Collar tie Question

[Gilman]

I'm making Kathy (wife) the garden shed in Timber Frame Construction, Soban and Schroeder.

On page 124 they show the detail of cutting the collar tie.

In the upper right corner of the page shows the Rafter Detail.

Is the mortise in the rafter cut square like shown in the upper right corner, or do you cut one edge of the mortise at 45 degrees?

[alpmeadow]

Hi
I guess we are undertaking the same project.  Here is my drawing of the collar tie in my rafter.
Hope it helps and good luck.
alpmeadow

[Gilman]

Do you have your book handy?  If so, look on page 124.  Their cad drawing shows the mortise cut square. I'd see how that would be easier and assemble easier, but wanted to make sure which way it should be cut.

[alpmeadow]

Hi
I believe you have both options.  Talking to a local timberframer about cutting brace pockets in posts, he advised it is easier to cut it out square, as the open pocket will not impact the tension and compression forces in either timber.  The same for the rafter may apply to cut it square, unless the rafter size is too small or a heavy snow load means cutting the pocket on angle to keep the rafter and collar tie stronger and more stable.  I defer to our more experienced members to comment.
alpmeadow

[moonhill]

If there is a great load on the rafters, the collar is in compression.  In that case the bottom of the collar and the bottom of the pocket (the square end) are in contact.  The upper part (the beveled end) actually has a gap.  Notice on page 124 the upper drawning,  and the gap.  The gap is there to allow for rough size stock and its variables.  It should not be a tight fit, no contact.  I find it easier to rough out the mortice full depth, instead of creating a bevel the whole depth of the mortice.  I don't waste time checking the beveled end of the mortice for square like you do on the square end.   

The term collar tie is some times misleading, in this case it isn't actually tieing the rafters together it is a compression member.  This collar tie will do nothing for the spreading of the rafters.  The spreading of the rafters is handled with the dropped tie beam.  I wonder if we change the words around, tie collar, if it would than keep the rafters from spreading.  I'm just practicing my typing.  Ill stop now.  Tim B.

[Gilman]

Thanks Tim, squareish it will be