Modified membranes are designed to be installed over an approved surfacing fastened to specific guidelines. If modified membranes are installed over a surface without the proper fastening the manufacturer’s warranty is voided and the roof assembly is out of code. This also leaves the roof susceptible to roof blow-off in the event of strong winds.  In 1994, when a new code system was introduced, the requirements became much more strict. However, most of the good roofing contractors go above code and install more fasteners than required due to lessons learned after the storms.