Wooden Gravel Boards 1.8m 6ft To be used under a 6ft panel. 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.