Field-Treating Doug Fir Timbers for Insects

      Impermeable woods such as Douglas fir are very difficult to successfully treat with insecticide. March 3, 2009

The foundations of my client's 25 year old residence on a cliff top have around twenty douglas fir columns from 3 to 14 feet high above bedrock. The architect and engineer originally specified pentachlorophenol-dipped 8x8's, or equivalent round columns.

Currently half or more of the columns are showing numerous holes from buprestid beetle infestation. The client wants a treatment to prevent further damage (copper napthenate), and if possible then to frame in and stucco from bedrock to sill beams, enclosing the columns to deny further access to pests. Any advice would be very much appreciated.

Forum Responses
(Architectural Woodworking Forum)
From Gene Wengert, forum technical advisor:
Dipping provides very little protection, in general, for most species. Douglas fir from the Rockies is known to be very impermeable, so dipping would have little benefit for controlling insects, as the dip would not penetrate very far at all. So, after dipping, a few small cracks (which can be expected to develop) would allow the insect’s access to untreated wood. Note that because of this impermeable nature of douglas fir, it is incised before pressure treating; the incising allows preservative to penetrate. So, the bottom line is that with the species chosen, dipping was not the correct choice for providing any significant protection. Some other species or preservative technique was required. Even then, dipping would not be a reliable method of providing much protection. At this point, a commercial service could fumigate the house and kill any existing infestations. Future infestations would be difficult to prevent by applying something to this impermeable wood. Perhaps a commercial firm can treat the soil.

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