Tarps and fly sheets? Ridgeline strength?

[Moondog55]

Also partly a question on how big is too big.
I scored an older style cheap tent at an op-Shop last month and it needs a tiny repair in one corner.
These cheap tents and flysheets almost never seem to have centre seams, being pieced in the other direction.
How important to the strength of a nylon fly or tarp is a centre seam?
My machine will sew this stuff easily enough but as they get larger the stresses on a tarp get quite large and I was thinking that a 5m * 6m tarp needs a centre seam for strength.
The alternative would perhaps to sew in a length of tape and I just had the thought that I could fold the fly in half; fit in a length of tape and then pin and sew. Resulting in a vertical seam.
My sewing machine doesn't have enough reach for me to easily sew 2 parallel lines of stitching in the horizontal
This fly has a short beak too.
My thinking was that if I made the ridge strong enough I would not need a separate ridge rope, i am not sure how strong this polyester tape is, it is lightweight Grosgrain 26mm wide so probably about 90-120kg static

[Mark F]

The trick to what you propose is getting the tension right between the fly and the tape and this requires an understanding of the different stretch in both materials and this is not straightforward. The simplest solution is a ridge rope with a bight in each end but not attached to the fly. Tie one end of the fly to one of the bights and at the other end have an adjustable guy linking the fly and the second bight that is used to tension the fly once the ridge rope is in place and tensioned. This way you can really tension up the ridge rope and then more gently tension the fly for good pitch.

If you want a sewn in ridge tape then use this method with the tape in place of the rope to determine the tensions. Then pin before sewing.

[Moondog55]

The alternative view could be that all of the tension would be taken by the tape and therefore placing no additional stress on the lightweight nylon on that axis.
I think this is the same tape that is used on TerraRosa tarps.
I was thinking of pre-stretching the nylon by 10% to allow for taught pitching however as you describe.

[Moondog55]

Another question.
To attach guy points which would "you" use, a long length of woven tape along the edge seams in the manner of Bill Moss or a large triangle of doubled fabric and a short tape loop at 45 degrees??
I don't have the degree in engineering to even know where to start working out which is stronger but I know which is easier, and Bill Moss used both when really strong attachments were needed as do a lot of US tent makers.

[Moondog55]

Well in the end I used a short loop of 20mm webbing along the outer seam, couldn't find my lightweight 12mm tape so I used the bigger stuff, way too strong but it spreads the load over 120mm of each edge at the corners.
I am going to run a length of tape along the ridge line so I don't need the extra cord between trees/poles.
I'll post pictures when finished.

[Moondog55]

My machine needs a service and adjustment after all the heavy work I have been doing but this shows the edge sewn guyline attachments