You're correct that when it snows, the weight of the snow pushes the ice down and water seeps up around the edges to create slush. It's not that the boards aren't allowing the water to freeze...it's just that it's very deep there. Think about a pond...if you go ice fishing, you might only have 10" or 12" of ice, as it freezes from the top down. Same deal with your rink...you have 18" of total depth, but maybe half that is frozen. It's been very cold this year, but the only way to really freeze all the way to the ground is by leveling your yard a bit to reduce the depth in your deep end.
I'm Joe from Backyard-Hockey.com and EliteRinks.com.
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