"Caution - Low Ceiling, Watch Your Head" Sign By SmartSign | 10" x 14" Plastic DURABLE PLASTIC. Signs use 55 mil thick HDPE (high density polyethylene) and can last up to 2 years outside. Signs have a 100% recyclable base material. ROUNDED CORNERS. Signs have rounded corners and burr-free corners for safe handling, longer life and a professional appearance. INSTALLATION. Signs have pre-punched and pre-cleared mounting holes for easy installation. Signs mount to walls, fence posts and doors, and can be used indoors or outdoors. SAFETY AWARENESS. Alert everyone to exercise caution while taking some burden off your shoulders, regarding safety on your property.