Tuesday, October 23, 2007

Wood Truss

Of all the wood trusses that are constructed for use in a cabin, as an engineer my favorite is the wood truss that is shown in the sketch.

As noted in the sketch, the span of the truss is 24'-0". Of course, this truss would work between 20'-0" to 28'-0" of span. The spacing of the truss is 12'-0" on center.

With the roof slope of 12 on 12 and the heights involved, a cabin using this truss could lend itself to a story and one-half with the first story composed of the living room, kitchen, bathroom and lower main bedroom or bedrooms. The upper half story could house at least two bedrooms with a bathroom.

The truss shown has two large 10" x 14" horizontal timbers that carry the roof loads. The lower timber is placed at 7'-0" above the finished floor. This would match the height of the exterior doors and windows. The space between the lower timber and upper horizontal timber could house a strip of 2'-6" windows giving significant day light to a living room space. That 2'-6" space between the horizontal timbers could also be filled with a decorative wall surface.

The lower horizontal timber can be a member that raps around the entire structure adding a decorative feature to the exterior. Of course, the upper timber also raps around the entire structure and carries the roof rafters.

In order to provide day light to the upper one-half story, dormers with windows could be used and windows could be placed on the gable ends.

Both the interior and exterior can have more architectural interest by the use of this arrangement of the horizontal members matching the horizontal members of the truss. Other wood exterior surfaces and logs can be worked together to create horizontal, diagonal and vertical shadow lines on the exterior walls.

A massive stone fireplace could fit into an exterior or interior wall of the living room space.


Janet said...

Hi. My husband and I are building a log cabin out of 5 x5 logs that we are harvesting from our farm property. I love the drawingg you have done of the roof trusses. We are going to do something similar and have 2 30foot long black spruce as support beams from front to back. I haven't finished reading the blog yet but I'm hoping there will be ideas on how to do things before we screw them up. One question. Do you know how to make the dovetail joints at the corners?
PS my blog is This Log House. We are only at the tree cutting stage now.

Deer Tracks & Trails said...

To use the truss noted in this post would require Douglas Fir-Larch or Southern Pine timbers. These specie have premium structure qualities. In other words, these specie are stronger than your proposed black spruce timbers. The black spruce timbers may be used, but may require larger sections.

The cutting of any log into square or rectangular pieces may lead to potential problems. Branches on the tree grow out of a knot on the main trunk of the tree and when the log is quartered, these knots go through the quartered sections of the cabin wall. The knot will shrink when the log dries and may fall out. Also, the quartered sections may shrink differentially from the adjacent section causing longitudinal cracking.

The quartered sections should have a tongue & grove on top and bottom of the sections to interlock with the adjacent sections on the top and bottom. I suggest the books "Building with Logs" by B. Allan Mackie or "Your Log House" by Vic Jansen.

A study of the above referenced books may help to determine the type of joinery used at the cabin corners. Full dovetail and half dovetail corner notching is a common historical method of joining the logs at corners.

Hope this points you in the right direction!

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