Durrock is concrete and will not burn, however the wall behind it will. Here are a few ideas of what you need to do to reduce the clearance to the flue 1. install a 1/2 round pipe shield, available from wood stove dealers. 2. Install durrock on heat shield spacers, to hold it 1" away from the wall. 3. Use double wall pipe or metalbestos pipe. Check the specs with the stove manufacturer as to distance the stove requires for clearance.