I am making a homemade truss for an addition to an existing barn. I
have never been any good at math. The barn roof was built to be a
4/12 pitch. The barn walls are 107 1/2 inches outside front to
outside rear. I measured the existing rafters, and they are 113 3/8
inches long from the highest part of the rafter (at the peak). I used
an angle tool (tool to set to the same angle and transfer it to a
board), and I cut the boards at the same angle. OK, I transferred the
angles to the boards and cut them the same. Then I loosely nailed a
piece of plywood across these 2 planks where the angles meet (at the
ridge). I left the lower ends (overhang) a little long to adjust the
height, so it's right. That's where I get puzzled. I want to be sure
the height is the same. How do I figure this out mathematically, or
am I better off just stretching a string across the existing roof at
the top of the walls, then measuring from the peak to the string, and
moving the string to the homemade truss, and make sure the height from
string to peak is the same, before I nail the horizontal board across
the truss.

If this was an outside wall, it would be fairly simple to make a 2x4
from the top of the wall to the peak, and be sure its the same as the
existing roof. However, this truss is being placed from pole to pole,
in the middle of the barn.

If I am not exactly clear, here is what I am doing.
I bought and moved an existing pole barn. To move it, I cut it in
half because it was too large to pull down the road in one piece. I
now have both halves. One half is in the ground, the other half is
still on the wagon frame that I used to move it. I am adding on to
the barn as I put it back together. But I am adding in the middle.
I dont plan to put the half still on the wagon into the ground until
the new portion in the middle is built. The reason for that is so I
can shift around the other half in their (oversized) post holes until
the steel roofing and siding sheets fit together properly.

The reason I am telling all of this, is because I can not just run a
string from the roof of one half to the roof of the other. I wish it
was that easy......

Yes, this has been a complicated ordeal, but it will work out in the
end......

Thanks

Mark