Q. On elevated decks, the redwood decking often causes black streaks on the joists below. How can we prevent this?

A.Redwood contains tannins and extractives that are water-soluble. The extractives will wash down onto the wood below, leaving brown or black streaks. The best way to minimize this streaking is to treat the redwood with a water repellent. If the water does not penetrate the redwood, only a minimum amount of extractives will dissolve to wash down on the wood below.

According to work done by Brian Buchanan of the Texas Forest Service, the best water repellents are clear or cedartone Amteco Roof and Deck Sealant (Amteco Inc., 815 Cass Ave., St. Louis, MO 63106; 800/969-4811), ABR Natural Seal X-100, (American Building Restoration Chemicals, 9720 So. 60th St., Franklin, WI 53132; 800/346-7532), Seal-Treat II (W. M. Barr Co., P.O. Box 1879, Memphis, TN 38101; 800/238-2672), and WoodGuard (Chapman Chemical Co., 6075 Poplar Ave., Suite 306, Memphis, TN 38119; 800/238-2523). For color maintenance, Amteco TWP tested best.