The Forestry Forum

General Forestry => Timber Framing/Log construction => Topic started by: bigshow on June 04, 2007, 12:53:44 PM

Title: Post to Plate joint
Post by: bigshow on June 04, 2007, 12:53:44 PM
I know i'm going to have to use metal to secure my principle and common rafters.  So as per my diagram.  I have a housed birdsmouth, with a 1/2 cylinder representing an approximate bolt/lag size.  this is at the gable end of my frame.  I have created a 'blind housing??' so that part of my roof doesnt slide off.  So, i have my bolt location going thru the center.  what should i do for post to plate joinery at this location?  or should i just offset the metal or run it in at an angle to provide more room for tenon.  plate is 8x10, post is 8x8.

See, one thing i cant seem to find..is if want my tenon parallel to plate, or perpendicular...i see it both ways.  My frame is comprised of principle rafters at each bent (4 bents in total) and commonn rafters in between.  I also have 8x12 principle purlins running the length of the frame.  So, rafters are supported on both the principle purlins and plates on my outer most posts.  So, that being said...its my guess that i have plenty of support for roof and the system i have in place should reduce outward thrust..in which case i'm inclined to believe a tenon perpendicular to the plate would be stronger when outward thrust is concerned...but i have a feeling i have that beat and could use whatever works.  I have put way too much thought into this....havent I?

(http://www.forestryforum.com/gallery/albums/userpics/14035/post_plate.jpg)

pre/post thanks: Jim, before i posted this i realized i never replied to the last email you sent me a couple weeks ago regarding a previous question/post.  I take pride in providing thanks when i receive help...i dont believe i thanked you for your last email reply.....thanks!!

and, thanks in advance for anyone that has input.
Title: Re: Post to Plate joint
Post by: Jim_Rogers on June 04, 2007, 06:32:01 PM
what should i do for post to plate joinery at this location?  i see it both ways.
Well if you ever see a mortise going cross grain and not with the grain then that timber framer didn't know his a$$ from his elbow.
YOU never ever go across grain with a mortise.
(http://www.forestryforum.com/images/03_21_04/amortisesrightandwrong-s.jpg)

Just follow my example to you the last time; offset your post tenon two inches off the outside face and two inches off the end of the plate....

Jim Rogers


Title: Re: Post to Plate joint
Post by: Jim_Rogers on June 04, 2007, 06:43:54 PM
Like this:

(http://www.forestryforum.com/gallery/albums/userpics/10095/post%20to%20plate%20joint.JPG)

Only center the peg hole on the tenon width, not the post width....
Title: Re: Post to Plate joint
Post by: bigshow on June 04, 2007, 08:40:11 PM
oh man.....thanks for that.  looking at illustrations of english tying joints had me all in a fluster.  I kept thinking i saw mortises going cross grain. Now, sitting there and thinking - no way thats possible...i understand what i'm seeing perfectly.

I have a tendency to not keep it simple.

Title: Re: Post to Plate joint
Post by: Don P on June 04, 2007, 09:19:25 PM
I'm wondering if the birdsmouth notch can be in the bottom 1/4 of the rafter depth rather than near middepth. I'm trying to visualize shrinkage and whether it will shrink off the lower housing, throw the load to the top of the plate, then split the rafter till the bottom hits the lower housing again  ???
Title: Re: Post to Plate joint
Post by: Thomas-in-Kentucky on June 05, 2007, 08:39:03 AM
I wondered that myself Don - especially since this is the exact joint that I used in the common rafter part of my house.  (and lagged it from above as bigshow illustrates) Like you, I figured that if the top of the birdsmouth became loaded enough to split the rafter, the bottom of the rafter would settle on the housing before splitting got out of hand.  In fact, it's almost impossible to get the top of the birdsmouth and the bottom of the birdsmouth precise enough, even at initial assembly, so that they both bear weight at the same time.  Shrinkage (between cutting and assembly) results in angles that are no longer what you started with, so that the end of the top of the birdsmouth wants to hit before the inside, regardless of what the bottom of the birdsmouth is doing.  Then, to beat all, the housing makes it very difficult to get in there and kerf.  By "kerf", I mean to adjust with a saw after partial assembly.  Just put it together, do your best, it will work.  :)

So far, splitting doesn't seem to be an issue.  I'll take a close look at these joints this evening and see how they're behaving so far.  They have been together for well over a year now - maybe the top and bottom of the birdsmouths are finally both touching.  :D :D :D :D

FWIW, I laid out my post tenons just like Jim suggests, but I used double tenons.  I had in my head some nonsense about the top plate rolling... but I'll now admit that I don't think it was necessary.   ::)  For the top plate to roll, it would have to raise the rafter feet vertically, which does not seem likely.  I wonder if anyone has ever found evidence of this failure mode in older timber frame buildings?  Probably not.

-Thomas
Title: Re: Post to Plate joint
Post by: bigshow on June 05, 2007, 11:20:37 AM
how about setting birdsmouth on a haunch?

(http://www.forestryforum.com/gallery/albums/userpics/14035/housed_birds.jpg)
Title: Re: Post to Plate joint
Post by: Jim_Rogers on June 05, 2007, 12:17:56 PM
Stay with the way you have it drawn in your first post.
When cutting the birds mouth on the rafter end you have to take into consideration the shrinkage factors. The rafter may shrink and lift off the rear housing seat but hopefully the plate will shrink nearly the same amount so that the rafter still sits in the rear housing seat.
Sometimes plate housings are dished so that when the plate shrinks the housing is then flat.

Jim Rogers