Protecting Cutoff Tops of Douglas-Fir Piles from Decay

Along the Oregon coastline, unprotected cut-off tops of pressure-creosoted Douglas-fir piles rot within 4 years. When the unprotected wood exposed by cutting is brush-coated with creosote, piles are infected by decay fungi within 1 to 2 years. A test plot was established in 1976 to evaluate 22 different methods for protecting tops of cut-off piles. Ninety-nine 5- to 6-foot-long pressure-treated piles were cultured for decay at midlength, cut in half, and placed treated-end down in the ground. Simulated pile tops were cultured for decay fungi before treatment and annually for 2 years. All piles were free of decay fungi before treatment. Two years later, all nine piles without top treatment contained decay fungi. All piles with a preservative and a coal-tar cement cap were free of decay fungi. Of piles without a cap, only solid ammonium bifluoride, or a paste of flour-chromearsenic-phenol (FCAP) and water, prevented infection by decay fungi. Pentachlorophenol (penta) in oil, penta in bitumen (Pole Nu), a boron formulation in water (Polybor), a cap alone, or cellulose felts soaked with either FCAP or penta reduced but did not prevent infection by decay fungi. Making 2-inch-deep sawcuts in pile tops improved protection by penta and Polybor.

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