Roof Pitch; 4/12 or 3/12?

[BukitCase]

Play with this awhile
Maximum Span Calculator for Wood Joists and Rafters
(it's also available as a phone app)
You can plug in the species that're available in your area, then enter the same variables and see the differences.

board length - if you draw out (to scale) your trusses, keeping chords so the rafter span is divided into fairly equal parts (I'm assuming the long length is for the BOTTOM chord) the drawing will suggest to you which shorter lengths should fall where your diagonal chords will fall. You can tweak the chord angles a little either way if it helps things come out better. I personally do NOT buy the exact length I need anymore, there's too much crappy wood out there to guarantee the ends will not be screwed up (knots, cracks, splits, etc) - Also, the sad state the lumber industry's in these days I ALWAYS calculate with #2, then spec #2 and BETTER (it's usually NOT much better)

If your yards will let you pick thru stacks, you might get away with actual length.

If you're spanning 26 feet and using a gable end, you're really only spanning 13 feet on each side; so if you're using (say) Douglas fir, 2x6 on 24" centers, a quick check shows 9'6" max span - if I were doing that, I'd have two diagonals per half of the roof span - they would meet the rafter in the center of its 13' span, and the bottom chord at 1/6 and 1/3 of that half's 13' span. Same on the OTHER half span (the opposite side's roof slope)

Remember, all the angled chords and the rafter (top chord) connections will be mostly in compression, and the bottom chord (splice) will be in TENSION - If you use nails, you'll want ring shanks the same length as your total thickness (for 2x material + 2 thicknesses of 1/2" ply, that'd be 2-1/2" nails ALWAYS DRIVEN SO THE NAILS WILL SELF-TIGHTEN, NOT LOOSEN.

when I built mine in place, I set them up so the two chords on either side of roof peak met in the center of span, as did the bottom chord. Those got a 2'x3' truss plate of 1/2" ply, plenty of titebond, bunches of clamps, and lots of toenailed ring shanks. That joint especially didn't get un-clamped til the glue squeeze-out started turning color (since it's the ONLY one in TENSION).

At this time, that roof has been thru 11-12 winters (on my hill, that pretty much always means several 70-80 mph storms) - it hasn't moved a bit... Steve

 

[Diggin It]

This is sort of what I'm looking at. This way, no two splices (where the colored lines meet) on the bottom chords would be at the same point on any side by side trusses.

 

[Diggin It]

when I built mine in place, I set them up so the two chords on either side of roof peak met in the center of span, as did the bottom chord.

The top/angled chords will be 2 x 6 x 14 boards, no splice. They measure out to be about 160", so I only lose 8" or so off a 14 footer.

 

[BukitCase]

There's no reason that won't work; I would just get all 14' in that case, and pick the best ones for your bottom chords. Those are the ones that'll be in TENSION, so they're gonna want to pull apart. The rest will be PUSHING. CDX ply 1/2" for your truss plates, a bit larger ones (both sides of the joint) and glue/ring shanks (most of the nail HEADS further away from the splice point than their points are) and you should be golden... Steve

 

[the old grind]

I was about to suggest, and refer to the pics of, buckitcase's joist plates as shown in post #28 as he posted all that helpful stuff since. :thumbsup: My 40'x40' barn's trusses are 42' long on 4' centers with 2x4 'webs', and horizontal nailers above to support the sheeting. As bc said, all truss components are either in tension or compression.

So my bottom chords are made up of 3 16' 2x6s ea and are joined by 'butt-connecting' plates only vs 'corner' or 'tee' plates. So what Diggin It shows above could be like mine by not have a bottom chord (tension) plate on the center of the truss where verticals likely converge. I might then use 16' and 12' to allow 6" overhang on the trusses and eaves.

 

[Diggin It]

Hate to keep buggin' ya'all, but this is what I came up with after studying a bunch of truss maker diagrams and what I have in the garage that was stick built. This is sort of a modified design based on a few others. Bear in mind this was sketched out in MS Paint, so it's not even close to scale. Second picture is an attempt at a top view top show that the webs would be nailed to the sides of the top and bottom chords in an alternating pattern to hopefully help avoid any future twisting. One would be nailed and glued across the splice between the 12 and 14 footers with a truss plate on the other side, nailed through both.

Turns out the piece I cut off the top chords fits perfectly as a wedge between the top and bottom at the ends, so that will be added and glued in place.

Will this work? Do I need more webs or rakes?

 

[BukitCase]

Top chord (rafter) looks fine, longest span is under 7' - The "mini-spans" on bottom chord are a bit long, I marked up your sketch a bit.

It's fine to put your cutoffs on the end like that, but don't expect it to do more than just fill the corner - for strength on those end joints, you will need to make truss plates shaped to that end - top edge of plate follows the outer edge of rafter, bottom edge follows the bottom edge of horizontal chord.

Looks like you're using MS Paint - couple tricks I use on that (I do more precision stuff in Cad) - if you want lines at perfect 90 or 45 degree, start the line and hold the shift key - mouse position will determine angle, and will shift between horizontal, 45* or vertical - try it, it's handy. Also, if you're semi-**** like me you can "sorta" SCALE a drawing - move your mouse while looking at the borders, you'll see a pixel count change as you move the mouse. Choose a rectangle shape, click on far left of the screen, drag right and a little bit UP - one of the small indicators will show the size of your rectangle. If you drag full width of your screen, that will tell you how many pixels the width is. If you make that skinny rectangle come out to an even # of pixels, you can draw subsequent shapes to 1/2, 1/3, 1/4, etc, and your drawing will be less likely to "fool" you.

Also, the ellipse tool will make a true circle if you hold the shift key while drawing it - if you want several identical circles, you can copy ONE circle with the select tool; that puts your selection into the clipboard, and you can copy and drag it anywhere on the drawing.

I spent breakfast time looking thru pics, must have a few HUNDRED of the shop and NONE show much of my trusses. sometime today I'll go take some pics of mine for you.

As I mentioned before, I would NOT try to use the entire length of lumber, ESPECIALLY on your bottom chords - there's too much likelihood that you'll end up using ends that are split or knotted, which will SERIOUSLY WEAKEN those TENSION joints. You'd be better off centering all your bottom chord splices and making that joint with THREE inside chords - one vertical to roof peak, and two more angled outward @ 45*, then dividing the remaining rafter length. I'll explain more later today, after I get a few pics for you... Steve

 

[3Ts]

You don't have triangles in your truss. Your truss should look like one of these.

 

[BukitCase]

T's, absolutely; the ones I haven't posted pics of (yet) are a modified Queen. The changes I drew (above) are just a little improvement over the original. And DigginIt does have some triangles, just not everywhere... Steve

 

[Diggin It]

A lot of those don't really have triangles. What I did is based on those designs, dividing the spans into manageable sections and combined them with what I see in my own garage that has been standing for close to 20 years. That is sort of an inverted Fan with a vertical down from the ridge to the center of the bottom chord and just one web angled down and outwards from near the ridge to the center of remaining width of the bottom chord.