How Logs are Prepared

log-full-scribe-roundThe logs in log houses have a varying degree of moisture content; all logs have moisture when freshly cut. In the case of 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.

Green logs

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 (manufactured logs) are 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 (MC).[5] The actual moisture content of “green” logs varies considerably with tree species (cedar, fir, spruce, pine etc.), 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% (the oven-dry method of measuring MC).[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 (EMC) of between 6% and 12%. The actual EMC varies with local climate, season and location.[7]

Air-dried logs

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.

Kiln-dried logs

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).

Since logs reach equilibrium moisture content at about 6% and 12% (in North America);[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.

Glue-laminated timber

“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.

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.

Source: This information is fromĀ https://en.m.wikipedia.org/wiki/Log_house

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