Flat Felled Seam Question

[Hoosier T]

I usually use a French seam for shelters but am considering trying a felled seam. My though is when holding and sewing the two panels together for the first stitch line, you usually line the raw edges up then trim the one that’s inside the fold. However, I was thinking about holding the panel you would normally trim a half inch inward of the other edge. This would not only prevent the time and wasted material from trimming, but leads me to my next hunch. Wouldn’t this automatically center the seam for you? If you do a normal FF seam and don’t accommodate for the overlap when cutting (I.E. the two panels are of the exact same width) the center of the seam does not end up being exactly centered on between the two panels. The true center would normally be where your first joining stitch line was sewn then when you fold the seam over and sew again, you’re actually creating an offset seam.

Am I thinking through this correctly? If I cut both panels to the same width and pre-offset the edges when starting the seam, wouldn’t the seam end up being center? Anyone else do this?

[jimmyjam]

With silnylon, there’s enough stretch to it that the seam being slightly off center has never given me a problem.

[Hoosier T]

Yea, I have never worried about it until now because I’m going to make my first cuben shelter. It is beaked, similar to a hexamid and I am most worried about the seam on the beak being center since the two panels are smaller than the others.

[Mole J]

The offset way you describe,  is the way I learnt to do it from Ray Jardine’s Tarp Book.  Don’t really see the point of not doing it that way, as trimming a long edge of silnylon is not my favourite job.

My tarps appear to be equal each side, but I can’t guarantee the theory.

[Hoosier T]

That’s good news! I actually ended up confirming that the halves match after I posted this using some paper but it is reassuring that this is already being done. I couldn’t think of any possible caveats but you never know. So I’ll just mark a half inch in from the edge of the one piece, line the second edge up with that mark, then I’ll sew all the way down a half inch in (one inch from the longer piece). This will leave me with a nice and centered 1/2″ seam that I can tape over. Sounds easy enough!

[Hoosier T]

FWIW, I have tried hard to talk myself into bonding the seams but it scares me. I’m actually MUCH more comfortable sewing, especially with cuben, it’s just so easy to sew and Zpacks construction proves it will hold up when taped after sewing.

[Lance M]

I’ve done it the way you describe on a half dozen tarps and tents.  Equal seam allowance all the way around both fly halves.  Halves are offset 2/3 of the seam allowance.  The first stich line is 2/3 of the seam allowance from the ‘short’ half and 1-1/3 of the seam allowance from the ‘long’ half.  For instance, if you used a 3/4″ seam allowance the offsets would be 1/2″ and 1″.

In these pictures, the seam allowance was a full inch (I think most folks use less, but it makes a nice 1/2″ rolled edge all around). Edges are offset 2/3″, first stich line is 2/3″ from ‘short’ edge and 1-1/3″ from ‘long’ edge.

Second stitch line after folding the longer seam allowance over the shorter seam allowance:

Seam comes out centered which facilitates centered reinforcements, tie-outs and beaks.

(Edited to 1-1/3 from incorrect 1-2/3)

Hope this has been  helpful.

[Hoosier T]

Awesome, that is very helpful! Thanks everyone!

[Author]

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