WHIRLYBIRD TURBINE VENTS - Permanently lubricated upper and lower ball bearings that ensure long life and no maintenance. aluminum rust free construction. Rigid spider-type structure. Riveted at every connection. 21 air foil curved vanes with rolled vane edges to deflect water. Exclusive vari-pitch base adjusts to 12/12 roof pitch. Large flashing for easy installation. Tested to withstand winds of 147 m.p.h. Forever Guarantee residential use only. Easy installation. Reduces energy bills. Reduces winter ice build up. Big Whirly - 14 ft.is the same as the Whirly bird, but moves up to 37% more air . 8 turbine base units available. Design differs slightly.