RCM Cad Design Blog » Log Home Design - log home design

By: Rcm Cad Design  09-12-2011
Keywords: Construction, Wood, home design

RCM Cad Design Blog » Log Home Design

Topic: Settling for log home construction styles

Definition:
When a stacked log wall reduces in overall height, it is called settling, a well known occurrence in log home construction.

Introduction:
Settling in log homes can occur in three ways: compaction, slumping and most significantly, radial shrinkage of the logs.

  A-   Compaction occurs mainly in log construction using cope style lateral groves where the interior and exterior sharp corners of the cope are the only contact with log below. The cumulative weight of logs, roof and snow above crush these contacting lines of wood fiber into the log below, creating a tight seal between stacked logs.
Compaction can account for up to 1% height reduction of a log wall.
Compaction is minimal for logs with large contact areas like tongue and grooved flat to flat log profiles.

B-   Slumping is an old problem applicable to cope style grooves, that is practically eliminated with a kerf cut at top length of log to control cracks or checks in the wood as it loses moisture content.
If no kerf is cut, check will appear at cope as it is closest opening to core of log and the log will slightly open up at cope creating a slumping of the log over the one below.

C-   Most settling in log homes happens because of radial shrinkage of the logs as the wood dries out.

Moisture content (MC) of wood is defined as a percentage of its wet weight (water and fiber) against its oven dry weight (fiber).
Depending on the wood species and world location, living trees moisture content varies from 50%, up to 200%.
It is quite common for living trees to have 100% moisture content, which means it is half water and half fiber in weight.
Wood cells are like interconnected pipes with sugary water inside the cell and in the cell wall.
When a tree is cut down, the water in the cells readily flows out and evaporate.  This process can take months or even years.
When the cell water is gone the wood has reached its fiber saturation point(FSP) usually somewhere around 27% MC.
Above that point, wood is dimensionally stable.Wood does not shrink until moisture content gets below its fiber saturation point.
Only then is the water locked in the wood cell wall able to drain which causes the cell to start collapsing on itself, just like an apple left to dry will shrivel onto itself.

Logs mostly shrink radially.  Soft woods like Pine, Spruce, Fir, Cedar… which are commonly used in log construction, lose about 4% of their diameter to shrinkage.  That is, from FSP to the Equilibrium moisture content (EMC), which is determined by where the log home is built.
Logs also shrink in length, but it is almost negligible, at up to 0.2% maximum.  A 10′ tall log post will shrink in height by less than 1/4”.

EMC varies greatly by location ranging from 4% to 20% in North America.  It also can noticeably swing between seasons. 
In Beijing, China, EMC is 7% in January and 14% in July.
That is an unusually wide seasonal EMC differential!
That would translate to roughly a 1% change in log diameter between winter and summer, which is more than 1 inch difference for a 9 foot high log wall between a swollen to a shrunken state depending on the season.

Radial shrinkage (%) = S x (MC-EMC) / FSP 

To evaluate the % of radial shrinkage to expect for a log at a certain location, multiply S (total radial shrinkage for wood specie used) by the difference between MC (wood moisture content taken with a moisture meter) and EMC (equilibrium moisture content at specific location), and divide by FSP (fiber saturation point for that wood specie). 
S is a known value specific to each wood specie, ranging from about 2 to 5% for soft wood and between 3.5 to 7% for hard wood.

D-   Why should we care about this radial shrinkage of wood in log and timber construction?
Homes that are built with horizontal logs will lose wall height.
If well known design and construction practices are not applied to counteract settling, windows and doors will be damaged and crushed, plumbing stacks and water lines will shatter, beams will fail under too much load, second floors will warp and roofs will leak…
Construction practices such as adding settling space above doors and windows, adding screw jacks under posts to lower them as log walls settle, adding slip joints to plumbing stacks and flexible water lines en route to second floor baths, are a few of the many proven building solutions to log home construction.

E-   Settling and log construction styles

   1-Handcrafted log construction
Two basic types…

The scandinavian scribe fit has cross corners and laterals that stay tight during the settling process.
Usually built with green logs that are seasoned from 20% to 35% MC. Estimated settling is 6% with (4% shrinkage, 1% compression and 1% safety allowance)
The lowest logs in the wall (sill log) must start at same height and top logs (cap log) must complete the log wall at about same height to avoid settling differential in the house.

The chinked style has tight cross corners but laterals are not closed.  An elastomeric compound called chinking is applied to close gaps inside and outside.  Chinking material and labor costs are high as well as upkeep costs to reapply chinking.

Using dead standing logs does not eliminate any settling issues as logs MC is still much higher than EMC to be reached.

The MC of logs that are kiln dried gets down to, at best, 18% at the center, because massive logs and timbers can not be fully dried at the core.  Again all settling issues are in effect.

The log standard defines that a log wall with no more than 0.5% settling can be designed and built as a non-settling wall.  However, when more than 0.5% settling occurs, screw jacks and settling spaces above all static building parts of the home must be added to the design.

A log post and beam structure is considered to be non-settling construction, even when a double cap log is used.


A fusion style log home
is a mix between a full stack log home and a post and beam style.  It is a settling log structure, as three stacked logs from floor up, then a short post to a large horizontal log beam at top adds up to four logs stacked, and this equals 1 to 2% settling to the total wall height, too much to be allowed to pass as a none settling log wall.
  A piece en piece log construction home is a post and beam structure with horizontal log fillers between posts from sill to cap log. 
Those filler logs will settle by about 5%, so design steps must be taken to counteract the settling.Static horizontal stacked log assembly can be achieved by simply pounding lags or rebar from one log to the log below.
In this case, the log wall will not settle as the lags or rebar will lock each log in a static position to the log below to form the wall.
However, tight corner wood joinery (cross corner, dovetail…) will be lost as the drying logs will separate at corners and laterals when they shrink away from each other.  The loss of the tight corner weakens the log structure and chinking is a must to fill all those gaps!
Another problem with adding all that steel inside the log walls is condensation. In winter, moisture loaded air from heated living spaces meet the cold dry air from outside at the center of the logs. That moisture will condense on the steel and slowly rot the wood around it, thus weakening the tight fit between steel and wood.

This problem is mostly overlooked as it can take years to be noticeable, but it is real and documented.

Some log manufacturers advertise non-settling patented log wall systems using some rather complex steel pipe assembly inside the log wall to lock the logs in position.  The same problems mentioned above with lags and rebar is present in this system.
These patented log wall systems are merely a marketing gimmick.

A note about thru bolts…
I highly recommend thru bolts, usually 5/8” threaded rods to tighten log walls from sill log to cap log as a proven engineered system for seismic and wind lateral loads.
Although it means adding steel inside the log wall, there are self draining by simply placing a split washer at bottom to allow moisture to drip away.

Laminated logs, or sometimes called engineered logs, are truly the only non-settling stacked log construction available on the market.

  The laminated log is composed of glued layers of kiln dry boards (usually 1 1/2” thick), milled into a common round or square profile.  Each board, being so thin, can easily be kiln dried down to 8 - 10% MC, making them very stable and almost check free in the process.  They do not settle as they are already so dry and the flat contact area between stacked logs means no compaction or slumping.  However, laminated log walls exposed to the outside air will regularly contract and expand with seasonal EMC.  Windows and doors still need to be independent of the log wall.  This is achieved by using spline boards and a small settling space above, as they are fragile, non-structural, and expensive house building components. (See detail below)
Laminated logs do have glue lines visible at log ends which can be considered unsightly to some.  The glue lines can be somewhat hidden with fine sanding and a darker stain at those locations.
Because of all the extra manufacturing work needed to produce them, laminated log home total prices can be up to 15% more expensive compared to standard milled log homes.

1- Laminated log wall
2- header log above window
3- Box liner header (same width as log wall thickness)
4- Spline board loose fit in slotted log wall
5- Back rod foam gasket
6- Slot in log ends
7- Foam gasket at interior and exterior in compression between box liner and log wall to stop air leaks.
8- Box liner vertical jamb.
9- Header trim board nailed directly to log header and overlapping trim board below nailed to box liner header only.
10- Spacer trim board nailed to header log to offset Header trim at 9.
11- Vertical trim board nailed to vertical box liner jamb only.
12- spacer vertical board if window jamb width is smaller than window log wall opening width. Using a box liner allows using a smaller jamb width window than wall thickness.
13- Window jamb nailed to box liner at vertical and top. Window sill is nailed directly to log wall.
14- Window glazing
15- Sill trim is nailed to window sill.

  Last word…
Settling of a log home is unavoidable unless logs, as dry as the local environment, are used (laminated).
Hiring an log home experienced and knowledgable designer / architect to prepare your log home plans is a safe step to ensure success for your log building project.
Working with trades that have already been involved with that type of construction is a big plus.
Do not let anyone push you in making snap decisions, and question those miraculous non-settling systems. Another sales speech…

Sources:

1987 Wood handbook: Wood as an engineering material. USDA,
Agricultural handbook #72

Log building standard (International log builders association)

Equilibrium Moisture content of wood in Outdoor locations in the USA and worldwide.  USDA Forest service.

Keywords: CAD design, Construction, home design, Log Home, Log Home Construction, Log Home Design, Wood

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