Service Life of Douglas-Fir Piles: Methods for Protecting Cutoff Tops from Decay

Various treatments for preventing decay in Douglas-fir piles were studied at an experimental plot in Corvallis, Oreg., and at a coastal marina in Brookings, Oreg. At the experimental plot, four preservatives were tested on piles with and without coal-tar cement caps. Other caps were also tested. After 8 years, pile tops treated with the water-soluble preservatives fluor-chromearsenic-phenol or ammonium bifluoride, or capped with Pole Topper, were free of decay. Coal-tar cement caps alone did not prevent invasion by decay fungi but did prevent development of advanced decay. The effectiveness of the caps can be attributed to their ability to keep the wood moisture content below 30 percent. At the coastal site, semipermeable bags of ammonium bifluoride placed on cutoff tops of creosoted piles eliminated decay fungi within 6 months and prevented reinfestation for 3-1/2 years.

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