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House built from wooden logs From Wikipedia, the free encyclopedia
A log house, or log building, is a structure built with horizontal logs interlocked at the corners by notching. Logs may be round, squared or hewn to other shapes, either handcrafted or milled. The term "log cabin" generally refers to a smaller, more rustic log house, such as a hunting cabin in the woods, that may or may not have electricity or plumbing.
Log construction was the most common building technique in large regions of Sweden, Finland, Norway, the Baltic states and Russia, where straight and tall coniferous trees, such as pine and spruce, are readily available. It was also widely used for vernacular buildings in Eastern Central Europe, the Alps, the Balkans and parts of Asia, where similar climatic conditions prevail. In warmer and more westerly regions of Europe, where deciduous trees predominate, timber framing was favoured instead.
The two initial primary styles of log houses included:
Handcrafted log houses have been built for centuries in Fennoscandia, Fenno-Ugric areas such as Karelia, Lapland, Eastern parts of Finland. Also in Scandinavia,[1] Russia[2] and Eastern Europe, and were typically built using only an axe, knife and log scriber.[3]
Settlers from northern Europe brought the craft of log cabin construction to North America in the early 17th century, where it was quickly adopted by other colonists and Native Americans.[4] C. A. Nothnagle Log House, built in New Jersey circa 1640, is likely the oldest log cabin in the United States.
Pre-fabricated log houses for export were manufactured in Norway from the 1880s until around 1920 by three large companies: Jacob Digre in Trondheim, M. Thams & Co. in Orkanger, and Strømmen Trævarefabrik at Strømmen. They were factory built from sawn or milled logs, numbered and dismantled for transportation, and reassembled on the buyer's site. Buyers could order standard models from catalogs, custom-made houses designed by architects employed by the companies, or houses of their own design. Log houses from Thams were exhibited at the Exposition Universelle (1889) in Paris.
During the 1920s, the first American milled log houses appeared on the market, using logs which were pre-cut and shaped rather than hand-hewn. Many log houses today are of the milled variety, mainly because they require less labor-intensive field work than handcrafted houses. There are about 500 companies in North America which build the handcrafted, scribe-fit type of log house.
The logs in log houses have a varying degree of moisture content; all logs have moisture when freshly cut. Handcrafted logs moisture will naturally leave the timber, drying it out until it stabilizes with its climate. This drying-out causes movement and shrinking of the log's diameter. As logs and timbers dry, the differential shrinkage (radial versus tangential) causes small cracks (known as "checks") to open slowly over time. Checking is a natural process in both air- and kiln-dried logs. This occurs in all log houses regardless of construction method or how the timber is allowed to dry, and is considered normal.
Milled logs are processed in a different manner from handcrafted logs. Logs destined to become milled logs may become one of several types, depending on the desired quality and results.
Logs that are cut from the butt forest, brought to a mill or to a log-house construction yard, have their bark removed and are used to build a log-house shell, handcrafted log houses, or sent through profiling machines usually referred to as "green" logs if they have not been air- or kiln-dried. "Green" does not refer to color, but to moisture content.[5] The actual moisture content of "green" logs varies considerably with tree species, the season in which it was cut, and whether sapwood or heartwood is being measured. Green logs may have a moisture content ranging from about 20% to 90%.[6]
One type of air-dried log is "dead standing," which refers to trees which have died from natural causes (bug kill, virus, fire etc.) and cut down after they died. Standing dead trees may be cut one month or several decades after they died, so the term "dead standing" does not necessarily mean the logs have dried down to equilibrium moisture content. Dead standing logs can be green, or more-or-less dry.
After construction, green logs dry in service in the log building. Within about four years, green logs which are part of a completed log house reach equilibrium with local conditions and have an equilibrium moisture content of between 6% and 12%, which varies by local climate, season and location.[7]
Some log home companies let the fresh-cut logs (or milled timbers) sit outside in the open air to dry naturally. The timbers may be stacked with spacers (known as "stickers") between them. This process allows the moisture content of the logs to naturally fall as the timber dries. In areas of high humidity, it may require as much as one year per inch of log thickness to dry. Arid climates may require less. A log with a diameter of 8 inches will usually reach equilibrium in about 18 months in the Rocky Mountains of North America. Air circulation is critical or the logs may begin to rot before properly drying, especially in humid regions. If the logs are to be dried to equilibrium with the local climate the process may take several years, depending on the location and size of the timbers. In some environments, the logs must be kept under some type of roof or cover to reduce the impact of rain storms on the drying process.
Once the logs have dried for the desired length of time, they are profiled prior to shipping. Profiling usually does not take place until shortly before shipment, to ensure that the logs stay as uniform as possible. It is uncertain whether this process is advantageous; it depends on many factors such as local climate, wood species, its size, and the location of the log structure.
Mills that have a kiln on site have the option of artificially accelerating the drying process. Green timber is placed inside a large oven, where heat removes moisture from them; however, they can suffer severe checking and cracking if the kiln controls are not properly monitored during the drying process. Use of a kiln can reduce the drying time from many months to several weeks. Kiln-drying usually results in an average moisture content of 18-20% ("average" means the average moisture content of the outside and the center of the log).
In North America, logs reach equilibrium moisture content at about 6% and 12%;[8] since most kiln-dried logs are dried down to about 18% to 20% moisture content, kiln-dried logs can be expected to shrink and settle over time, but to a lesser extent than green logs.
"Laminated" or "engineered" logs are a different approach to log-house building. Full trees or (alternatively) sawn cants (unfinished logs to be further processed) are brought to a mill with a dry kiln, the bark is removed and the trees are sawn into boards usually no more than two inches thick. These boards are then taken to the dry kiln, where (because of their size) they can be dried without causing severe damage to the wood. Timber destined for glue lamination must be brought down below 15% moisture before the lamination process will work, so typically these timbers are dried to around 8-10% moisture. The drying process varies on the species of lumber, but can be done in as little as a week. Once the drying process is complete the planks are sent through a surfacer (or planer), which makes the face of the lumber perfectly smooth. These planks travel to a machine, which then spreads a special glue on the interior boards. Now there are also Non-Settling log profiles available.
Depending on the type of glue and type of mill, there are two ways to finish the lamination process. One type of glue reacts with radio-frequency (RF) energy to cure the glue in minutes; the other uses a high-pressure clamp, which holds the newly reassembled timbers under pressure for 24 hours. Once the glue has dried, the result is a "log cant" that is slightly larger than the buyer's desired profile. These log cants are run through a profiler, and the end result is a log that is perfectly straight and uniform. Some mills are capable of joining together small timbers by using a combination of face and edge gluing and a process known as finger- jointing. Boards which would be scrap at another mill may be used in the center of a laminated log or beam to minimize waste.
Milled log homes employ a variety of profiles, which are usually specified by the customer:
Nearly every profiled log on the market features an integral tongue and groove milled into the top and bottom of the log; this aids in stacking and reduces the need for chinking. Wood is not airtight, but caulking or chinking improves energy efficiency.
Once fabricated and assembled, the shell of the log house may be disassembled and the parts shipped to the building site. This allows for centralized manufacturing of the house, and relatively quick construction on site. Full-scribe-fit handcrafted log construction is a method of precisely marking where to cut each individual wall log to provide a tight fit between naturally-shaped logs along their entire length and in the corners. A high degree of craftsmanship is required for success in this method, and the resulting tight fit of naturally-shaped logs have aesthetic appeal.
Log houses which settle require slip joints over all window and door openings, adjustable jacks under vertical elements (such as columns and staircases) which must periodically be adjusted as the building settles, allowances in plumbing, wiring, and ducting runs, and fasteners for the walls themselves to prevent uplift.[10]
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