Base Sealing Ceresit BT 21

Hubby's dad and I scrapped the excess parts of the Base Sealing (Ceresit BT 21). We need to only have atleast 3 to 4 cm over the edges, so that our outside wall plaster will be done soon.

Hubby and I installed these Base Sealing almost two years ago. We thought that making it longer protects more the edges of the house. Actually it did. Unfortunately, we need to cut it out to 4 cm from the edge . It is because the wall plaster have no possibility to hold with the Base Sealing.