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Proper name of notch and depth of

Started by Sedgehammer, November 25, 2019, 12:16:32 PM

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Sedgehammer

Sorry if this has been answered, but i cannot find it.

Notch is on the interior side of the top plate and there is no rafter tail. 3" x 12" rafter sets on top plate to the inside. Rafter is notched to put pressure on interior top plate. What is the name of said notch and what would be the proper depth of the notch with a 3" x 12"? Thanks
Necessity is the engine of drive

Jim_Rogers

Just about anything cut into a plate for the rafter connection would/could be called a rafter "seat".
Something like this:



 

or:


 

 

As to how much, that's an engineering question.

I believe there is a rule or code on how deep the bird's mouth cut can be made into the rafter. I think it is no more than 1/3 but it could be less than that, like 1/4 the depth of the rafter.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Don P

There are sections of code that apply here, the caveat is that these codes apply to light frame construction, a 3x12 is light framing, remember heavy timber starts at 5x5 and larger. What this joins may be heavy timber. Anyway, here's some stuff to think about. I'd let the engineers do the detail though.

From the code then, this part applies to a rafter with a tail and here is the 1/4 max depth notch provision. This is relatively new, maybe 10 or so years. I don't agree with this but here is that section;



That doesn't really apply here but there is another section that this came from that would apply, it's actually in the floor chapter;








So there is the 1/4 depth limiting language, I'll show you where that comes from in a second but while we're in the codebook the minimum size of the bearing is here;





The document most of the wood portions of the building codes reference is the NDS, The National Design Specification for Wood Construction. In other words this is where the rules, the span tables, the engineering, comes from. Here is their blurb on notches that generates the codes above;






That's the laws, now for a bit of common sense. Just because you are allowed to make a particular notch doesn't make it a good idea. The end notch in the timber above is called a re-entrant corner, it concentrates splitting stress to that inside corner. I cannot tell you how many times I've seen floor joists split with the split emanating from that corner, some split to failure as the slope of grain runs out. The same can happen with a rafter if it is shaped like the left hand example of a birdsmouth in Jim's post above. NDS 3.4.3.2 goes into that in more detail than we should need, don't go there.

The right hand example is better, but only if the load is carried on the notch in the plate, if the load is on the wider face the problem is the same as the left example although if the rafter splits it won't fail, it'll land in that pocket. View the code above as a minimum but quantify load and check the bearing area capacity. Think about what you want tight and what you want loose as you are fitting up.

The beveled shoulder is a way of lessening that stress concentration. Going into another section of the NDS 3.2.3 "A gradual taper cut from the reduced depth of the member to the full depth of the member in lieu of a square cornered notch reduces stress concentrations." You'll actually see that concept in good older work, they probably didn't use the fancy language or write a novel about notching but they certainly had common sense :)

Sedgehammer

Thanks for your reply Don and also to Jim. Ben did mention that it was possible to split, but didn't mention an alternative. to save money he suggested I do all the design work and he'll check off on it if it passes, as he said everything we've done to date was good. I am drawing everything by hand and then having my brother to it in sketchup, so I'm trying to get an acceptable way of doing it, so it's not an back and fourth. How would you set it on the 2x6 top pate? It is also resting on the purlin timber, so not all the force that would typically be there, is there.
Necessity is the engine of drive

Hilltop366

Not sure if you will be seeing the rafter ends or not but I have used joist hangers in a horizontal position to receive rafter ends. Perhaps now there are purpose made metal fasteners that would work.

I cut the rafter ends so the heal of the rafter was fully on the plate and added a smaller 2x for the overhang. (This was on a loft floor)

The building inspector at the time seemed to like it and it was easy to build.

Question: Will you be required to have hurricane straps or something else to prevent uplift?



 

Sedgehammer

This is for a cathedral ceiling and the rafters will be exposed. Yes uplift is a concern. We are in tornado country. I think 120 mph is our wind requirements. The plate area will not be seen. What I think you're showing is an angle type plate that is screwed in both directions, correct? What types of screws/bolts did you use? Yes it will have tails, but they with go on top of the decking, as these rafters will terminate at the top plate. Another way of preventing uplift would be to use a 'strap' on the ends of the rafters and go down the exterior, but that doesn't prevent outward thrust.
Necessity is the engine of drive

Hilltop366

Quote from: Sedgehammer on November 26, 2019, 11:13:24 AMWhat I think you're showing is an angle type plate that is screwed in both directions, correct? What types of screws/bolts did you use?


It was a standard 2x6" joist hanger, I can't remember how I fastened them, most likely I used hot dipped galvanized nail. (the short joist hanger ones)

Don P

That's a great solution. If the 3x12's are an actual 3" thick a double hanger would fit perfect. A 10dx3" nail would work in a double top plate.

Hilltop366

A bit pricer but I have seen some outdoor grade (meant to be seen) hangers that would look much better if in view.

Also some galvanized hangers that the sides are folded under instead of out so that the joist hides the "wall" part of the hanger. I'm thinking that these could be painted or at least trimmed over easier.

Sedgehammer

Thanks for the replies! i'm thinking of leaving the notch, as it's a natural stop of the rafter pushing out. Then i'll add a decorative plate that covers the entire side of the rafter from the bottom of the notch straight up to the top of the rafter. .375 plate that is maybe 5" wide. Will be laser cut, so it'l have a decorative cut outs in it. Maybe a lattice type look. 5/16 bolts that will go through the 3x12. 2 bolts 1' from the bottom, in the middle and 1" from the top. Plus i'll most likely add the double joist hanger as you both suggested and add a strap on the end of the rafter down to the studs. What do you think? Over kill or?
Necessity is the engine of drive

Don P

It's hard to say what you actually need without seeing the current plan. The purlin description sounds like there is no thrust, not sure. A strap at the end for uplift is good, look through the uplift connectors at strongtie.com, Simpson's site. They also have the concealed hangers and architectural products group of connectors Hilltop mentioned.

One thing came to mind after seeing your drawing Hilltop. I've done similar as well as just nailed a birdsmouthed rafter to a plate on a floor before. It is good for raising the interior height and overhang height. I was taking a short course at the U and the professor brought this up. When you look at the heeljoint tables it specifies nailing the rafter to the side of the joist. When you set the rafter on top of the joist really scrutinize that connection all the way through from rafter to the tie, it is hard to develop the same strength.


Hilltop366

I can see what your saying Don P, I hope it never falls apart because I'm sitting about 12 feet from it.  :D

I can say that of bunch the roofs I shingled years ago the worst were some built in the 50's/ 60's, there were some that you didn't dare drop a bundle of shingles on for fear the roof would collapse but my roof was as steady as a rock when the bundle hit the deck.

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