How would you make this timber joint??

[Terry King]

I need to decide how to join the gambrel roof rafters on the barn I'm building to the top plates running along the top of the side walls.

The top plates are 8" by 8" beams. The rafters are 2" by 8". (These are all actual sizes, native lumber). The angle they meet at is 55 degrees.

I'd like to do more than the small 'birds mouth' that occurs the way the pieces meet, by some amount of cut-in to the plates. But what's the best compromise of attaching strength VS minimizing the material removed from the plates? What's a strong geometry here? I can rout some shaped cut-in in the plates, but I'd sure like to do that on the ground, soon now.

I will have 1" thick purlins on the outer surface, and so I can set the rafters in 1" more than shown to get more material at the inner end of the joint.

The max. bad wet snow load is 1500 pounds vertical, and 1000 pounds outward. I plan to use a 1" wooden peg or a large lag screw in each joint; this is not necessarily traditional timber frame construction totally...

Take a look at:

or for what this looks like.

Suggestions, pointers, comments appreciated! I have some books but I haven't found this joint discussed.

[Lobby Dosser]

Terry King wrote in news: [e-mail]:

See if you can find something on Japanese joinery. I've got a vague recollection of something similar in a book on Japanese construction.

[bridger]

first, my framing experience is almost entirely with stick framing, so I may not be following what you're doing entirely, nor may I have good solutions for you. but hey, it's usenet....

you show neither ridge nor collar ties in your drawing. I hope you're not building it that way...

you could mortise the tails in, but make sure you leave plenty of material at the outside of the mortise. make sure that any cuts you do make seat in towards the inside of the building. it might be necessary to cut a shoulder on the tennon to leave enough wood around the mortise

if you move the rafters in to account for the purlins the horizontal amount will be more than the 1" thickness of the purlins. I'd bet it will give you plenty of strength in the birdsmouth. if you're still worried about it, add an fha strap from the rafter over the top plate and tie it in to the wall framing.

[Terry King]

What's an FHA strap? Some government required anti-hurricane roof-liftoff device, maybe?? Are these standard items, or might steel strapping be OK??

The max uplift in this design is only about 200 pounds as I recall, unless you use South Florida data...

Thanks for the ideas....

[bridger]

yep. a length of sheet metal a couple of inches wide by various lengths with holes for nails. bendable enough to frinstance hook one end over the top of a rafter, get a few nails there, run it down the face of said rafter (more nails) give it a twist to lay flat on the face of the top plate (more nails) another twist to lay flat against a stud (more nails)..... in situations where tension loads are a factor they can add a lot to a connection.

there are lots of versions, fha (federal housing authority) required and others. lots of use-specific ones and lots of general purpose ones too. a place to look into it would be:

you mean steel strapping like what pallets are bound with, I wouldn't. too light, no nail holes and no inspector will pass it.