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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:
- Use cedar, which is not as strong as other woods but smells nice and looks good.
- Fir heartwood is stronger and also somewhat resistant to bugs. Several eastern species perform similarly. If you have a preference, your local lumber yard can advise you on available choices, if any.
- Use treated lumber and re-treat all end cuts and holes (the original treatment penetrates only partially into the wood and a cut or hole will allow organisms a path throught the treated shell into the untreated interior of the wood). Weaker sapwood is generally selected for pressure treatment because heartwood doesn't accept treatment easily.
- Seal all ends. The ends are the easiest way for moisture (and bugs) to enter the wood. Little moisture is absorbed from the other faces of the wood. Sealed ends will keep the wood drier and less attractive to bugs and rot.
Sealing the ends will also allow the wood to dry more slowly and evenly until its moisture content reaches equlibrium with the local climate. This will reduce the checks, splits and warping that tends to develop if the ends dry quickly while the core dries slowly. New cracks and splits will be another avenue for moisture and nasties to enter your wood.
- Any penetrating wood sealer will keep the wood drier and increase the life of the structure. Surface coatings, like paint, are generally not recommended as they have a tendency to peel and blister, retaining moisture, which could be more harmful to your wood than no coating.
- Space your decking planks an inch or so. Not only does this make it much easier to keep the deck clean but tight deck planks, especially with debris in the cracks, don't dry well and invite bugs and rot. As a bonus, spaced deck planks reduce the dead load, give you a bit more traction on a rainy day and save you money.
- Separate the arch feet from concrete footings with metal.
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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