Is the seat cut always 3 1/2″ long for a 2×4 wall plate and 5 1/2″ for a 2×6 plate?
Does the length or height of the cut ever change?
Thanks
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Replies
We ALWAYS customized our seat cuts to fit whatever situation we encountered.
Bob's next test date: 12/10/07
Thanks Jim. What method do you use to figure out the cut?
You may need to think about the height of the fascia relative to the top of the windows on houses with a steeper pitch. A shorter seat cut will also give you a bit more room for insulation. Lotsa things to consider...
I woke up and smelled something today. But I don't think it was coffee
"Lotsa things to consider..." I agree Boss. That's why I was wondering if there was a formula to figure out the cut depth and length.
I assume the 3 1/2 and 5 1/2 is pretty much a general size if no custom work needs to be done? Is this correct?
"I assume the 3 1/2 and 5 1/2 is pretty much a general size if no custom work needs to be done?"
It probably is, just because it "looks right".
Vision without action is merely a dream. Action without vision just passes the time. Vision with action can change the world. [Joel Barker]
I assume the 3 1/2 and 5 1/2 is pretty much a general size if no custom work needs to be done? Is this correct?
Yes, you can use that and have no problems. Also, just because a wall is a 2x6, you don't have to make the seatcut 5-1/2" when using ceiling joists. You should however if it's going to be a cathedral ceiling. Is this for your garage with the 4/12 pitch roof? If so, what size rafters and ceiling joists are you going to use? Joe Carola
Hello Joe. Yes it is in a way related to my garage. Since I have never framed before I have been studying alot and have not really come accross a good explanation on how to figure the height and length of the seat cut.
The only thing I have read on that is, 3 1/2 inches on a 2x4 plate and 5 1/2 inches on a 2x6.
I'm one of those people that has to know why it is done the way it is. I don't know if that is a good thing or not.
I have found that in framing there are so many variables that it could explain why the seat size is not set in stone. I guess it can't be since you may run across many different situations that may require a differnet h.a.p. and the seat needs to be cut deeper or more shallow etc.
As far as my garage goes I think I am going to follow your advice on the joists. I was going to use 2x6 rafters. Did you get a chance to look at the last post I replied to you on my garage question?
Generally the birdsmouth seat cut is the width of the top plate, as this maximizes your bearing surface and allows good attachment (min. 3 8d nails). But it's also important that your HAP (height above plate) leave approximately 2/3 of the rafter width intact.
What some here have referred to as "heel" is the HAP.
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Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I think that so often now hurricane clips are required, the connection part of the issue is moot. As long as the load is transferred properly, ie 1 1/2" or greater.
But according to the code the 2/3s rule can result in a condition where the rafter is overcut on a steep roof. Having said that, I personally believe that not to be an issue as long as the seatcut doesn't extend past the inside of the wall.
There is a JLC article written by David Utterback of the WWPA that adresses this. It has to do with the grading of the lumber and the fact that it could result in the stock splitting.
We always consider supporting the short side of the rafter in some way too. In the case where it might extend inside the plate, there are options. Sometimes, we've had the ability to support the stock hanging over the plate if the ceiling is getting a soffit of some kind. Other times, we've notched the rafter and been able to nail the extended bottom edge into the sides of the plate. I've never been knocked down when I did either. Basically, for every problem, there is a solution. It might have simpson stamped on it. Bob's next test date: 12/10/07
Basically, for every problem, there is a solution. It might have simpson stamped on it.
That is a great statement. I love it :-)
Thanks Riversong. That explains it perfectly to me.
That's what i have done. 3 1/2" + 5 1/2" so it sits on the top plate.
Right now I'm in house that gets ice dams so I would strongly consider getting the most distance you can from the top plate to the bottom of the roof sheathing.
Do you guys put blocking in between the rafters on top of the plates? It would block any ventilation from the soffit though. But it sure stiffens up the rafters and connects them to top plate.
Edited 3/13/2008 3:10 pm ET by popawheelie
Do you guys put blocking in between the rafters on top of the plates? It would block any ventilation from the soffit though.
Maybe that's why you're getting ice dams. Gotta have that ventilation.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I didn't build the house we're in right now. The one that's getting ice dams. I grew up in So Cal and did my framing there so and ventilation wasn't a big issue as I remember.
The way houses are built has changed.
If you want to see the house we are in right now go to the general discusion 1950's By-level house thread and My pics are at #45.
If you look at the intersection of the hips and the shed roof there's one of our problems. The house has a bunch of little problems like the roof.
Edited 3/13/2008 4:28 pm ET by popawheelie
We work from the fascia up. Our fascia is governed by the window heights and belt lines, projection of soffit and of course the Architects specs.After calculating all this, it usually dictates to us what our heel will be. If the heel meets our standards (4" minimum) we go with what ever it calculates out to be. Keep in mind that a 4" heel doesn't work very well for insulation purposes. Most heels are now "energy" heels which means they are much greater than 4". Bob's next test date: 12/10/07
Jim forgive my ignorance but when you say 4" heel is that on a 2x6 top plate that you have that size heel on?
"Keep in mind that a 4" heel doesn't work very well for insulation purposes. Most heels are now "energy" heels which means they are much greater than 4"."
A 4" heel would drop the rafter lower onto the the plate correct? Would'nt a greater than 4" heel do the same?
When I talk about heel, I'm talking about the minimum distance that "should" be left to properly carry the roof through to the overhang. In theory, you could safely create a heel with 0 height. I've had to do that in some situations with bay windows and applications of that sorts.When we calculate our heels, we only give a small consideration to the size of the wall plate. It does not matter to us that the inside of the rafter meets the inside of the plate like so many people desire. To us, that is an irrelevant detail. Bob's next test date: 12/10/07
I gotcha now Jim. Thanks for all the help.
The code is somewhat difficult to understand on this sometimes. Basically you need a minimum 1 1/2" bearing or seatcut. After that, make sure that the underside of the rafter doesn't extend in past the inside of the plate.
There is a D/4 notching rule that can be applied (12/4 for a 2x12, the 1/4 being the max you can take out) but this comes into play if you have a large overhang or heavy snow loads.
If you just keep the rafter from sticking into the house, and have a min 1 1/2" seatcut. you'll be fine. Check with your inspector first though.
I nearly always use a 3" seatcut because that keeps my heel hight tall and I like that look. If we close the soffits, usually I can still have a decent amount of room from teh soffit to the head of the window.
There is a D/4 notching rule that can be applied (12/4 for a 2x12, the 1/4 being the max you can take out) but this comes into play if you have a large overhang or heavy snow loads.
Actually, the D/4 rule applies to end-notching with or without a cantilever. For instance, a joist that is notched to drop lower onto a wall plate cannot be notched more than a quarter of its depth. This is to prevent the notch from getting close to the center of the joist, which is where the maximum horizontal shear stress is and the point of likely splitting under load.
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Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 3/13/2008 2:34 pm ET by Riversong
Actually, the D/4 rule applies to end-notching with or without a cantilever. For instance, a joist that is notched to drop lower onto a wall plate cannot be notched more than a quarter of its depth. This is to prevent the notch from getting close to the center of the joist, which is where the maximum horizontal shear stress is and the point of likely splitting under load.
IRC
R802.7 Cutting and notching
R802.7.1 Sawn Lumber. Notches in solid lumber joists, rafters and beams shall not exceed one-sixth of the depth of the member, shall not be longer than one-third of the depth of the member and shall not be located in the middle one-third of the span. Notches at the ends of the member shall not exceed one-fourth the depth of the member. The tension side of members 4 inches (102mm) or greater in nominal thickness shall not be notched except at the ends of members. The diameter of the holes bored or cut into members shall not exceed one-third the depth of the member. Holes shall not be closer than 2 inches (51mm) to the top or bottom of the member, or to any other hole located in the member. Where the member is also notched, the hole shall not be closer than 2 inches (51mm) to the notch.
Exception: Notches on cantilevered portions of rafters are permitted provided the dimension of the remaining portion of the rafter is not less than 4-inch nominal (102mm)and the length of the cantilever does not exceed 24" (610mm)
I think we are saying the same thing. So I'm not sure why you used the word "actually". That implies to me that I misstated something. If I did, please tell me what I said incorrectly.
I talked to an engineer about this subject. This is the understanding that I'm left with after talking to him and posting this on the ICC forum and seeing the responses there.
Assuming no tail, if the heel of the rafter is bearing on the plate 1 1/2" -to the inside edge of the plate, the notch won't weaken the rafter. When it hangs inside the wall, there is the chance of a split or crack that could follow the grain.
There are conditions where you had a full 2x6 bearing and that would weaken the tail of the rafter. But in a practical sense, that isn't too big an issue for most of us.
So again, the conclusion seems to be, make sure you have 1 1/2" of bearing minimum and maximum to the inside of the wall plate.
Or just don't notch more than 1/4.
So I'm not sure why you used the word "actually". That implies to me that I misstated something. If I did, please tell me what I said incorrectly.
It's not that you mistated anything, but you said the D/4 rule comes into play if you have a large overhang or heavy snow loads, suggesting that it's a rule only for cantilevers. I merely pointed out that it applies even without any cantilever to any end-bearing member that has some horizontal component.
The 1½" minimum bearing rule, however, ignores the need for proper fastening. To put even 3 8d toenails, as required, into the rafter-plate connection would result in splitting if there is only a 1½" seat.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Thanks for clarifying that.
About the 1 1/2", since hurricane clips are required (I believe) under the IRC, the connection issue probably isn't that big a deal. You could easily get 1 3"x.131" nail from each side without splitting and then let the hurricane clips and the solid blocking or birdblocking help with that issue.
I personally wouldn't go for the 1 1/2", but its acceptable under the IRC.
I personally wouldn't go for the 1 1/2", but its acceptable under the IRC.
The codes set minimum standards, and I think few of us build to those minimum standards.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
You are right about that, but I have before :-)
Thanks Tim. I got it now. Thanks again for the help.
there is an excellent book on the subject that explains things in great detail.
" a roof cutter's secrets" by will holladay. it is a journal of light construction book.
there can be many reasons and many correct ways to do anything, but maybe one way will stand out as better than the rest. i find will's book to be the best but i like to approach things in a similar manner, and also cut my teeth in the central coast of cali.
someone raised and trained in another area of the country may not like the techniques as well. with modern autocad programs (sketchup etc) you can play with full scale layout on the puter.
Thanks segundo. I just purchased a copy of the book. I'll look into it.