Glass does not flow. If it did, the high precision glass lenses used in astronomy, etc. would not work after just a few days. (Well, technically they would, but the accuracy would be v. off)
The windows in old houses sag because the manufacturing techniques of the day were not nearly as good as the methods we use today. (Currently, we make glass panes today by floating molten glass on liquid metal.)
The windows in old houses sag because the manufacturing techniques of the day were not nearly as good as the methods we use today. (Currently, we make glass panes today by floating molten glass on liquid metal.)