I rarely have water accumulating on my slide topper since I rarely have the coach completely level on purpose. I do so in order to keep the A/C condensation running off the roof in the opposite direction of where we are sitting and have camp setup....usually have the front elevated slightly above level to make it run off the back. Unless there is REALLY heavy rain, the water doesn't build up on the topper. And when it does, it dumps right off when I roll in the slide. I guess my topper is sized correctly and my sweep seal does a good job sweeping since I haven't had water come inside from the slide.