We like our pleated day/night shades. In 11 years I have had to restring one shade twice and the other once due to frayed strings. In all cases, the strings had frayed because one of the nylon grommets that provide a slippery surface where the strings go through the metal top, middle or bottom bars popped out. This allowed the string to wear and fray against the sharp edge of the hole in the metal bar and eventually break. A little epoxy glue to better hold the grommets in the metal bars resolved the problem.
The other reason for breaking strings is that they are tightened too much. The strings should be just tight enough to keep the shades up going down the road.
Restringing these shades is a relatively easy DIY job. After doing it once, the next one gets even easier.
How to do it
here.
