Dovetail Wedges for Floor Joists?

[jander3]

I plan on using dovetail joints to install the floor joists for my sauna.  I've read a few things on wedges, but I don't understand how they are used.  Anyone have a photo or drawing that shows the wedges and how they are installed?  What material is used?

[shinnlinger]

Since 50 people have looked at this, but no replies, I will say I too am unfamiliar with what you are asking.

Is it about wedging the dove tail if it is an imperfect joint to make it tighter?  I suppose I wouldn't be to worried about it unless most of your joints are pretty sloppy and you have a real aversion to using nail plates on the rim joist where they won't be seen.  One bad joint won't effect the overall structure.

Dave

[Jim_Rogers]

The problem with using dovetail joints for floor joists are that the dovetail is going to shrink.

If you're looking down at the dovetail from above the shrinkage is going to happen from left to right. This makes the dovetail smaller than the dovetail pocket cut into the side of a sill, for example. If the dovetail tenon is smaller then the dovetail pocket then the joist can pull out of the pocket.



In the above drawing the red dimension is what is going to shrink. To offset the expected shrinkage hardwood wedges are driven in on the sides shown here at the blue lines.

I hope that helps.

Jim

[jander3]

Jim,

Thanks.  That is exactly what I was looking for.

Is there a better joint for floor joists?

Jon

[Alexis]

from your explication Jim,

the framers needs to deliberately leave a gap to have space to put the wedge, is that standard practice?

Alexis

[beenthere]

The problem with using dovetail joints for floor joists are that the dovetail is going to shrink.
..................
In the above drawing the red dimension is what is going to shrink. To offset the expected shrinkage hardwood wedges are driven in on the sides shown here at the blue lines...........

A little misleading, as the red line is shown running along the grain where there is essentially no shrinkage. The shrinkage explained is for across the grain on the joist.   

[Jim_Rogers]

The problem with using dovetail joints for floor joists are that the dovetail is going to shrink.
..................
In the above drawing the red dimension is what is going to shrink. To offset the expected shrinkage hardwood wedges are driven in on the sides shown here at the blue lines...........

A little misleading, as the red line is shown running along the grain where there is essentially no shrinkage. The shrinkage explained is for across the grain on the joist.   

The red line is suppose to be a dimension line showing that in this dimension the wood will shrink.

It was a quick sketch to show the part of the dovetail tenon that will shrink, and it was intended to show which way it will shrink.

It's my understanding that because of this type of shrinkage most shops have now moved away from using dovetail joints, especially those not housed. If the dovetail tenon is housed into the sill, tie, plate, or rafter (as a dovetail ended purlin) and you look up to see the bottom of this joint it will look normal even if the dovetail has pulled out a very small amount.

However, if the dovetail tenon is not housed and you can view it from the bottom, and it has shrunk you will see a gap between the timber with the dovetail tenon and the piece with the dovetail pocket. Which is poor workmanship and sheds a bad light on timber framing.

My experience was with talking to others who have used this joint, and that they have done this "wedging" with timbers that were both eastern white pine. They do not leave any gap anywhere in cutting these joints. And pound in a hardwood wedge with the intend that the pine will be somewhat compressed by the wedge and when it shrinks the wood will then expand what was compressed and not pull out.

If a tenon needs to be secured to the piece with the pocket then another tenon design should/could be used.

It really depends on the "why".... that is why does this tenon need to be secured? There should be other members in the frame that have the duty to hold the frame together. Most floor joist's jobs are to just hold up the floor.

Of course unless it is a tying joist. If it is a tying joist then it should be made to tie the joist to the sill, tie beam, or girt that it will be attached to. This is usually done with a tenon, sometimes known as a tusk tenon.

Like this:



and the pocket would look like this:



With the appropriate peg hole coming down through the top of the girt/tie/sill through the tenon, not showing in this drawing.

I have real pictures of these joints if you want to see them.

Hope that helps....

[DWM II]

Jim, those look familiar! These tusk tenons are easier to cut with precision as well.

[Brad_bb]

Obviously dovetail and tusk tenon joists do the same function of supporting the floor load, but the dovetail  in addition, resists spreading or movement of the beam they are let into, adding some rigidity.  Which you use would seem to depend on your design intent.
But here's a question Jim, When you layout a dovetail, do you layout the dovetail shaped tenon the same dimensions as the corresponding mortise?  The reason for the question is, what if the wood only shrinks a little?  Could the gap be too small for a wedge?  It would seem you'd need at least an 1/8th inch wedge for the integrity of the wedge as you drove it in, no?  So would you intentionally offset the dims of either the tenon or mortise?

[shinnlinger]

When you say sauna, I am thinking small.  If it were me, I would just make joists that drop in pockets and have the rim joists or perimeter pegged to hold it all together.  By the time you get your flooring and siding on, you will be all set.

Of course I would cut all my joists with my chainsaw too so maybe you don't wan to follow my advice.

Dave

[jander3]

Jim,

Thanks for the drawings and clarification.

shinnlinger,

I am also thinking about just pockets and tenons. I only need to span about 8 feet.

[shinnlinger]

Jim,

Do you have any sketches on how your sill/plate corner intersection would look on a small frame.  I have done a few different things and have never been entirely satisfied.

Dave

[Jim_Rogers]

When you layout a dovetail, do you layout the dovetail shaped tenon the same dimensions as the corresponding mortise? 

.
As mentioned I have not ever cut these joints, and those who explained it to me said they do not cut any joint with any gap and pound in the wedge to intentionally compress the eastern white pine timbers. I have never seen this done, only heard about it. I have never seen the gaps they talk about when it's not done. I honestly don't even know if it works or not. Or how it would work with other types of wood.

Most shops have moved away from this system because of the potential problem it could produce. Again if the frame is being held together by other joints then these joist may not need to be dovetail tenons.

[Jim_Rogers]

Jim,
Do you have any sketches on how your sill/plate corner intersection would look on a small frame.  I have done a few different things and have never been entirely satisfied.
Dave

Yes, for general information, the sill that is going parallel with the ridge is known as the "long sill" and the sill that connects the two long sills together is usually known as either the "short sill" or the "cross sill" as it goes across the frame.

Normally the long sill is the same length of the building. And the cross sill is the width of the building less the width of the two long sills reduced at each end for the square rule joint plus each tenon. (You need to get the shoulder to shoulder distance first and then add the tenon length back on to get the exact overall length of the cross sill. More on this if needed.)

Next you need to apply your general frame rules or standard timber framing rules to determine the exact placement of your tenon and mortise.

For example if you're building with 8x8's then the tenons are 2" thick (one quarter standard rule, mentioned in other posts on this forum). And usually these are laid out the same dimension, so that's means the tenon is 2" off the reference face and then 2" thick. (For a smaller frame 6x6 pieces these are 1 1/2" x 1 1/2").

Next, due to the fact that the tenon of the cross sill will be right at the end of the long sill, the tenon is cut back a bit so that there is some "relish" left at the end of the long sill:



In the above drawing this may not be easily seen, but you should plan on it.

So, you can see the housing in the long sill and the mortise and peg for the cross sill tenon.

And one thing I learned a while ago about this mortise in the side of the end of the long sill is that the long sill isn't cut to exact length until after the mortise is done. This provides more wood beyond the end of the long sill for the "relish" to stay intact. If the long sill is cut to length first then this "relish" is only 2" long and it's mostly cross grain wood and can be easily by accident chipped out when cutting the mortise.

To prevent this short piece of "relish" wood from not being accidently knocked out we cut the mortise first and fit the cross sill to it and then we cut the long sill to length. It works for me.

Keep asking questions....

Jim Rogers

[moonhill]

Notice the nice simple drop in pocket just down the long sill from the post. 

Dovetails I have cut were smaller than the mortice, once in place it would rattle around with out the wedges.  The wedges are smashed in and deform the wood.  I think it is rather crude.  I find dovetails time consuming in comparison to the simple drop in or the tusk tenon mentioned earlier.  I only use them when forced into them.  A dove tail on the end of a summer beam on occasion is fine but to do all the joist and then the purlins is border line torture.  I don't believe they were use all that much traditionally, it is a modern framing addition.

Tim

[shinnlinger]

Jim,

I like your sketch and I like your explanation.  I did a half lap thing to avoid what your relish answer deals with, and I wasn't happy with my answer.  I like your's better.

Thanks.

Dave