Why should you buy a Structurally bonded rooflight rather than one that isn’t? In order to answer this question, it’s important to know what is meant by “Structurally Bonded”. ‘Structurally bonded’ refers to the load-bearing structural adhesive used to form a bonded joint, capable of holding two or more substrates together under stress. In high-quality rooflights, this is the bond between the glass units or the glass unit and its framework.