OffSet Tenons

Hi. In some of the Timber Frame designs, I notice offset tenons. Are the tenons offset due to structural strength or are they offset as not to interfere with an adjacent tenon? If they are offset as not to interfere with an adjacent joint, is there a rule of how much wood should separate adjacent joints?
Lastly, when cutting a tenon, should a tenon always be 1/3 the thickness of the beam?

I believe the thickness standard is 1/4 the thickness of the beam.
So for an 8x8 it's 2" and a 6x6 it's 1 1/2".

As for offset tenon they are usually location specif due to the frame design, I believe.

Jim Rogers

In my practice (square rule), I place tenons at a consistent distance from the reference edge, such as 2 inches from the edge with a 2 inch tenon. I try to have the mortises in line on the receiving timber in order to maintain strength and continuous grain. Others may have a centerline layout where tenons and mortises are on the centerlines of their respective timbers. As a practical matter, many different nuances of layout are employed to reduce interference of joints ie, half height tenons and free tenons to keep as much continuous grain as possible and avoid adjacent offset tenons. But I am unsure what you mean by offset tenons, I think the sense of the term is vague and can be taken in a number of fashions. Different framing systems have a different sense of regularity.

!/3 width is not common in tradition, I have seen 2 inch tenons common in 8 and 10 inch timber in local barn building.

Hi Hemlock,

Offsets are design, region and vernacular specific to the style of timber frame it is found in...

"Rules of 3rds" is more found in the finer member sizes of furniture...not in Timber frames as Jim stated.

In larger timber work, it is a "rule of quarters" but this is a very loose rule and more to do with structural dynamics within the joint intersection complex of a given union...

I work in mostly Eastern European, Middle Eastern and Asian modalities of timber framing. As such, I use ancient Line ruling methods and standards that tend to be more centralized on a timber (but not always center)...I also tend to use splines much more often and these can be of stronger more robust species...so dimension is often diminutive compared to American and Western European standards...

1" tenon thickness is not rare, nor are offsets and other anomalies compared to the standard American timber frames. We don't use oblique bracing, nor pegs very often...just gravity, and wedge joinery for the most part...

As you get deeper into the craft you will note the vast nature and stylizations within it...

Regards,

j

Allow me to present a general case where there is both consistency in layout and a variety of appearance by use of offset tenons and predictable layout of mortises. Suppose we are building a barn with vertical siding nailed to wall purlins (4 x 5) at 24 oc flush to the frame and has bracing (4 X 5) in set 2 inches quarter lapped with the purlins at their crossing. The posts are marked out for a series of mortises 2 and 4 inches from the reference side (the building line or the outside surface of the frame) for the braces, wall purlins and wall girts. The flush members will have the tenon cheeks marked at 2 and 4 like the mortises and so getting flush members. Since the brace will be inset 2 inches the tenon cheek will be marked only 2 inches from the brace's outside surface. The quart lapped joint will be marked at 3 inches from the purlins outside surface and the brace will be marked for the lap 3 inches from the building line or 1 inch from the brace's outside facing surface. Basically all this is managed by ALWAYS having in mind the building's reference planes. All the mortises in this scenario will be in line but the position of the members is dictated by the location of the tenons.

For information, here in France a carpentry tenon is 3cm (approx 1 3/16"). Regardless of the species of wood and the design of the frame. Generally they are centred but when a certain face alignment is sought they are often 3cm from the edge. A standard french carpentry tool is the jauge which is coincidentally 3cm large. Pegs are placed at 3cm from the edge of the timber containing the mortice.

...do you see a pattern here?

Great Post Jon!

Thanks. Interestingly this is not a recent thing. I worked on the restoration of 17th century manor house near me and a part of the work involved repairing the numerous dormer windows. The posts all needed replacing and so we kept the carved lintels. The tenons were 3cm wide and 3cm from the external (reference) face which meant that the faces were aligned. While I'm not certain that the dormers were original, they were certainly centuries old.

I've subsequently used the same technique when building a frame which was to be glazed directly as this kept the glazed surface planar.

Interesting indeed!

I am fascinated by the broad spectrum of modalities we see in our craft from culture to culture that has timber framing within it architectural forms both historically and/or currently. There really isn't a "hard and fast" rule for tenon size and location, with so many variables at play...

Given the common practices here of 2" and 1.5" tenons, it's often a shock when I use much smaller or even way larger in a design to outside observers. Primarily because of the current and historical trends here in North America. Now with modern young Timberwrights applying multicultural influence to the craft, we are seeing more and more of different systems and modalities being employed...


Regards,

j