Laying glass block can be tricky. Because glass units don't absorb water from mortar, the work has a different feel and the mortar takes longer to set than with most other masonry materials. For these reasons, masonry contractors sometimes opt to install prefabricated glass block panels rather than to lay glass block onsite. WHY PREFABRICATE? Prefabrication can offer several advantages. Using a prefabricated panel reduces the field time required for a contractor to install a glass block basement or bathroom window. Prefabrication also can improve quality control. WHEN TO PREFABRICATE Prefabrication is most beneficial when the panels can be handled by two people without mechanical assistance. According to one manufacturer, a 12-square-foot panel is about the largest that can be handled comfortably that way. Larger panels can be made offsite and moved by crane or hoist into place for installation.