Q: On elevated decks with redwood decking, we’ve sometimes noticed black streaks on the joists below. Are they caused by the redwood, and if so, how can we prevent them?
A: Charlie Jourdain, president of the California Redwood Association (calredwood.org), responds: The dark streaks may be the result of water-soluble tannins (also called extractives) in the redwood leaching out during rains and then “bleeding” over the joists and structures below. The issue is cosmetic rather than structural, and typically, the bleeding will stop after the first rainy season, once the surface tannins in the redwood have been exhausted.
One way to reduce redwood tannin bleed is by applying a quality finish to all surfaces (top, bottom, and sides) of the decking prior to installation. In addition to prefinishing the decking, placing some type of flashing or deck-seal product along the top edge of the joists before installing the decking may have the additional benefit of directing rainwater runoff away from the sides of the joists, thus preventing discoloration. And once the decking has stopped bleeding, you can remove the dark tannin stains from the joists with a commercial wood brightener containing oxalic acid.