The best way to use the HeatTrak mats is to turn them on before the snow begins to fall. This way, there will not be any water accumulation when the snow melts, because the snowflakes will evaporate as soon as they land on the mat. However, you can turn the mats on after the snow has already fallen on them. In this case, give the mat a few hours to melt through the snow. When the snow melts, there will be a slight water run-off, but the run-off will go under the snow that surrounds the mat. Also, there will be small puddles on the mat until the water completely evaporates. But have no fear, these small puddles will be beneath the non-slip treading, so there will not be any slip hazard. Eventually, all the water will evaporate.