Building a Timber Frame
Copyright 2008 by Morris Rosenthal
All Rights Reserved
Mortise and Tenon
The smaller post to the right, know as a "prick" post, is an 8"x8" with a
2" tenon extending the whole width of the post. Prick posts are usually located
in the gable end to support the outer wall tie beam. The principal post,
18"x12" at the bearing surface at top of the jowl, has two tenons carved
out of it. A 2" tenon run the full 12" width of the post, and the10" long
teasel tenon is centered lengthwise is also 2" wide. The principal post has
been treated with tung oil. This early application of oil probably won't
prevent checking and may even encourage it, according to Jack Sobon, in his
"Build a Classic Timber-Framed Home"
The peg holes have been drilled in both the mortises and tenons for the draw
pegs. Test fitting tenons in mortises is always a good idea, especially if
they're as long and deep as the 19" long, 4" deep tenon on our arch brace.
Kevin is primarily using scribe rule framing in this house, which means a
lot of handling of very heavy pieces. It took several fittings and some minor
chiseling to get the tenon seated all the way in the mortise. The jowel on
the principal post (flaring at the top) is more typical of English timber
frames, which made wide use of White Oak, which is a tap root tree, and therefore
has a natural flare at the base of the trunk.
Since beams shrink far more in the width dimension (against the grain) then
the length dimension (with the grain), tenons length tends to stay pretty
constant, while the mortise may lose depth as the wood dries. Since this
would have the effect of pushing the tenon out of the mortise, the mortises
are cut extra deep to compensate for the shrinkage. The next step will be
to fit the principal post to the 32 foot long, 12"x12" tie-beam, and then
to scribe the tenon on the other end of the arch brace to the tie-beam. The
posts, beams, and braces must all be numbered in the scribe rule system,
since they are all custom fit. The more standard American system uses the
square rule, and modern framers often use what they call the "mill rule."
Mill rule basically relies on planed timbers coming out of the sawmill so
exact that as long as you measure right, the frame should snap together like
Lincoln Logs. Modern timber framers using the mill rule often cut their joinery
so fine that the frame components have to be come-alonged together, the fit
is so tight.
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