Timber Framers Guild Forums General General Forum Questions Lapped Dovetail vs Tenon&Mortise Knee Braces

Hi everyone,

I'm new here so let me introduce myself, I'm french living in Norway, basically engineer in industrial maintenance and interested in renewable energies, but 3 years ago I took the Pat Wolfe Log Building Course in Ontario CA and since then I'm motivated by working into that field and learn more about timber framing as well.
I have been studying books at home, practising training alone and right now I'm building my own house, timber frame based on the Steev Chappell book plan of the high post cape 24' x 24' which I downsized to 20' x 20'.

Well, I have a question today since I started cutting my Tie Beams and I didn't cut mortises inside the width of the timbers regarding the knee braces, but on the outside face, planning to fit some Lapped Dovetail knee braces. This is for the bents 1 and 3. For the middle one, bent 2, I cut mortises to fit tenon and mortise knee braces.
I read in the Tedd Benson's book (Building the Timber Frame House) that tenon&mortise braces were better without a bent and both lapped dovetail and tenon&mortise could be used between bents.
So question: is it really an issue that I'm doing lapped dovetail knee braces for my bents 1 and 3, first floor at least since it's cutted now!?
I coulnd't find so much information about the reasons for it..

I hope I can get some good inputs from you guys

I'm looking forward to hear more about it.

Have a nice day, Neil.

The use of lapped brace vs a morticed brace depends on tooling. A lapped brace can be cut with just an axe and saw, while the mortice will require more time/effort and tools.

How are you holding the lapped braces to the frame? pegs, nails, screws and boarding?

Lapped braces may have a tendency to twist, and come out of plane, if the wood is not near perfect, species choice could have impact, too, like spruce will twist more than pine.

Braces are typically designed to function under compression not tension. Dove tailed half lapped may help in tension. In compression the lapped section is using the fastening, peg, to resist tension along with the shoulder. A morticed brace has more surface to resist the compression.

Thanks for your answer Timbeal

I'm using pine 4 x 8 braces and they have been dried down to 18-20%, so hopefully twisting will be reduced then! I though about it, like in extrem cases the brace could pop out!

I'm holding the braces with 1" pegs, and, i'll think about it while framing, add some extra protection in those areas (thinking about larsen trusses solution for my walls). But, tell me, why do the lapped dovetail would have less surface in compression? excatly like the tenon&mortise, half of it is going to be in contact with the associate timber, the other half being in the mortise and partly on the timber surface too.

Well, my frame is over dimension for a 20' x 20' high post cape but I'm just thinking through all details!
Also, I join my tiebeam to the post by wedged half dovetailed through tenon. That might help the bent to be more stable too maybe.
I'll fit the braces after assembling the bents, then I'll be able to sbribbe fit them better, and probably get them more tight.
One more question: are two 1" pegs possible for such a small case (3 1/4" x 7" tenon approximately)?

Lap dovetails vs. tenons is an interesting subject.

Personally, I like the laps, you can make a quite elegent look if you wish it with these, or alternately by using a simple dovetail design with a steep angle, can get a rather archaic, almost medieval aesthetic.

It seems to me, lap braces and bents are usually not together. But I am used to a different tradition as well.

My like of the laps may also be culturally biased, it's common practice in the Swiss Canton of Bern where we come from.

I've seen a lot of these braces, so long as the stock is good (it does not have to be perfect) they are fine.

Make the braces 4 inches deep and the lap 2 inches, for example. a Depth of 2 inches is unlikely to twist out of the pocket.

In general, if you make the depth of the brace equal to or greater than the width of the brace, then twisting will not be a serious problem. Also, I usually see the depth of the dovetail as one half of the depth of the brace, or even a little more.

As for bearing surface, this is not as much of a concern as you might think, because the braces act in both compression AND tension. With a tenoned brace, you have the problem where they cannot resist tension, except by the strength of the pegs securing then. I've seen where a strong wind has gone under the roof of an Indiana barn and pulled the braces free, because they are not capable of resisting the resulting tension. The braces are designed to act in series, under normal circumstances the compression of one brace will cancel the tension that would be placed on a corresponding brace.
The lap dovetail works similarly, any excessive compression on one brace is counteracted by the corresponding brace's ability to act in tension. The difference though is that any single brace can be expected to act in both cases.

My opinion is that lap braces are superior in roof structures, where loading is uneven and unpredictable. You can not always expect a series of braces to be able to handle a given situation (such as the wind torn barn previously examined) and I feel much better knowing that every single brace has the ability to act in any possible scenario. But it's all a matter of opinion, BOTH systems have proven themselves to work MOST of the time, and BOTH systems have proven to be flawed PART of the time, a very SMALL part.

So in the end, I think it all comes down to what you like. I pick lap dovetails at a steep angle, because I think they look nicer. Structurally, I have more bracing than is technically necessary anyway.

Give that 18-20% moisture count another 5 years and it will shrink even more.

I don't employ a lot of half lap stuff, from time to time, I like how it works. In two of my next three jobs I will have some half lap stuff. Scribed while on the floor not in the upright position.

I can't help but enjoy the contained joint and the work being done by a mortice and tenon brace. I can be sloppy and neat at the same time. I can be a 1/4" off on the tenon and no one would ever know, almost. I am going by my gut feelings on this, as DL points out they all will work wonders.

The housed morticed brace has the housing and the tenon for bearing. depending on the type of half lap it may be similar or less.

Yes it is true that shrinkage can be a major concern. This is a reason why brace stock should be quite dry, and also a reason for the development of the refined dovetail, which has a compound angle to counteract shrinkage (often manifested more gracefully as a curve)

I will try and find a picture of this

Another thing.... two pegs driven in opposing directions can assist with the half lap coming undone.

I've seen some pretty wild pegging on these braces.

The best practice is to use well seasoned timber. This of course is not always practical.

In some cases, the bracing was actually added well after the building was raised and allowed to settle. Temporary bracing may at times have been used in the mean time.

Hello guys, nice, nice,

it looks like I'm pretty right on my choice then
My braces are inside the barn right now, they are dryer every day but how dry, time will tell! I guess they will be dry enough to scribbe them when mounting the bent on the floor as one of you mentionned. Also for the brace specifications, let's see, maybe I can join a sketchup, but I need to check how it works first..

I have 3 bents spaced 10'. My braces lenght is about 42" in first floor.
I have one center post connected to the tie beam and two queen post connected to the collar tie, so in total 4 braces for the first floor and two for the second (braces are a bit shorter on the second floor, about 34").
my center post is 7" x"7 so I'm limited for the braces depth if i want some wood left to support the compression without having braces pushing on each others. if you know what i mean... the same situation in between bents for the second one since I'm using 7" x 10" exterior posts.

I'm considering seriously those details since I'm building in the Lofoten islands in the north of Norway ,and let's say that it can be very windy up here since they are big mountains opened on the atlantic ocean.

If wind is a concern, I recommend horizontal girts. These tie the bents together and help the frame to resist wind forces.

Just curious, is there a particular reason why you have chosen an Anglo-American style of framing over an indigenous Norwegian method, or a French style? I understand a few reasons why you wish to do so and am not saying you shouldn't, just wondering what your thought is there?