I use 1.2mm cord for blinds with the cord lock system. I use the cord lock pictured below purchased from M&D who are also the cheapest for this system and fibreglass rods
I've found when using these cord locks that the blind drops slightly after it's been fully raised, as the brass barrel falls into position, so that the top most fold of the blind sits higher than the others and therefore the folds don't all hang level. I've calculated that you need to add an extra 1/2" to the headrail allowance to compensate for this. Has anyone else come across this problem?
Comment