A house typically made from logs
that have not been milled into conventional lumber. The term log
home is contemporary and preferred by most log home builders, while
log cabin indicates a smaller, more rustic, log house, such as a
hunting cabin in the woods.
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Contents
1 Components
1.1 Green Timber
1.2 Air Dried Timber
1.3 Kiln Dried Timber
1.4 Glue Laminated Timber
2 Methods of log cabin construction
3 Corner styles
4 Other methods
5 Structural types of log cabins
6 Types of milled logs
7 Fastening systems
Components
There are two kinds of log cabins: "handcrafted" and "milled"
(also called "machine-profiled"), made with a log house
moulder. A handcrafted log cabin is typically made of logs that have
been peeled but are otherwise essentially unchanged from their original
natural appearance when they were trees. A milled or machine-profiled
log cabin is one constructed of logs that have run through a manufacturing
process to remove natural features and imperfections of the log
and convert them into timbers that are consistent in size and appearance.
Many handcrafted log builders do not consider milled logs a log
at all, a position with which milled-log manufacturers disagree.
Handcrafted log cabins have been built for centuries in Scandinavia,
Russia and Eastern Europe. The Scandinavian settlers of New Sweden
brought the craft to North America in the early 1700s, where it
was quickly adopted by other colonists and Native Americans. In
the 1920s, the first milled log houses appeared on the market, using
logs that were precut and shaped, rather than hand-hewn. Most log
cabins today are of milled variety mostly because they require less
labor intensive field work and there are many more options available
than with handcrafted alone.
Any timber from large to small will have moisture in it when it
is fresh cut. Log cabins of all types experience varying degrees
of moisture content. In the case of "handcrafted" logs
this moisture will naturally leave the timber, drying it out until
it stabilizes with the climate it is in. This drying out causes
movement and adjustment in the timber. As the wood dries the individual
cells on the exterior of the crafted log will seal up. The remaining
moisture in the center of the timber keeps trying to escape and
will eventually open a crack in the crafted log. This crack, also
known as a "check," can continue to the heart of the timber,
sometimes leaving a large crack on the side of a cabin. This occurs
in all log cabins, regardless of construction method or how the timbers
are allowed to dry and is considered normal as well as part of the
charm of owning a log cabin.
Milled logs are processed in a different manner than hand crafted
logs. Logs destined to become Milled logs can become one of several
types depending on the desired quality and end result.
Green Timber
Logs that are cut from the forest, brought to a mill and profiled
are usually referred to as "green" logs. These logs will
have a higher moisture content, sometimes upward of 25%. After construction,
the timbers are allowed to dry in place.
Air Dried Timber
Some mills elect to let the fresh cut timbers sit outside in the
open air to dry naturally. This process allows the moisture content
of the logs to naturally come down as the timber dries. This process
can take several months and requires the mill to have space to let
the timber air out. Once the logs have dried for the desired time
frame the logs are profiled and shipped to a customer. Profiling
usually does not take place until right before shipment to ensure
that the logs stay as uniform as possible.
Kiln Dried Timber
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 the logs. They can suffer severe checking
and cracking if the kiln controls are not properly monitored during
the drying process. Kiln Drying can cut down the dry time required
by the manufacturer in order for production, from many months to
a number of weeks, and usually results in an average moisture content
of 18-20%, average being the mean moisture content of the outside
of the log and the center of the log.
Glue Laminated Timber
"Laminated" or "Engineered" logs are a quite
different approach to log cabin building. Full trees are brought
to a mill equipped with a dry kiln, the bark is removed and the
trees are sawn into boards usually no thicker 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 even 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 to cure the
glue in a matter of minutes and the other uses a high pressure clamp
which holds the newly reassembled timbers under pressure for 24
hours. Once the glue has dried the end result is what is called
a "log cant" that is slightly larger than the buyers 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 quite small timbers
by using a combination of face and edge gluing as well as a process
known as finger jointing. These boards which would be scrap to any
other mill could be used in the center of a laminated log or beam
to bring waste to a minimum.
Methods of log cabin/home construction
Scandinavian Full-Scribe (also known as the "chinkless"
method) where logs are scribed, custom fitted to one another, and
notched where they overlap at the corners
Flat-on-flat (logs are flattened on top and bottom and stacked)
Milled log cabins often are constructed with a Tongue and Groove
system that helps to align one log to another as well as create
a system for sealing out the elements.
Tight-pinned butt and pass method where logs are not notched or
milled in any way, logs in a single course do not overlap, and vertical
pairs of logs are fastened with tight, load-bearing, steel pins.
Corner styles
Butt-and-Pass where unscribed or milled logs butt up against each
other at the corners without notching
Interlocking Saddle Notch - Normally seen on "D" or Full
Round Profiles where a notch is cut into the top of one log and
bottom of another, these two logs then interlock creating a tightly
sealed corner.
Dovetail - Typically seen on square or chink style logs. A special
dovetail is cut on the end of a log where it would rest in the corner.
One to the right, and one to the left. This also creates a nice
tight interlocking corner.
Saddle Notch - Typically used where two round logs overlap each
other near the corners. It is common with the swedish cope profile.
Other methods
Half-Log where the structure is built with conventional building
techniques and then a "Half-log" siding is applied to
the exterior and interior wall to replicate the look of full-log
construction. Some Half-Log sidings can also have saddle notch,
butt and pass, or dove tail corners to give a more realistic appearance.
Structural types of log cabins
From a structural perspective there are two fundamentally different
types of log cabins. In the most common type, the load forces of
the building are transmitted to the foundation through the wooden
logs. In the second type, the load forces are transmitted to the
foundation through steel columns. These two types will be referred
to as "Chinkless" and "Tight-pinned" respectively.
Chinkless cabins may have some chinking or sealant between the log
courses, but the logs are typically milled or scribed to fit closely
enough together so as to minimize the chinking requirement. Since
the logs bear the weight of the building, any shrinkage of the logs
is cumulative and the shrinkage results in the settling of the building,
that is, the vertical dimensions of the building shrink as the logs
shrink.
Tight-pinned cabins typically are built with unmilled logs, and as
a result, there are large gaps between the log courses which must
be filled with some kind of chinking material. The log courses are
fastened to each other in pairs by steel pins which are driven through
the logs. These pins are tight enough, and spaced closely enough
(typically every 20 inches), so that they bear the entire weight
of the building. The pins are typically 1/2 inch in diameter and
are commonly made of rebar. When the logs shrink, each one shrinks
around its own center line with the center line remaining fixed
in its position in the building. As a result, the geometry and the
dimensions of the building are unaffected, but the gaps between
the log courses widen. Typically, the logs end up not touching each
other so they play no part in the transmission of load forces. The
logs do, however, prevent the pins from buckling under the load
for the portion of the pin inside the log. The portion of the pin
in the gap between the logs is a short column and thus is able to
withstand the load without buckling.
Advantages of the Chinkless method:
Once fabricated and assembled, the shell of the log cabin can be
disassembled and the parts shipped to the final building site. This
allows for centralized manufacturing of the cabin and relatively
quick construction at the final site.
Disadvantages of the Chinkless method:
Special consideration must be given for the settling of the building.
This includes such things as slip joints over all the window and
door openings, jacks under any vertical element 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.
Advantages of the Tight-Pinned method:
No allowance for shrinkage need be considered or made, thus simplifying
design, construction, and maintenance.
The building is stronger than the Chinkless buildings.
Disadvantages of the Tight-Pinned method:
The building must be fabricated and assembled on the final building
site.
Chinking maintenance is higher than for the Chinkless buildings
during the period of major log shrinkage. That period may be several
years for green logs.
Typically, the Chinkless method is preferred by log cabin manufacturers
and the Tight-Pinned method is preferred by many do-it-yourselfers
who build their own cabins. Individual log cabin builders who do the
work on the building site will find either method suitable for their
purposes.
Types of milled logs
"D" Shaped Profile
Square or Chink Style Logs
Swedish CopeMilled log cabins have an assortment of profiles that
are usually picked by the end customer. Just about every profiled
log on the market today features an integral tongue and groove milled
into the top and bottom of the log that aids in stacking as well
as eliminates the need for chinking.
'D' Shape Logs which are round in the outside and flat
inside.
Full round Logs which are fully round on the inside and out.
Square Logs which are flat on both the inside and out and may be
milled with a groove that could be chinked. When dealing with milled
logs chinking is more of a personal preference and is not required
to seal the cabin; however, A log cabin will eventually leak if it
is not properly sealed.
Swedish Cope logs which are round on the inside and out and include
a half-moon shaped groove that is removed from the bottom.
Fastening systems
These are some methods for connecting the logs together keep in
mind each manufacturer usually recommends their own way of sealing
and securing their logs.
Spikes: Basically a large nail, usually spiral, driven into the
logs nailing them together.
Lag Bolts or lag screws: This system uses a large threaded screw
to fasten the individual logs together.
Through Bolts: This method uses a continuous bolt from the foundation
to the top course of logs. A spring may be used at the top to maintain
downward force holding the logs securely in place.
Citied from wikipedia
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