Unprotected timber surfaces develop checks and cracks due TO Ultra-violet rays THAT break down lignin, and cause degradation of wood fibre. the repeated wetting and drying of timber which alternatively causes the timber to swell and shrink can allow exposure of untreated interior sections of preservative treated timbers.IN time THIS CAN result in surface degradation AND dimensional changes in TImber. These are warping, grain-raising, cracking, splitting, surface checking, splintering and cupping.