Ridgeline

with that seam, the red row of stitches is peeled apart, after it separates, then the green row of stitches is peeled apart

with a flat felled seam, two rows of stitches are slipped apart sharing the load, so a stronger seam

But, with your seam, after the red row rips the green row maintains the seam integrity, when you get back home you can re-sew the red row.  So it has value.

if your flat felled seam is 1/4 inch wide, then the center is offset by 1/8 inch.  You won’t notice this.  You could make one side 1/4 inch narrower, but errors in layout will exceed this.  You could just sew any guyline webbing to the center of the flat felled seam.  So, one side will be 1/4 inch wider but you won’t notice.

with your seam, the gap for rain to come in is at the top (ridge) so little water will flow through

a french seam is very similar to yours.  except the raw edges are hidden inside.  which would be an advantage.  would otherwise work the same.

I’ve seen several tarps with that type of folded seam. They have been pitched with the seam on the outside and leakage didn’t appear to be a problem that a little silicon can’t fix.

It is also easy to insert tape into that type of seam for strength. I do think tho that the French option is neater looking

I’ve used that seam on half a dozen tarps so far.  Standing French Seam with a ridge cap of grosgrain ribbon.

The cap acts as a shingle over the last row of stitches and effectively sheds water.  I never seam sealed any of them and never had leaking.

Additionally, I extend the grosgrain ribbon cap about six to seven inches past the end of the tarp and then fold it over itself and box stitch it to create the end loop that becomes the ridge tie out point.

Can post some pictures if it would be helpful.

I am now looking closer at your diagram and that’s not how I do it…  in your illustration water could penetrate between the two pieces regardless of the stitching.. seam sealing is most certainly needed.

As an alternative…  position you pieces with out side in towards eachother and tun a seam about half an inch in from the edge.  Then fold the pieces so the outsides are facing out and run another seam about three eights in from the edge.  Then keeping the fabric as is, cap the seam you just made with the last seam being positioned between the first two… so about one quarter in from the edge.

By flat felled seam, I assume you mean a top stitched french seam like this one? https://www.youtube.com/watch?v=LN5aOHmCrDM

A true felled seam doesn’t create an offset because it’s interlocked evenly.  It’s also the strongest and most waterproof with each line of stitching going through 3 or 4 layers of fabric.  Of course, this seam is hard to achieve with a single needle and without a folding plate.  With stiffer material that can be folded, ironed and pinned it’s doable.  With lightweight slippery material like silnylon not so much, unless you glue the fold together or use double sided tape in the fold.

I haven’t tried this, but in theory pinning your folds around a strip of stiff reinforcement tape or grosgrain before stitching could work to create a true felled seam (also more holes to seal, and I’m not sure how well it would work with cat curves):

I believe Roger was referring to an additional layer of fabric looped up to hold the poles on his tunnel tents.

A flat seam or “french” seam as Aaron shows doesn’t have a top or a bottom. It is symmetrical. As was said, it can be difficult to do on a standard machine, with slippery fabrics. I add a stitch just for the ease of holding it all together, rather than fiddle with trying to fold it as I go.
1) Just run a straight stitch about 1/16-3/32 in on the attachment sides.
2) Roll it over itself once, stitch one side
3) Flip it over and stitch the other side.
You will find that flipping the tarp over will let you see the edge of the other side. For longer spans (10′ and more) I roll it twice. It strengthens the ridge considerably.

The third “starting” stitch is just to make things easy, it is not really necessary if you have all the gadgets. (Yeah, like that is gonna’ happen.) A simple straight stitch works well. Keep tension rather low to avoid any wrinkles, puckers. I try to hold the stitches 1/16″ from the edge you can see on the seam. For ridge lines, I always use a flat seam. When the whole tarp is finished, I seam-seal with thinned mineral spirits/100% silicone caulk at about 5:1 to 10:1. For coating the whole panel, closer to 15:1 to 20:1. Too thick, and it can peel loose after it sets. I usually use poly thread. For DCF, you really want glue-tape. But I roll and stitch them also. Yup, right through the tape. Every 10′ or so I have to clean the needle with some acetone (nail polish remover.)

“Cat” curves are a bit of a cheat. They are supposed to compensate for the stretch of material. But, I only did two. Never did them again and eventually gave the both of them away (one to a friend, one to my daughter.) They interfere with many set-ups I use a flat tarp for. And they let in a LOT of wind. I am not a fan here in the NE USA.

Nylon (and poly) will eventually stretch, mechanically. In high humidity, nylon stretches a lot more than poly, as much as 4-5% of the fabric width. Poly doesn’t stretch so much in humid conditions but can still be stretched mechanically (the “pull” from staking it out.) With a flat felled seam on nylon or DCF/cuben on the ridge line, you will find there is plenty of strength. With poly, you might want to put an extra roll at the ridge since it is a 25-33% weaker than nylon.

I just use a few heavy duty hair ties in a loop-to-loop configuration. Roger says to ONLY use them on the down-wind side. I really don’t care since I stretch mine out like a solid line. One or two together will easily hold a tent in a 40mph wind with a few gusts. More than 40, you probably want storm lashings, anyway. (To prevent flapping, wind-hammer and just pulling loose.) This compensates for the flat tarp as well or better than a “cat” cut tarp.

Anyway, for hems, I fold these, stitch, roll and stitch twice. These are wider than ridge seams. When I add my loops, I tripple stitch to the hem, and box stitch to the panel. For poly/DCF, you need some sort of reinforcement at all the loops. For DCF, the hem is usually unglued, then sealed over the threads. The panel reinforcements, ARE glued.

I have made a few, but, I do not do it commercially. Just a few friends and family members. I really cannot sew, I just make the damn machine thing go…

(I worked as a sheet metal worker when I was 8-14 years old with my father/grandfather. It helped a LOT with basic fabric layouts/specs/calculations…)

This is what I am describing for the tarp ridge..  Not quite what is depicted in the first video because the seam is a standing seam with grosgrain ribbon encasing it.  No idea what the proper name is, though I’ve referred to it as a Standing French Seam.

On a different note…  I want that sewing machine!!  Holy smokes….

I should note I use a vintage Singer 221 and have absolutely no problem with the above seam.  It’s never leaked, offers a bit of abrasion resistance if you’re using an over-the-tarp pole and has the end tieouts integrated directly into the length of the ridge line.