Need specific mortise and tenon advice

[tonatonion]

Hi,
I am just about to start building a single-storey timber frame house. I'm new to this kind of building, and I have a couple of queries relating to the mortise and tenon joints, and the pins.
In my plans I have 3 kinds of tenon - a standard horizontal through tenon, a blind horizontal half tenon that will butt another of the same through the post, and a square, centered, vertical tenon on top of the post (onto which will slot the top plate with its mortise, the other end of the top plate having the blind half tenon mentioned).
In regards to the second (blind) type of tenon - I've read that I should leave 3 times the width of the pin in the tenon, after the pin. My stock dimensions are 105mm x 105mm throughout, so I calculate that, with a blind half tenon of 47mm I can allow for a pin thickness of maybe 10mm, which would leave 30mm of tenon after the pin, but only 7mm between the pin and the shoulder. Is this too close to the shoulder, and more specifically, is this pin dimension (10mm x 105mm) too thin?
2. Is there any objection to a square, centered, vertical tenon at the top of a post? I have dimensions of 50mm x 50mm for the tenon, centered in a 105mm x 105mm stock.

[Brian_Weekley]

Welcome,

105x105 mm (ca. 4x4 inch) posts seem very small for timber frame house construction.  Where are you located?  Typically, you wouldn't use square tenons.  In your example, the tenon at the top of the post would use the entire width (e.g. 50x105, located 50 mm from the outside edge.  It would be very helpful to see a plan of what you're building so the experienced folks can provide better feedback.

[tonatonion]

Hi Brian

I'm located in Japan. 105mm squared is the standard post dimension in these parts, and since our house will be single storey with a light, low-pitched roof, I've seen no reason to use larger dimensions for the beams. Our plans are all on paper (we did try to make sense of AutoCAD, but decided we'd probably have the house half-built by the time we figured out how to use it).
I'm primarily concerned about the pin spacing and dimension issue on the blind tenons.................

Regards
Iain Andrews

[Jim_Rogers]

There have been some new standards about peg/pin locations that the guild published a while ago.
And you may need to consult these and see how they will apply to your design.
You can access these standards at the Timber Framers Guild's Engineering Council's web site at:
http://www.timberframeengineeringcouncil.org and you may need to look in the "library" section.

Normally when we layout timbers using the square rule method we have a housing that you may or may not use with your timbers.
If you don't use a housing, because all your timbers are planed smooth, then your edge distance from the edge of the mortise timber maybe different then if it was housed.
This will determine how far off the shoulder of the tenoned piece the peg/pin should be.

In case you didn't know the tenons are usually 1/4 the thickness of the piece, which it would appear that your design meets this. And then the peg/pin would be 1/2 the tenon thickness.

In imperial terms your 4x4 tie beam would have a 1" tenon with a 1/2" peg/pin.

If you use a general standard peg/pin layout of 1 1/2" off the shoulder, and a 1/2" housing, then your peg would be about centered on your post for your through tenon joint. Using a 1/2" peg your distance would be 4 peg diameters from the end of the tenon. Which exceeds the 3 minimum required. If you don't use a housing then it is even further meaning it exceeds the 3 minimum more.

Also, you have again expressed a design of two butted tenon inside a post. Of course you will not have this 3 diameter distances here. This is why I suggested an over and under tenon to you on the other forum. Doing so will again allow you to have the 3 diameters distance needed.

Now about your square tenon on top of your post to the above beam. This size 50mm x 50mm doesn't leave much wood to peg/pin the tenon to the beam. This is one reason why we don't use square centered tenons. You should have a tenon that is wide enough for the peg/pin as well as far enough away from the end of the mortise to allow some relish on the mortise.



In the above picture the beam has be rotated 90° to show the mortise.

In the above drawing you can see that the post meets the beam at the beam end. And the post has been reduced on one side to a standard width of 1/2" under the actual size of the timber. This is what square rule layout means.

The tenon is laid out off the face of the post the same distance as the tenon thickness. This makes it easy to layout the tenon and the mortise to match, by using a steel framing square.

You can also see that mortise doesn't go to the end of the beam. This makes the beam stronger.
It is set back from the end the same distance as the tenon thickness or more.

If you still want to use a square tenon then you'll most likely have to use a metal fastener to hold the beam to the tenon. You could drill a larger hole and counter sink the head of the fastener and hide it with a wooden plug, but that's for you to decide.

I'm not sure if you're concerned about roof up lift or not, but normally we do peg or secure the beam to the top of the post.

And we need to have enough wood there to make it all work.

Thanks for asking.
Good luck with your project.

Jim Rogers

[Brad_bb]

Why is he using such small timber?  I'm used to seeing 8"X8" timbers or so.  Maybe you know more about this Jim?  A 4"x4" tie beam?  Even regular stick built floor joists are 6-10" tall.  A 1/2" (12.5mm) peg?  Can it handle the loads of a home?  With such small timbers, will it be exposed on the interior?  It won't have much visual impact that size.