A 150 ton dual tubing spider and manual hand slips were being used for making up and standing back stands of 2-7/8” tubing, in the derrick. After running 18 joints of tubing and 6 blast joints into the hole, the driller started pulling and racking back whole stands of pipe. After the driller pulled and racked back the second stand, he started to lower the dual spider when it struck the derrick diving board. The blow to the board caused it to shear off from the racking board and fall 85’ (28 meters) on to the strong back walkway located outside the rig floor area. Fortunately there were no injuries to personnel and limited equipment damage.