It depends on how long the mats have been turned on and what the ground surface is made of. If you turn the mats on a few hours before a snow fall, chances are you will have snow build-up along the 1.5" perimeter of the mat. This is because there are no heating elements inside this area of the mat. However, if the mat has been turned on for a full day or so before the snowfall, then the heat from the mat will have absorbed into the ground and spread beyond the mat by about 1 -2". In this case you will have snow melting coverage well beyond the size of the mat. This is especially true if the ground surface is cement or concrete, where heat is easily retained. This will not be the case if the surface is wood.