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:
www.terryking.us/public/gambrel3.jpg or
www.terryking.us/public/gambrel2.jpg
for what this looks like.

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

looks like a step-lap would work
shaped like stair steps, this joint should be in most tf books
a step lap can let a tail run through the plate if you want support for your eaves.
Could send a drawing if you can't find it
-Mark in Ontario.

Terry, What you do not want to do is cut grooves across the top of your plate. I would route small housings on the inside of the plate no deeper than 3/4". This housing will help keep the rafters standing and give you a positive layout as you assemble your roof. Uplift should be a concern so I would recomend using Oly log screws to attach the rafters to the plate; or use a Simpson A34 clip and nail them down. Pre drilling and laging them down is great but takes alot more time.
I see no advantage to moving the rafters to allow for your 1" strapping.

Bake

Mark, BAKE, thanks for the ideas... I can see I don't want to take material out of the plate all the way across, for sure..

So, a small housing at the inner corner, maybe with a couple of steps to maximize strength VS removed material seems right. I want to have a significant surface of the rafter bottom/edge bearing for both downward and outward force.

The max. uplift on this design is 200 lbs, so a lag or a strap is in order, I think.

Any other suggestions or pointers appreciated...

I would be curious as to how you arrived at your Roof design, and, why that wouldn't have suggested, or forced, a rafter to Plate connection?

Most of the Traditional Gambrel Barn roofs I have seen in the NorthEast U.S. have had the rafters broken across Continous Purlin Posts. But, your rafters are only 2" x 8"'s so it would seem, to me, that this question could Just as easily, if not more easily, be solved by asking, What would a conventional Carpenter do?

I would certainly not use a 1" peg, for you would, at absolute best, have but 1/2" checks on either side of your peg hole.

And, as Bake points out, 1" Purlins are, better refered to as Strapping.

I am all for Step Laps, but, it would seem, given the nature of the roof framing, and the dimension of the members, that Bake's Advice will likely be the truest for your particular circumstances.

Tim, You said:

>I would be curious as to how you arrived at your Roof design, and, why that wouldn't have suggested, or forced, a rafter to Plate connection?

Well, I basically needed good headroom on the second floor, and after a lot of looking I found barn plans from the Extension Service Out West that had both the gambrel spans I needed (32 feet) and snow load rating that I needed. Those plans spec'd the 2 by 8's . I'm pretty sure I'm going to combine the housing suggested earlier with the conventional carpenters birds-mouth to get 'enough' side-force bearing area, which I think is 4 square inches (Taking the compression load rating of wet hemlock at 360 PSI) to get a rating of 1440 lbs side force. The Plans I have show a side force of 1000 pounds in the 100 year worst-case wet snow load of 75 pounds per square foot. (The US Weather service says 50 pounds is OK here, but I have some data from nearby New Hampshire through the Army Cold Weather Lab that says 75 is possible.)

One of the books I read early on said something like "We should build barns so that 200 years from now they'll say 'Those Old Geezers knew how to build 'em!' ". So I'm aiming at that.

Yeah, the 1 inch peg is a Bad Idea. Can someone point to an online supplier for those "Oly Log Screws"?? I've never actually seen one. Our log cabin has these huge 14" spikes...

Thanks for the help; I'll put some photos up WHEN I get this farther along.

my teacher wouldn't have hesitated to use small pegs in this situation.......
i would say 1/2 inch or less for 2" material
the wood doesn't need a lot of strength in that spot...

Terry

Here are a few links for screws http://loghelp.com/fastener.html
http://www.fastenmaster.com/terracomm/cat_detail.epl?product_id=3
http://www.olyfast.com/products.htm