Open Mortise Common Rafter

[Brian_Weekley]

I remember learning that the mortised end of a common rafter should be placed on the north side.  I always assumed this had something to do with the joint being better able to withstand forces from the prevailing north/west winds.  I would think forces from either direction would push against one of the mortise sides in a similar way.  Can anyone provide a reasonable explanation or alternative reason for this orientation?

[Jay C. White Cloud]

Hi Brian,

I have seen them on both sides in vintage buildings and have come to the conclusion it was more about symmetry than anything else.  On some barns the "tongue" was "housed" for added strength and to compensate for variations in the sizes of the rafters, they could be on either side, but usually follow a constant theme, depending on the barn wright that built them.  Again, some always one side or the other, I'm not sure cardinal direction was the consideration, per say.

I know that themes are common among different traditions, for example in Asia, most often beams are oriented according to the growth of the tree.  Tree top always skyward, or North and/or East if in a horizontal configuration.  Planks and boards, most often "bark side up," seldom is the "pith" exposed.  I still follow this tradition in my frames. 

Regards,

Jay

[Jim_Rogers]

No special rule about which side.

But the layout should follow the standard rule. For example 1 1/2" off the layout face and then 1 1/2" thick. Unless the rafters are thinner then 4".

Jim Rogers

[Jay C. White Cloud]

Hi Jim,

I of found "tongue and fork" rafter tops in a multitude of configurations; from Canada to the Carolina's, then West.  They can range from 35 mm to 55 mm off a reference face and in some configurations, (seldom but not rare,) centered.  I do agree with Jim that many follow close to 40 and 40 rule.

[Dave Shepard]

I was taught to put the mortise to the North. In theory, you have the two sides of the mortise to resist the wind forces, whereas from the South, you have only the tenon. In the Dutch barn I am working on now, they alternate, M, T, M, T all the way down the ridge. They are scribed assemblies, with uneven purlin post heights, so it was intentional.

[canopy]

If this is square rule, wouldn't you also add reductions to the rafters?

[Jay C. White Cloud]

Hi Dave,

Good to point out the alternate pattern, when I mentioned symetry that was part of it.  Systems I've seen,

All one way, not all barns are oriented with the ridge facing a set direction, but oriented to the locations particular wind events is common.

Alternate, as Dave pointed out.

Double and triple skip, meaning 2 or 3 one way then then the other.



Rare: t&f mixed with half lap, (no not a moved or repaired frame,) all original work, t&f on gable half lap in center of frame.

Hi Canopy,

Could be "edge rule," but you also had variants of "scribe rule" and a few looked to be some form of "line rule."

Regards,

Jay

[Jim_Rogers]

If this is square rule, wouldn't you also add reductions to the rafters?

That's the way I draw them and cut them.
Because the top edge of the rafter is the reference edge and the bottom edge is not accurate enough to make a good joint to the opposite rafter.
I did say that the layout should follow a standard rule.
And I probably should have said "standard ruleS".....
The second rule it should follow is the joint reduction rule.

Almost every timber in a timber frame should follow all standard rules.
I post these standard rules on every page of a plan set I draw.
The only timbers that I don't "reduce" to a standard size are braces. We elongate the mortise instead of reducing the brace.

Jim Rogers

PS. in review, the standard rules that I use, and call "General frame rules" or "General frame notes" are these:

General Frame Notes
1*.) Joint spacing is two inches off layout face and two inches wide.
2.) All joints are framed down to the next half inch in size.
3*.) All bents are laid out from the North face except for the South bent.
4*.) Braces are framed to four inches thick.
5.) All tenons ends are trimmed back 1/8"

* some rules are frame specific. And what I mean by that is if this frame is all made by 6x6 stock then the rule would be 1 1/2" off and 1 1/2" thick.
And some frames are laid out east to west instead of north to south.
Also, these compass directions are not to the specific site where it will be built in all cases. Some are if I know where it will be built. If I don't know where on the lot it will sit or what direction on the lot is north then the compass shown on my drawing are "project" north as it is drawn.

[timberwrestler]

Brian,

I don't think it's that they should be placed on the north side, it's that they often historically were.  Dave Carlon brings this up, a side note of his that in most old buildings he sees the mortises on the north.  I don't buy into any strength arguments.