You don't say when it sags but if it's just sagging a bit in the middle when it's rolled up then there isn't much you can do for it.
If it sags on one side check the arms to make sure they are both at the same adjustment height.
When electric awnings are out, there's nothing like the old manual awnings arms to force them into being taught so we just have to put up with some sag unless you wan't to buy some of the new "rafters" that are available
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