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Some thoughts on the durability of lumber

Wood is the most versatile construction material ever invented. Wooden structures can last a thousand years or rot in months. Some woods like cedar and redwood, have a natural resistance to bugs and rot but, as it comes from nature, the degree of resistance is not predictable and varies a lot.

The heartwood of a tree is generally more durable than the sapwood but who can tell the difference, who drew the line and what did you actually get at the lumber yard?
If you buy pressure treated lumber you can be pretty sure it's all sapwood because heartwood doesn't soak up treatment very well.

The one thing that is certain is that bugs, visible or invisible, need moisture to live and to enjoy eating your wood. No part of the wooden bridge should touch concrete footings or dirt. A metal barrier will prevent moisture from wicking out of the ground, into the wood and will create a speedbump for soil bacteria and bugs.

My design is airy, with no damp, dark cavities to retain moisture. The structure should dry out regularly, even in damp climate, and provide the longest life possible for any wood you choose

If you want to maximize the life of your bridge, some of your options are:

Treated lumber has a generally accepted life of over 20 years, even in damp conditions. Other lumber may or may not last that long, depending on local climate and finishes.

Many untreated lumber structures have stood for hundreds of years but you may have seen your neighbours wooden fenceposts fall over after only a year or two.

A very informative page on the durability of wood is:

The design and construction techniques presented on this page are protected from patents by prior art & copyright and I reserve all rights regarding this design. Feel free to utilize this information for personal use but applications involving the exchange of money, require my approval. If you'd like to build bridges for profit, I'd be happy to help you. :-)  Frank Petersohn

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