Building a Timber Frame
Copyright 2008 by Morris Rosenthal
All Rights Reserved
Scribing the King Post to the Summer Tie Beam
The first step to fitting our arch brace to both the summer tie beam and
the principal post is simply getting the timbers into the same plane. Since
our summer beam is a 32 foot long 12"x12" and our principal post is an 18"x12"
gunstock (jowled) chunk of oak, even with shims and tweaking, this can be
an impossible task. Minor twists in the timbers mean that getting the whole
face of a beam in the same plane is primarily a question of averaging. Although
the timbers were planed at the saw mill, checking and drying leads to crowned
surfaces, further complicating the leveling process. That's where the straps
Since the timbers are flexible, the idea is to cut the joinery for where
you want the timbers to end up, and to muscle them into place, if necessary.
Once the arch brace has been test fit and tweaked several time, it's time
to clamp it into place with a cargo strap so the holes for the pegs can be
marked and drilled. Kevin is using the drill to mark the arch brace tenon,
but the hole will not be drilled in line with the holes in the timber. After
marking the tenons on both ends of the arch brace, the brace is removed and
the holes are drilled at an offset from the marked location. This mean that
when an oak peg is driven home when the frame is stood, it will draw the
tenon into the mortise and provide a continual spring force to keep the joint
tight as the oak continues to dry and shrink. This is known as draw boring,
or the draw bore method.
In order to hold the principal post to the summer beam (after the beams are
leveled in the same plane) a steel peg is used to draw the post into the
summer beam. The reason for using a tapered steel pin rather than an oak
peg is because the steel peg can be removed without destroying it.
An exploded view of the whole brace system is shown below. Note that the
large half-lap on the end of the tie beam is there to tie the principal post
and the summer beam to the top plate. A principal rafter (see the king post
in the scribing yard) will sit on top of the half-lap, joining four main
timbers in the frame.
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