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Cord wood, cement eating wood

 
steward
Posts: 979
Location: Northern Zone, Costa Rica - 200 to 300 meters Tropical Humid Rainforest
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Hi, I am very curious about this system (I think it looks GREAT), here is my question. We have found that if you put a post in concrete, it will eventually rot the wood, I suspect due to the lime in the concrete, but it could be other factors. What have you found regarding your system and how to avoid this problem?

 
gardener
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Location: PNW Oregon
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Hi Fred,

Depending on the type of wood, wood used in cord-wood projects will shrink, split and crack this isn't because of lime in cement though.

Most concrete and cement these days do not contain lime. When it contains lime its called Limecrete and generally it is used as a type of mortar for older buildings.



That being said, wood posts in the ground with cement can retain more moisture than those with rock for infill, and moisture is what rots the wood. Most recommend using toxic substances to preserve the wood you put into the ground, but I wouldn't do that with a building.

I had one post on the property I just bought 'rot' out - it was one of the very few in cement, but it was also under a fir tree completely shaded. So it's exposure to wet was great and the cement just held that moisture in. I just read where a natural builder recommends calking where the wood and cement meet when posts are vertical, and repeat yearly as needed. This prevents the rot you are speaking of.

Your cord-wood building should not suffer as your fence posts do, especially with a good roof overhang, air movement (keep brush back) and sun exposure.

All the best ~
 
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Jami McBride says: "Depending on the type of wood, wood used in cord-wood projects will shrink, split and crack this isn't because of lime in cement though." Jami is absolutely right. How to minimize this shrinkage and checking is a whole other topic, however. For the moment, we need to discuss wood deterioration. It is true that wooden posts in the ground will rot, particularly at ground-level, where we have the perfect combination of food (the wood), and the air and moisture to promote fungal growth: rot. The reason that this does not happen with a cordwood wall is that the wood is breathing through the end-grain longitudinal fibers. Moisture, the bane of rot, is not trapped. The wood gets wet, dry, wet, dry, ad infinitum. The conditions for rot - fungal growth - are not present. Little baby fungi cannot propagate.
 
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