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Author Topic: bird mouth for 8x8 rafters  (Read 7227 times)

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Offline laffs

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bird mouth for 8x8 rafters
« on: January 12, 2010, 10:10:59 PM »
im new to this. for the most part im self taught. i plan on building a 26x28 barn this spring plan on using 8x8s for rafters. would it be reasonable to use conventionable birds mouth? then anchor with lag bolt.if not shove me in the right direction please ive read some timber frame books and havent found the answers im looking for. ive looked at sobons rafter seats and they look difficult and time consumeing.any help? anybody
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Offline moonhill

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Re: bird mouth for 8x8 rafters
« Reply #1 on: January 13, 2010, 07:16:33 AM »
Can you share more information on the whole frame?  Tie beams, dropped or at the plate level?  Is there a truss in the roof system?  Rafter spacing?

Is the birds mouth you are wanting to use land on the inside of the plate or the outside of the plate?

I recently timed cutting step lapped rafter seats for 6x6 rafters,  I could easily do one while 2.5 songs played on the radio.  At an average song play time of 3 minutes that is 7.5 or 8 minutes.  That is right on time with Jack Sobon's time for joint execution.  I did vary from his directed play list and drill 2  holes to ease the deep corners and I used an axe to remove the bulk wood where the rafter tail sits.  That is after layout. 

Tim
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Offline matt eddy

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Re: bird mouth for 8x8 rafters
« Reply #2 on: January 13, 2010, 09:13:12 AM »
if the rafters sit on/into the plate i typically house the 90 degree cut into the plate and let the top sit on the plate.  i try to get the intersection of the 90 degree and seat cut in the approx center of the 8x8.  to do this you can adjust the depth of the housing.  i usually put a timberlock screw into the top of the rafter after words.  however i have been successful in just pegging them with a 3/4 peg.  I started using timber-locks when i took down a barn a few years ago ago and the pegged bird-mouths where releasing from the plate as we pulled the roof boards up.  Good examples of birds-mouths commonly used check out : Jack Sobon's book Historic American Timber joinery, page 34.  This is a great book i often refer to it as the timberframe bible! 
Good luck
Matt

Offline laffs

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Re: bird mouth for 8x8 rafters
« Reply #3 on: January 13, 2010, 05:58:23 PM »
i meant the cuts on the plates looked difficult...probably i havent been at it long enough..gonna space rafters 8ft.pegged dovetail collar ties and purlins. tie beams below plates...i havent got it all sawed out yet *DanG snow ..and logs just show up in the middle of the night..*DanG customers
brent
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Offline matt eddy

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Re: bird mouth for 8x8 rafters
« Reply #4 on: January 13, 2010, 09:52:54 PM »
router or axe  your choice i find them both equall in speed, however the axe is much more fun!
matt

Offline laffs

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Re: bird mouth for 8x8 rafters
« Reply #5 on: January 14, 2010, 09:43:59 PM »
timber frame construction pg 46 step laped rafter seat . thats the one that looks difficult. what i proposed to do was cut a birds mouth in the 8x8 like in stick built and then anchor with lag bolts 5/16
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Offline moonhill

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Re: bird mouth for 8x8 rafters
« Reply #6 on: January 15, 2010, 05:38:31 AM »
Page 46 example is the 2.5 song version I spoke of.  The example below that is what Matt is referring.

It is my understanding the the typical birds mouth, in stick built structures, is cut into the underside of the rafter and rest on top of the top plates.  The plate is not cut.  From your description Laffs,  you want to cut the a joint similar to what Matt is referring?

Maybe someone will post some pictures.

Tim
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Offline matt eddy

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Re: bird mouth for 8x8 rafters
« Reply #7 on: January 15, 2010, 09:21:27 PM »
i tryed to do a pic, but i am computer dumb.  Jim prob has a good pic.  I personally do no like the rafters to sit ontop of plate with out any plumb cut on inside plate.  I have seen it done but structural i think its bad.  laffs email me or send me your email and i can send you a pic of a typical birdsmouth i do.  I believe i even have a cad drawing.
matt   pinetreetimberframes@yahoo.com

Offline laffs

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Re: bird mouth for 8x8 rafters
« Reply #8 on: January 15, 2010, 09:33:27 PM »
me too mat i did a cut out and took pics..might have done something to my pc..but anyway maybe could always add a heel on rafter
timber harvester,tinberjack230,34hp kubota,job ace excavator carpenter tools up the yingyang,

Offline Jim_Rogers

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Re: bird mouth for 8x8 rafters
« Reply #9 on: January 16, 2010, 06:35:54 AM »
Send any pictures or drawings you want posted to me and I'll post them for you, anytime.

Here are the examples we were talking about:





The rafter seat with the small notch out of the plate locates the rafter and prevents it from moving side to side. It also gives the rafter more bearing surface with the small flat part at the heel that rests in the notch. This is better then the pointed heel.
The bevel seat is easier to cut and it locates the rafter and prevents it from moving side to side.

You have to be careful where you create the birds mouth in the rafter as this can cause stress in the rafter and it may split at this point. The bevel seat would be the one most likely to not cause a split.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline moonhill

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Re: bird mouth for 8x8 rafters
« Reply #10 on: January 17, 2010, 07:00:21 AM »
The first picture which says step-lapped rafter seat is a little deceiving.  The step-lapped rafter seat is pictured above that which is not shown, Jim?

Tim
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Offline Jim_Rogers

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Re: bird mouth for 8x8 rafters
« Reply #11 on: January 17, 2010, 09:01:59 AM »
corrected thanks for the note
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline laffs

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Re: bird mouth for 8x8 rafters
« Reply #12 on: January 17, 2010, 01:05:09 PM »
makes more sense to me on here for some reason. dashed lines are just where the rafter sits? when i looked at them before i referred them as a hidden line,like something else needed to be cut. i sent Matt some pics of stuff i cut from scrap.
so if ya do it like these pictures then your using sips for facer and soffit. correct?
timber harvester,tinberjack230,34hp kubota,job ace excavator carpenter tools up the yingyang,


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