It's not suppose to but will eventually. It's going to take several layers of glass to make it water tight so if its something you are not going to make too fancy just pour lightly shape and replace every few seasons. If you are going to want a nice clean finish then you are going to need to seal it with glass. Use 3 layers of the lightest thinnest weave you can find and so not stretch the weave when you install it. The glass has gaps that wi have pin hole leaks so several thin layers will seal better than on thick layer. Example if you use 1 layer of 18oz it will likely let water in but 3 layers of 6oz is less likely. Depends on the quality of your work. Keep the weave tight, don'tess with it too much.