Why does a wide roller blind sag in the middle?
A wide roller blind sags in the middle because its tube bows slightly under the weight and tension of the fabric. It is called tube deflection, and the wider the blind, the more of it there is. It can show as V shaped shadowing or faint diagonal lines, and it is normal, not a fault.
A roller tube is supported at both ends and carries the fabric across the span between them. Like any beam, it deflects in the middle, and the fabric tension pulling down on it adds to that.
What you see isn't usually the bow itself. It's the uneven fabric tension it creates across the width, which shows up as V shaped shadowing or faint diagonal lines running out from the centre.
It's a normal characteristic of roller blinds rather than a manufacturing fault. Physics doesn't care which workroom made the blind.
Roller blinds also aren't factory balanced. Levelling and balancing on site is the installer's job, and tracking or telescoping caused by an out of level installation is an installation issue rather than a manufacturing one.
This answer is general information. For your own windows, your retailer's advice and your order with them come first.
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