Wooden Gravel Boards 1.8m 6ft in Natural | Green Wherever possible it is advisable to fit Gravel Boards under your fence panels as this is the most vulnerable part of the fence. A softwood board pressure treated with TANALITH Extra for longer life Brown colour TANALITH Extra is an effective water repellent additive that can be used with TANALITH wood preservative within the treatment process. It gives an added built-in weathering protection to the treated timbers. TANALITH Extra treated timbers have all the same preservative protection properties as TANALITH treated timber. The water repellent also helps maintain the dimensional stability of the treated timbers.