Question on Seams

why are you putting grosgrain?

you still have a row of stitches on one layer of fabric, and that fabric could rip out

if you have a flat felled seam, there are two rows of stitches that share the load so it’s actually stronger.  Or sew a third row of stitches down the middle of the flat felled seam and it would be even stronger

Do you intend to sew on the grosgrain so that it is only stitched to the seam allowance portion of the gray + blue fabric initial seam? In other words, is the grosgrain to be attached by sewing it to the fabric edge above the dotted line in the first image?

Or, do you intend to press the combined gray + blue panel flat and then sew the grosgrain flat over the top of the initial seam, with one row of stitched going through the gray fabric and the grosgrain, and the other row through the blue fabric and the grosgrain?

Either way, what’s the intended purpose of using the grosgrain instead of a tried and true option like the flat felled or french seam?

If you look at any good quality cottage or commercial shelter they will be using a flat felled seam. It’s been the seam of choice for many decades, because on balance it’s the most functional solution.

My suggestion would be to bite the bullet and learn to sew it. I use the standard basting tape from my local sewing shop and stick everything in place before I start to sew – particularly on longer seams. This greatly deskills the whole process and makes it relatively easy to get a good result, even with slippery silnylon.

Anything else will give you an inferior result, especially if you want a shelter that can cope with a bit of weather.

(The only exception is a seam where you are also sewing in a pole sleeve. If that’s what you need, Roger Caffin has written here about his approach, which has stood the test of time).

Warbonnet makes their tarps with grosgrain on the ridge line. I’ve made a couple that way as well. Put the 2 pieces of fabric together right side out, sew a basting stitch about 1/8″ from the edge then add the grosgrain over that and stitch it 3/8 or 1/2 from the edge depending on the width of the grosgrain. I’ve never had a Warbonnet tarp or one of my DIY tarps leak on the ridge line and I’ve never seam sealed them either.

If you want to learn an easy french seam for a ridge line search for 201-Top Stitched French Seam on Youtube. Yama Mountain Gear makes it look easy.

My question would be whether there is actually any advantage to adding the ribbon.

I would think the ribbon may be a small bit heavier.

In terms of holding power, it may not be as strong as a flat felled seam (the standard way in the industry), but still OK for the typical use of a tarp.

Advantages of the ribbon: style difference, E.g. by using a ribbon of the color different from the fabric.

The standing french seam with a grosgrain binding is perfect for a shaped tarp pitched in an A-Frame style. All the load along the ridge is fed along the grosgrain. It’s a proven design in hammock tarps that are always pitched in some variant of A-frame and I have also seen it used in other traps intended for ground use.

If your planning on making a tarp where you want more flexibility go for some form of flat sewn seam though.