Originally Posted by hangNyak
If you sew the ridgeline after, one side will not be the same dimension as the other. Thats because of the way a flat felled seam is done. I am not an expert, but it was much easier after I sewed the two pieces of fabric together.
Actually, a felled seam offsets the ridgeline to one side of the seam no matter when you do it since the first line of sewing is the actual seam. The rest is there to hide the rough edges of the fabric and to strengthen the seam by another two lines of stitching.
Now, getting two pieces of 10 to 12' of slippery fabric to end up the same length at the end of sewing it together is the actual trick no matter how experienced you are.
One of the tricks to that is to leave the hemming of both ends to the end but to do the bottom edges and tie out stuff before you join the two panels. That way if you get the ridgeline seam a little off you can always adjust the end.