With the roll over design of the bag tops, when you get heavy rain there MAY be some water ingress into the bag due to water laying on TOP of the bag due to the 'roll over' design.
It is possible that a puddle will form. But perhaps I should have said that the bags are not waterproof. Because of the design, it will be possible that a puddle will form on top. And perhaps some water will drip through the canvas. But canvas automatically become water-tight when it gets wet, because the material expands when wet. Thus automatically closing the small holes which were left when weaving the fabric. The problem is the stitching: No bag which is stitched together will be water-tight because of the holes made while stitching.
Really water-tight bags (& coats etc.) are either welded or first stitched & later taped.