You guys ask us good questions and sometimes we don't know the answer. When that happens, we ask Tom -- given that he has over 40 years of bag making experience, he usually has the answer if we don't.
Below is an email sent this past week to our customer service crew and assigned to Cody, who wasn't sure of the answer. Cody asked Tom, and the response below is what Tom shared with Cody:
I have several of your products and I've noticed that none of them have top stitching on the zippers like many bags do. Is there a particular reason why you guys don't top stitch the fabric on each side of the zipper?
Thanks! -- C
We typically sew coated zippers (YKK Aquaguard) into a bag without top stitching to minimize puncturing the fabric (and its coating) any more than necessary, thus maximizing water repellency. This doesn’t make the seam or the zipper “waterproof” but simply a bit more weather resistant.
We’ve always appreciated the esthetic of top stitched zippers, the way the stitching forces the fabric on either side of the zipper to lay flat; however, we’ve found in use that the fabric on a non-top stitched zipper will relax and lay flat pretty quickly. How quickly depends on the weight/stiffness of the fabric.
We do typically double-needle top stitch our non-coated zippers, where either the zipper has a flap for weather protection, or the expectation for a specific bag is that the zipper not be particularly exposed to the elements.
Furthermore, there are occasionally design and/or manufacturing constraints that nudge us one way or the other.