Guide to building a timber frame home

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.

Foner Books Home | Building a Timber Frame | Contact