Dovetail Mortise and Tenon for Timber Framing

[kristingreen]

I just wanted to get the group's opinion on this alternate method of joinery. Is this sacrilegious? Am I going to get kicked off the board for suggesting this? Has someone here used this type of joinery before?

http://www.woodlandia.ca/machines/arunda

http://www.timbertools.com/Products/LignaTool.html

There is a local company selling pergolas and pavilions that use this type of joinery. No doubt so they can pre-fab and deploy them quickly. Kind of seems like the fast-food of timber framing to me but I'd be interested to hear other's opinions.

[S.Hyland]

In my opinion it's pretty sacrilegious!  >:( ;D

Honestly a lot of the applications that they show on the websites seem pretty wanky. The rafter peak detail for the Arunda, for example.  They just don't have the redundant structural qualities, which in my experience looking at historical timber framing, allows it to survive under adverse conditions.

The other aspect of it, is that one of the things that drew me to timber framing is all the skill necessary to create elegant, functional joinery. Anyone can run a router, which is I suppose one of the advantages in some peoples minds. But I enjoy the satisfaction of a challenge well met, otherwise I'd just get bored!

[Dave Shepard]

That looks like it would be great to build a bunk bed, provided nobody actually got in it.

[S.Hyland]

Indeed, perfect for the burgeoning "Decorative Bunk bed Industry"! ;D How I wish I could get a foot in the door in that industry!  :D

[Dave Shepard]

There is no strength in that system. Even if you used dry wood. Green wood and dovetails aren't great together.

[kristingreen]

I agree with everything said here. So, how is it that this guy can build an entire business out of this type of construction method?

http://www.timberkits.ca/

Warning: Don't look too closely at those knee braces.

[S.Hyland]

   I suppose it's because they look high end to the people who are buying them, and they don't know the difference between a real timber frame and one of those pavilions. The market place is chock full of cheap disposable junk in every category. Speaking to the strength aspect...I'm sure that those joinery systems have probably been engineer approved. I would be very surprised if they weren't.
   However, even though I'm sure that those joints may perform well in a controlled testing setting handling downward force, they would be very poor in handling and kind of racking pressure. The dovetail portion only looks to be about an inch deep, which on the secondary member is one inch of fragile end grain. 
   The other point is that the slightest water damage or rot will cause that joint to fail very quickly, especially on an open pergola where that joint is going to be a funnel for water. Because of the siting on lakes and other open high wind areas, I would be quite surprised if these didn't start collapsing, possibly with deadly consequences, within 10 years or probably less. And because of the shallow bearing surface any failure will be total and catastrophic.
      As an example of the opposite... when I timber frame, I pocket in floor joists 1 1/2" for the bearing surface. Engineering really only requires that they be housed 1" typically. However, in 200 years if the frame has been neglected and has started to lean a bit, causing the joist to come out their pockets say 1/2", I want that margin to exist. It can be the difference between the frame falling in under neglect, or of being able to be salvaged and straightened.

[BarnMB]

I have been a long time lurker here but never posted before. We have been using this system for various projects on whit pine over the last few years and are very impressed with it. Due to the fact that the jig is tapered and dovetailed it has tremendous pulling power. I appreciate traditional timber framing a lot but this jig does have its place. 

[S.Hyland]

Hi BarnMB, thanks making the leap on the first time post!

I agree with your point to an extent. I think that this system could be acceptable if one wanted to use it in certain cases. I think that it lends itself fairly well toward secondary framing members such as floor joists and purlins. As you say, because of the geometry these are going to handle vertical loading well. If the primary members were traditionally framed I think that it would work. As has already been stated, there are other more ideological reasons one might not want to use them, but if one wanted to use them I agree that they could have a place.

I do take exception to a lot of the examples out there, such as the pergola company or a lot of the applications seen on the Arunda website. I see no way that it could be acceptable in a post to beam arrangement, for example. Under racking pressure it's either going to blow out the post face or the parallel grain of the dovetail. Do mechanical fasteners usually get speced for those connections? That's the only way I could see it being viable. I think one has to agree that the joint is a disaster exposed to weather. The geometry is going to suck water in and keep it there.

As with most things, for it's potential utility it has great power to be misused by those who don't know or don't care.

Funny coincidence... About a week ago I got a solicitation in my email by some log and timber company offering house frames built this way. I gave it a quick look and said "Boy, those look like some wanky little dovetails!" and deleted it as junk mail. The cost they were charging for kits...$3.00 a board foot. This was wholesaler pricing of course, not for the consumer. Now I have a better understanding of how they are so crazy cheap. It's really scary to think of a whole house being joined this way. I predict lawsuits in the future because of misuse of this system.

[Brad_bb]

It's not about sacrilege, It's about practicality and integrity.

Dovetails have been used in timber framing, but mostly in floor joists from what I've seen.  The problem with dovetails in green wood, when they shrink they are no longer tight.  You need to put wedges in the dovetail and tighten them after things shrink, which you usually can't do.  That is why floor joists are usually just simple tusk tenons or drop in and secured on one end only. 

One problem I see with trying apply dovetails to a horizontal member to vertical member joint is that the dovetail will seem strong if the force is applied in a straight line, but if racked and force is applied starting at top or bottom of the dovetail, well could start shearing the dovetail. 

It's not really practical either.  A dovetail will take more time to cut by hand.  Those examples you posted are all machine cut.  If you have a CNC or even just a router, you need to continually adjust the router for the width of the timber.  Timbers are rarely actually the same.  You could have 1/8" -1/4" of variation or more.  With traditional layout techniques, it takes into account that timbers will vary from the nominal dimensions.  It would take more work and time, I think to make sure machine dovetail joinery came out the same from piece to piece.  It's just not practical on a full size timber frame in my opinion. 

[S.Hyland]

Brad,  I'm sure with those dovetail systems they are relying on mill rule like they would with a CNC. Timbers are all S4S to exact specs before any joinery is done. I agree that it certainly wouldn't be applicable to rough cut framing.

Barn MB, could you give us more detail on the application of the system as you have used it. Are we missing something that makes the concerns less valid? I'd love to hear from someone with direct experience. Thanks!

[BarnMB]

The jigs are set up to use center line layout. Once you have the jig set you can easily cut a mortise or a tenon in 30 seconds. We have primarily used it for joists as mentioned. A couple of times we have used it when we have rafters running into a structural ridge beam. Make sense? I'll try to find some pictures and see if i can manage to figure out how to post them. Not sure what brad means by 1/8-1/4 making a difference, you could have 1/2" difference without changing.

The mortise jig is about 2 mils thinner then the tenon jig. As a rough rule you set the tenon jig @ 3 mils larger - taller then what will loose fit. Since it is tapered both ways to precision one or two got hits with a large mallet and it will be flush. It is easily possible to crush the grain it wedges so tightly.

I agree that it would not be practical outdoors due to moisture concerns.

We have used it with both rough and s4s timbers it works equally well with both as long you know which face you want it square to if they are a bit off square. And as I said earlier every joint is perfect once you have it set.