A roll-up door has a plurality of panels and a plurality of roller assemblies mounted to ends of the panels for mounting the panels in channels. At least some of the panels are formed of blow-molded thermoplastic material and have a relatively flat outside face and an inside face including ribs that extend longitudinally and transversely on the panel. A reinforced panel of thermoplastic material is made by blow molding a panel with a thermoplastic material to form a relatively flat outside face and a ribbed inside face, and with elongated channels along upper and lower walls thereof. The panel is trimmed while it is still warm to open the channels at one end. While the panel is still warm, reinforcing members are inserted into the elongated channels. The panels can be cooled to shrink the channels onto the reinforcing members.