Full-Dovetail Notch vs. Half-Dovetail Notch

[hlhill]

What are the pro’s and con’s of using Full-Dovetail vs. a Half-Dovetail Notch? Which is structurally better?
Which is easier to cut? From an aesthetics point of view which is more acceptable?

I have seen a lot of jigs for full-dovetail…but none for half-dovetail.  Is anyone aware of a jig for a half-dovetail notch?

I am going to square some beetle killed pine to use in the construction of some out outbuildings. I plan on notching them so that the logs sit snuggly on top of each other…thus avoiding any chinking. Will the corner joints separate if the logs are not completely dry…or will everything stay tight in the event shrinking does occur?

I am still undecided as to which type of notch to use…hopefully you all can help me make that decision.

The log size will probably be 5” x 10”…maybe a bit larger.  This means removing 5 inches of wood from the end of each log. Hence, the average would be 5 inches. On a full dovetail notch…the inside of the notch would be 6 inches and 4 inches on the outside. That puts the slope on the notch at about 15 degrees. Does this sound acceptable?

My plan is to pre-cut all of the notches in order to fabrication a bit easier.

Also, I am considering using some blind dovetail notches in order to tie some walls together. Does anyone have experience with doing this? I want to avoid handling extremely long logs. (I have discovered that the length of the logs that I want to use is somewhat inversely proportional to my age. In any event I don’t need a building that is eighty feet long.)

There is still two or three feet of snow on the ground here in the Colorado Rockies…not counting the 6 inches we got last night…so it will be a few more weeks before I get started…should have stayed in Florida.

Thanks…in advance for your comments/critique.

Dick

[fred in montana]

Half dovetails seemed to be more common historically. They also have the advantage that everything slopes away from the notch. Water would not get trapped in the bottom of the notch like on a full dovetail.

The full dovetail jig is cheaper to manufacture because you only need one jig- it can work on both ends of the log.

With the half dovetail, there is a left and a right. This works pretty well because you can cut both ends and then roll the log one time to make the remaining cuts.

If you have a helper, it is pretty fast to notch a bunch of logs.

Do a google search for log cabin dovetail jig and you will find several different ones.

[jander3]

I think 1/2 dovetail sheds water better. 





You can layout with a simple template.

[PA141]

I too like the idea of 'pre notching' using a jig, and have been experimenting with jig fabrication. I think that squaring the logs with a chainsaw mill afterwards will be very slow, and you waste almost half the log. Chinking is not that bad of a deal--easier than chainsaw mill operation by a long shot.

[PA141]

This V notch is simple to jig cut, and shouldn't hold water. It

 works well with green wood because the notch settles into itself as the wood dries, if there is an air gap left at the peak of the V to leave room for the movement down.

[badger1]

Half dovetail should tighten up as it dries also

[Don P]

People say that about dovetail corners, I've yet to see it actually happen. I can usually see through them when they dry. A draftstop notched in does more to prevent that than expecting the logs to drift inward.

[badger1]

Sure, I guess initial precision of cuts makes a difference, but the theory of the joint should hold true just like a full scribed log, as the logs settle it should tighten. The design of the dovetail locks everything in to place, there shouldn't be gaps if cut well and gaps are small or not there to begin with. I suppose you may have varying degrees of shrinkage. I have built a couple dovetail structures and they remained tight (at least for the past 5 years). 

I have seen some builders put lag screws in the joints to hold them together (although i dont do this). 

[PA141]

Good input. I'll experiment a little with the 1/2 Dove tail. Round logs might work OK if I cut a flat on the back side (inside wall) of the notch. The cross V on the V notch is a little tricky, and the 1/2 DT might be simpler .. I'll check it out.

[Don P]

Badger, imagine the vertical line of the intersecting log, as that log shrinks in width it draws into itself, it is not a matter of how tight the gap was to begin with, the log becomes narrower. It does not "know" to shrink inward towards the interior corner line, it simply shrinks towards the center of the log and opens a vertical gap along each shoulder.

The concept of the draft block is not mine, I was playing around in that pic with making it a French dovetail. I had built a cabin kit machined by a Hundegger that had a small square vertical draftstop notched in the corners, flush on the interior face and notched in along the vertical line. It would be a bear to try to jig it up and that was not really my intent here I was really meaning to just dispel that old myth about dovetails getting tighter as they dry. One builder told me that the old timers would go around the house after it was dry and knock the corners together to drift them back tight against the shoulders, could be but how often does that section really dry straight.

PA, you were asking about intersecting walls, I've done that same French (sliding) dovetail or half dovetail, one face flat the other with a tail. Rather than driving it the entire way down if you flare it slightly to the top, say about 1/4" wider at the top than at the bottom of both the male and female pieces if drops in easily and wedges just as it hits home. And yes, just as with any drop in dovetail, be it there or in a TF drop in dovetail joist, if done green it allows some degree of withdrawal as the intersecting timber shrinks in width. When I've seen dovetail corners done in round log they had flattened the ends inside and out. There's a really nice example of one in the Black Hills, I think its a church near Nemo @Dakota. There are also a couple of exterior pictures of that in B.A. Mackie's "Notches of All Kinds". His next notch after that is a locked dovetail. Good book for the shelf if you can lay your hands on a copy. 

[badger1]

Badger, imagine the vertical line of the intersecting log, as that log shrinks in width it draws into itself, it is not a matter of how tight the gap was to begin with, the log becomes narrower. It does not "know" to shrink inward towards the interior corner line, it simply shrinks towards the center of the log and opens a vertical gap along each shoulder.

I see what you're saying..I guess my thinking was that assuming all things being equal, if you have a dovetail notch, and the upper and lower pieces both shrink inward toward the center line of the log, if the shrink rates are similar it should keep the notch tight. 
I do see what you are saying though, I might have been thinking of it wrong, or perhaps just because I havent seen it in real life, but Im not negating the possibility. Maybe the moisture content of the material I have used is much lower, maybe hasnt had enough time to reach full shrinkage etc many possibilities. 
Come to think of it, I guess I have seen older dovetail cabins that have chinking around the dovetails rather than just between the laterals, could be a reason why, while chinking noticed that the joints had opened up and wanted to seal them up as well. 

[Don P]

That is the reason for the vertical chinking you'll see in the notches. The end grain coming in from the right in my pic doesn't shrink, imagine the next log coming in from the left, tight up against that vertical end grain, that will shrink in thickness so there is where the gap forms. I think I still have a pair of corners from that hundegger draft stop notching up in the barn, I'll try to dig them up and get a pic. It does add complexity so not sure if it is worthwhile vs chinking later but just more stuff to see and think about.

[Don P]

 Here they are. Sorry didn't clean them up or pose them well, Michelle just dragged me out from under the knuckleboom for dinner and I snagged a couple of shots on the way up. I thought their draftstop notch was too small, when I was messing around above I made them too deep, there is a real risk of them breaking off on the ends at that depth, there is a happy place in between somewhere. Anyway, hope the idea is apparent there.


 

[badger1]

thanks for sharing

[PA141]

Badger: Thanks for the link to the video of the guy making a model of a 1/2 dovetail notch. I did that with 4x4 pieces to get the feel of the V notch.

Don : I ordered that Notches of All Kinds book just now. Thanks for the suggestion.