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Bird's Mouth/ Heel Cuts in stick built roof framing

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56K views 34 replies 15 participants last post by  Timuhler  
D
#1 ·
I was wondering how some of you guys are doing this and would like some "opinions" about this.

My main questions concerns the width of the level cut when you are framing with different wall thicknesses (eg. 2x4 top plate, 2x6 top, etc.)

I know this is rather rudimentary, but I as I have limited roof framing experience, I would like to be clearer on this subject, as it has come up on a job, and I didn't have "all" the answers.

So, I know your plumb cut is going to be 1-1/2" up. The argument is that should the heel cut be 3-1/2" at all times ( because of the way teh speed square is designed) on, say a 2x6 wall, or should it be cut at 5-1/2" long (as I think) to allow full bearing on the wall?

OK, guy's, let 'er rip, as I am sure I will get some nasty comments on this..:whistling
 
K
#2 ·
It will depend on many things, but as a rule of thumb, the heel starts at the inside of the framing wall. It doesn't HAVE to be, but that will be the general consensus that it should for proper framing.

Then you have instances where you will need to match the heel height of other roofs, most notably trusses, and if you're good enough, you know how to compensate the height of a wall to allow your full bearing to the inside of the wall.

If it's a vaulted ceiling, I'd make my best effort to get that heel right on the inside edge.

There will be no die hard "right" answer, there's many methods. There are some things you don't want to do like leave a tiny birdsmouth. There some formulas out there for this, but I don't have them. Just comes naturally to me to not cut a birdsmouth in a way that will compromise the structural integrity of the rafter.
 
R
#4 ·
Nothing really to add to what framerman said other than to emphasize that there is no 'right' answer to this question. When you are matching stick to timber framing for instance you often have to make it up as you go along. Make sure it bears appropriately and that you have enough surface to put enough fasteners in.
 
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J
#6 ·
For new work your level cut/seatcut can be 3-1/2" for a 2x4 wall and 5-1/2" for a 2x6 wall. There is no rule of thumb and cutting the level seatcut the width of the wall does not ruin the structural integrity of the rafter. How can it? If you overcut the seatcut past the inside of the top plate it will.

The only thing it can to is weaken the overhang. For example if you frame a 12/12 pitch roof using a 2x6 wall and make the seatcut 5-1/2", that will give you a H.A.P. cut (Height above Plate from outside of wall) of 2-1/4". If you had no overhang, that's fine, if you have an overhang you have to strengthen the overhang, it does not weaken the rafter, the rafter is sitting full bearing flush to the inside on the top plate.

You can have a 2x8 wall and cut the seatcut 7-1/2" and in wont weaken the rafter.

Working on existing houses and tying into existing rafters is a whole other thread.
 
F
#8 ·
In the few years that I have framed, compared to most of you, I have always been taught to come down the plumb cut 4 1/4" for a 2x6 rafter 6 1/4" for a 2x8 and 7 1/4 for a2x10 to start the heel cut unless we are trying to match something else. Also a vault might compromise this theory. On lowerpitched roofs where the heel cut becomes very long i just cut the heel to the inside of the wall and then cut a 2nd plumb cut for the inside......Many many ways to do it. Ben
 
B
#9 ·
you are mistaken joe.If you make your seatcut to long ,you are also making your plumb cut too deep.Measuring square from the topside of the rafter to the inside corner of the top plate is where you lose integrity.You may have a 2x12 rafter but with too much cut out only the strength of a 2x4. You Need to lower or raise plate height sometimes tocompensate for this espescially with vaulted or cathedral cielings.
Steve
 
#10 ·
Just another thing to consider to what's already been said is that alot of new homes use taller heel hights to accomodate more ceiling insulation at the outside walls. It' makes a huge difference in effective R values. My brother usually does his at 8" heel heights, as I did on my first personal home I built 30 years ago. I was a trendsetter!
Joe
 
J
#13 ·
big Steve said:
The more you cut into a rafter the more you compromise the integrity of the lumber doesnt matter what pitch what size lumber or what state you work in.
Steve
Click to expand...
Steve,

You're the one that is mistaken. If you cut the level seat cut the with of the top plate whether it's 3-1/2" or 5-1/2" the integrity of the rafter is not compromised at all. I've been cutting them like that for 24 years and never had a problem. Th only place that is compromised is the overhang.

If you overcut the set past the inside of the top plate, then you have weakened the rafter. Where from the same State and I've never failed inspection before and I've cut rafters a million different ways and always cut them the width of the top plate with new work.
 
B
#14 ·
OK ,if you have an existing 8' wall height with 2x6 rafters-not uncommon around here An 8 pitch roof and the new addition has 8' walls also. how do you keep the fascia/gutter line at the same elavation with existing with new rafters being 2x10 or 12.You have to cut so much out of the seat cut that you destroy the load capacity of that size rafter.Or lower your plate height which kills your 8'ceiling.Maybe were are not talking about the same thing here but I have been told by inspectors here to be careful of this exact situation.As far as the eves ,if you frame them in from the fascia back to the wall they cant sag or move downward.A 2x10 rafter with a 9"long seat cut is bad.You would be better off having used a 2x6 with a shorter seat cut. I have also been doing this for over 20 years and some times you cant get around .
Steve
 
J
#15 · (Edited)
big steve said:
OK ,if you have an existing 8' wall height with 2x6 rafters-not uncommon around here An 8 pitch roof and the new addition has 8' walls also. how do you keep the fascia/gutter line at the same elavation with existing with new rafters being 2x10 or 12.You have to cut so much out of the seat cut that you destroy the load capacity of that size rafter.
Click to expand...
Steve,

First of all I was talking about new work. Now if you want to talk about tying into existing houses, that's a whole other thread. I do additions all year around, tying into an existing roof using your example with a 2x6 existing rafter and a 2x10 new rafter is always a problem that some Architects don't pick up.

You can't overcut the seatcut and I don't. I told you that already that I don't cut past the inside of the top plate. You HAVE to lower the plate height. After that to maintain an 8' ceiling you will have a little clipped ceiling where the bottom of the 2x10 rafter is. Therre are a million scenarios about this topic. I frame additions her the top of the new 2x12 rafters have to plain in with the top of the existing 2x6 rafters and follow the same overhang and fascia. The new addition walls have to be cut shorter so that the seacut doesn't extend past the inside of the top plate.



As far as the eves ,if you frame them in from the fascia back to the wall they cant sag or move downward.A 2x10 rafter with a 9"long seat cut is bad.You would be better off having used a 2x6 with a shorter seat cut.
Click to expand...
First of all the 9" seatcut would never happen. If the plans call for a 2x10, that's what the addition is getting and the walls will be shorter. If no one likes that, then I call the Architect up and ask him if the rafters can be 2x6's to match the existing house and keep the same plate height. This is his problem now because I can't make any changes.

I frame additions where the whole back of the house comes out and the addition has a cathedral ceiling or clipped ceiling and the new addition is 2x10's or 2x12's and the existing rafters are 2x6's. Now the plate heights are shorter and we have to pack down the existing rafters to plain in with the bottom of the new 2x10's or 2x12's.

All this is because I can't compromise the rafter by cutting the seatcut 9" or anything else like you said.

Like I said before, I can cut a rafter with a 3-1/2" seatcut or 5-1/2" seatcut and still not compromise the rafter. If using a 2x6 with a 12/12 pitch it will compromise the overhang. That's no big deal because the overhangs can be built up and never go anywhere.
 
W
#18 · (Edited)
Like Joe said. as long as the seat is not cut past the inside of the plate, then your rafter has full bearing (vertically up the rafter from the inside of the wall).

If you need to lower the rafter any more than that...time to drop the plates. I have done many multi-pitch multi-plate height homes (silly architects...) I tend to keep all my HAPs 4 3/8" or 6 3/8" and adjust the plates to make it work the cornice. (not pronounced cor-NISH) That way my tails have at least a 2x4 thickness at the BM for support out over the overhang. But then I ladder block my soffet sub-fascia so that tends to beef up my rafter tails.

(what would it sound like if I said PORK up my rafter tails.....mmmmmmmmmmmmmmmm BEEF!!!) okay...long day

just my 2 cents

mike
 
D
#20 ·
Well, Damn!

I certainly DIDN'T mean to start a pissing match, but it was a Good One! Damn! I was waiting to see some names written in the SNOW!

BUT really, I appreciate all the input on the matter. Now for the next question!

Do you cut your rafters BEFORE or AFTER you set the RIDGE?

I ask, because I did a job, after the ridge was set, some of the rafters were notched for a 2x4 plate, which gave the correct pitch, but after I cut the rest, for a 2x6 plate, which was already there, well, you can imagine, my rafters were LOWER and at the wrong pitch. Needless to say, I recut the previous rafters (4) and set them back in place, albeit my pitch was not correct, as spec'd on the plans. (More like the Idea napkin, in this instance). Also there was a slight gap where the rafters meet the top of the ridge. Had I cut the re-cut the plumb cuts at the ridge, then the ridge would have been too high.

So, were are ready for pissing match Two!

Ding, Ding, Ding!
 
J
#21 · (Edited)
DEAD_ONConst said:
Do you cut your rafters BEFORE or AFTER you set the RIDGE?
Click to expand...
Rafters are all cut first and then the front and back rafters on each corner are set nailed into the top plate and then the ridge gets lifted up and slid in between the plumb cuts of the rafters. No need to set the ridge first t all.

The rafters set the ridge height.
 
K
#24 ·
Joe--

What it sounds like to me is he had a 2x4 wall and 2x6 wall and measured to the outside of each, or maybe the inside, and the cut the rafters with a full bearing seatcut.

Maybe the original had 2x4 walls and the addition had 2x6 walls, anyway the HAP's wouldn't match if he went with the full bearing technique.

For the OP, You don't always have to follow the full bearing rule. 3 1/2" of bearing on a 2x6 wall is fine. Most of the time I use less than that. I'm happy with 2" on a 2x6 rafter.

When framing roofs, all your plate heights should start with the thinnest wall, widest stock and steepest pitch. Everything else gets calculated from there.

JK
 
D
#31 ·
OK, The walls are 2x6!

The seats were cut 1-1/2 plumb and 3-1/2 level! "I" wasn't paying attention! I finished framing this roof, remember, it is a Bay window, so therefore has two hip rafters.

Now, in "MY" experience, knowing that I had 2x6 plates, I cut my rafters and hips with 1-1/2 x 5-1/2 birdmouths. I cut my sheathing and hauled it up the ladder and then found that the sheathing sat high/proud of "MY" rafters because "HIS" birdmouths were not cut as deep as MINE!

The pissing match between me and the HO was "I" screwed up, and therefore was in the wrong and the HO should not have to pay the extra labor to correct the situation. I fixed it anyway, NO CHARGE, and finished the job.

Mind you, I am 28' up, it is near or below 0* and with the windchill, blowing snow, and rain, IT was a SOAKING WET B!TCH! Not to mention, I was working of a 2x6 canitlevered scaffold, of sorts, nailed to the wall framing and climbing on ladders dug into ice/snow.
 
J
#32 ·
We all make mistakes and that's all a part of learning. I know you said you were 28' up in the air with bad weather. Now you know the next time(if there is one) that you will go up 28' in the air in bad weather and check the existing roof FIRST so that you don't have to fix this problem 28' in the air.

Exposing the existing rafter at the plate and overhang is the most important part of tying into an existing house. I opened one up a bout 2 years ago where the existing rafters were 2x6's and the HAP cut was only 2 with a 5/12 pitch making the seatcut about 9-1/2". My rafters for the addition were 2x10's and the top of the rafters had to be flush with the top of the existing rafters.

I had to drop the addition walls down about 6-1/2" - 7". That was a big problem with headers and window heights.......etc.

The main thing is you found out solution and it was a good one and it worked and everyone is happy.
 
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