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How long will douglas fir posts last untreated?


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As above really.

 

Last load of firewood had some nice long lengths of douglas that would make good poles for a small shed. Wondering if I give them a good coat of Creosote how long they my last? some are large enough to get a few plank out of as well to make perlins.

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Yes the bits above ground should last decades, depending on design.  Posts sunk in the ground will be rotten in 6 to 10 years.  Unless you can work out a clever way to protect them of course.  If you can bed them in large chippings (like railway ballast) and keep mud and sawdust away then they will not stay wet at ground level and may last better.  If they are embedded in concrete then at and just below ground level they will get wet and stay wet and therefore rot.

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The most vulnerable bit is not the bit under the ground, but the point a few inches either side of the surface. Below the ground, the environment is anoxic, and bacterial and fungal attack will be slowed as a result. Above the ground, the lack of moisture will slow degradation, but the bit in between will be the issue. Treat that bit most heavily.

 

Douglas is regarded as moderately durable, so you should be OK for a while. Cedar or larch would be preferable.

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