16 inches twice added for a grand total of 320. ???

Don
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Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #3 on: March 30, 2008, 02:18:23 pm »
How much math can I make you do  ;D
These are usually called "line length ratios". I've shown the ratios between sides for a 3 and a 6/12.



Is that enough for you to figure it?

Offline Don K

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Re: I need some help with scissor truss layout
« Reply #4 on: March 30, 2008, 09:23:32 pm »
 :-\ Don, I hate to sound stupid, but that is as clear as mud to me. :'( If I knew what all the numbers meant you might have to put on shades because of all the lightbulbs going off. I'm not much of a carpenter or mathematician. I did build most of my house and a tool shed and sawshed. If I have a good diagram in front of me I can figure it out.

Don
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #5 on: March 30, 2008, 10:50:44 pm »
Well, let's cheat  ;D
If you have sketchup on your machine I can send you a drawing that you can interact with (I think).
here's a snapshot of what I have


In a cad program you can click on a point and pull a tape to another point and get a measurement... hard to beat that if its accurate.

The ratios I gave you earlier are another way. In the drawing you can see some dashed lines that run from the outside wall corner (where I drew a short line to represent the outside wall surface) through the midpoint of the building. These are the planes I am measuring along.

The bottom edge of the upper 6/12 chord should be 144" (the distance from outside of wall to midpoint of the building) X 1.1180 = 160.992", Pulling the tape in sketchup it said that was 13'5". A track a couple of inches into the timber is the measurement line on the bottom chord in my drawing, I multiplied 144x1.0307=148.42" Sketchup called it 12'4-1/2". To avoid doing the math its hard to beat drawing to scale and using it for a cut list. We work beyond the reach of the computer most of the time, setting up the ratios and using them is fast and accurate.

I drew everything in #2 syp 2x6 for trusses 2' on center and thought 36 8d nails in each member in each heel joint looked good, all that's just my guessing though.

Offline Don K

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Re: I need some help with scissor truss layout
« Reply #6 on: March 30, 2008, 11:35:41 pm »
Don, That's getting closer to what I was trying to find. I was hoping to get a length  from edge of eave to centerline on top chord and of course the edge of wall to center on the bottom chord. I also know that the cut degrees where top and bottom meets at the wall will dtermine whether or not you get a screwed up pitch angle. The company that built my house trusses was 2.5 degrees off on the back side of my house. Luckily I cut a test porch rafter ( hand hewn white pine 4X6) for the back porch. 3/12 porch roof onto a 6/12 roof. The front porch was right on the money.

If I can get those two right, I think everything else will fall in place. I am cutting full dimension 2X4 to build the trusses on 2' centers.  I don't have any type of cad programs.

Don
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #7 on: March 31, 2008, 08:10:41 pm »
Quote
I was hoping to get a length  from edge of eave to centerline on top chord

We'll take 320"/2=160" horizontal run, 160" x 1.1180 = 178.88" ,  or 178-7/8" from plumb cut to plumb cut. The plumb cuts are at 6/12 or 26.5 degrees.


The mark measured from the short point of the upper plumb cut 161" down the bottom edge is where the point of the 3/12 bottom chord will touch.




Offline Don P

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Re: I need some help with scissor truss layout
« Reply #8 on: March 31, 2008, 08:55:07 pm »
This is the bottom of the 3/12 meeting the 6/12 at the point you marked 161" down the TC.


First mark the level seat cut at (90*-14*)=76 degrees. Measure from the heel back along that cut left 3-1/2" and make a mark.

A 6/12 is pitched at a 26.5* angle, a 3/12 is at 14*, so the angle between them is 12.5*. 90-12.5=77.5*. The top angle of that joint is swung at 77.5 degrees and intersects at the mark you just made.

Cut those 2 angles. Hook your tape on the point and pull 288/2 * 1.0307=148-7/16". Make a mark 7/8" in from the bottom edge that intersects this point (that keeps us on that straight dotted line). Swing a 4/12 plumb cut that intersects this point. As a check, from short point to short point along the bottom edge would be 144-3.5=140.5 X 1.0307= ~144-13/16"

Still with me?

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Re: I need some help with scissor truss layout
« Reply #9 on: March 31, 2008, 11:31:42 pm »
To make sure I understand your terminology. :-\

The level seat cut is where the bottom chord will sit on the wall. The heel is the edge of the board closest to inside edge ofwall. That mark at 3.5 is represented by the dotted line running the length of the 3/12. Are these measurements still based on 2x6's.

The 77.5 top angle represents the long cut next to the 6/12.

I think I'm getting it. Thank you Don for being patient with me. I know you have better things to do with your time. On each truss I will write designed by Don P. If it is screwed up it was cut by Don K. ;D :D
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #10 on: April 01, 2008, 12:29:58 am »
You've got it  :)

I was adjusting to 2x4 as I went.

Most of the measurements are along a "line". In this case the dotted line is the line I'm calculating to, it runs from the top outside corner of the wall. The seat cut means that the edge of the 2x is somewhere other than on our "line". To find that 7/8" offset look at the triangle formed by the seat cut the dotted line and an imaginary side plumb from the heel ...  3.5" level seat X .25 side ratio=.875" plumb from heel to dotted line, a true 7/8" but measured square to the seat. To convert that to parallel to the edge it takes another small imaginary triangle, divide .875 by 1.0307= .848" a shy 7/8". That gives me the location of the line in relation to the 2x4's edge for the top plumb measurement.

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Re: I need some help with scissor truss layout
« Reply #11 on: April 01, 2008, 12:49:02 pm »
Your killing me with this math. :D

Let me ask another question. Would another way to figure it be laying out a 6/12 rafter plus eave and then a 3/12 rafter and cut them where they intersect.
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline isawlogs

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Re: I need some help with scissor truss layout
« Reply #12 on: April 01, 2008, 01:46:54 pm »


  Don K   Thank-you , man I thought I was the only one with math developmentally delayed  issues  :'( :'(
A man does not always grow wise as he grows old , but he always grows old as he grows wise .

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Offline Don P

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Re: I need some help with scissor truss layout
« Reply #13 on: April 01, 2008, 10:24:02 pm »
Could be I just ain't no good at explaining this  :D
See if'n this helps.

Here's the basic top chord and bottom chord, hey I used scrap, its a 5' wide building  ;D


I think the top chord is pretty self explanatory this is the level seat cut being layed out, I'm swinging a 76* angle on the bottom of the bottom chord.


Making a 3-1/2" mark along that  seat cut;


Making the 77.5* layout for the top chord intersecting with the seat cut.


Showing that the intersection does occur about 7/8" from the edge of the bottom chord


At the top of the bottom chord the length measurement is 7/8" up from the edge, a 3/12 plumb cut is marked through this intersection.


Bottom chord hits alignment mark on top chord at the desired building width.




Offline Don K

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Re: I need some help with scissor truss layout
« Reply #14 on: April 01, 2008, 10:43:48 pm »
Don, You crack me up. I open this thread and see scrap 2X4's on your dining room table. The wife musta been gone. :D  As they say " A picture is worth a thousand words". You have gone above and beyond the call of duty. That is how I had it worked out in my mind, but now it is even better.

I've got two of those speed squares and all I ever use them for is marking a straight cut. Nobody has ever showed me how to use them to draw angles. Thanks a bunch.

Don
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #15 on: April 02, 2008, 08:28:36 am »
Hopefully that also shows that the joints are the same no matter what size the triangle, only the lengths of the sides chang, and they do so in the ratios described above.

To mock that up I went down to the barn with the ratios and a calculator and kicked up that scrap. I then figured it could make a 5' mockup with 6" overhangs.
top chord was 36"x 1.1180 overall long
Intersection mark was 30"x1.1180 down the underside
Bottom chord was 30"x 1.0307 overall long

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Re: I need some help with scissor truss layout
« Reply #16 on: April 02, 2008, 08:50:19 am »
Don, Since I don't know how to get the most from my square, what did you use to orient the square when you cut the 3/12 plumbcut in the sixth pic. Did you use a known degrees or what I use to cut rafters with a framing square... one edge on 3 and the other on 12.

Don
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #17 on: April 02, 2008, 08:40:56 pm »
I'm swinging from the edge of the lip where one side of your square will say "pivot". Mark on the left side not along the hypoteneuse


I've swung till it reads "3" on the "Common" scale, this also shows on the angle scale that a 3/12 is 14*


So you can use that square to mark off either in pitches or in degrees.

That other scale marked hip/val. Find some tape, the really sticky kind that never wears off. Cover that scale with it  ;D

That broken lipped square in the pics is a retiree, one like that won't pivot accurately.

You can double check pitches with a framing square. I think Jim Rogers has a file posted somewhere converting framing square pitches to degrees also.

Offline Don K

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Re: I need some help with scissor truss layout
« Reply #18 on: April 02, 2008, 09:47:19 pm »
Thanks. There is a lot I wish I knew sometimes.

Don
Lucky to own a WM LT40HDD35, blessed to have a wife that encouraged me to buy it.     Now that\'s true love!
Massey Ferguson 1547 FWD with FEL  06 GMC Sierra 2500HD 4X4 Dozer Retriever Husky 359 20\" Bar  Man, life is getting good!

Offline Don P

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Re: I need some help with scissor truss layout
« Reply #19 on: April 02, 2008, 10:06:43 pm »
Me too, she usually takes me back anyway  ;D.