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Rafter Blocking at Top Plate

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Kent Whitten

· The Duke
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Discussion starter · #1 ·
I have 2 questions.

1. Do you block your rafter tails (not trusses)
2. Is this code to block rafter tails?

The reason why I am asking is that today, a wide, wide gap in techniques reared it's head. I have always put rafter blocks in, every time, with a 1" air gap minimum at the top for airflow to the bays. Like this.

Image


I was told today that it is not done, it is not necessary, it is overkill, no one does it, it doesn't do anything, waste of time. Even on raised heel trusses, not done. Our raised heel is about 18" HAP.

Found a small section in the code book about it, wasn't too detailed.
 
#2 ·
framerman said:
I have 2 questions.

1. Do you block your rafter tails (not trusses)
2. Is this code to block rafter tails?
Click to expand...
1. I do not block rafter tails and never have or been on a job where others have. My framing experience is limited exclusively to remodels (additions, etc) not as vast and awesome as yours :notworthy

2. It is not code (at least in the areas I work). Though more times than not the eng/archi specs a 2x sub fascia.
 
#3 ·
I don't always do it, but more often than not, I do.
I think of them as 2x insulation stops. :p

Everything you do to a truss system helps. ie: truss blocking, spacer strapping, couple of diagonal straps, ridge blocking.
Yes, the truss plant sends out a strapping layout, but usually with too few to straighten the trusses to the boarder's satisfaction.
I also run diagonals on the king struts from the wall to as high up as the length of available strapping allows.

no, not code here.
 
#5 ·
Never heard of this, what is going to keep the rafters from rolling.

Did a quick web search and found this from the National Institute of Building Sciences.

"Blocking and Lateral Load Paths for Roof Systems

Rafters and ceiling joists having a nominal depth-to-thickness ratio exceeding 5:1 (e.g.,2x10) need blocking at their points of bearing to prevent them from rotating or displacing laterally from their intended position. Rotation loads on rafters occur when the roof sheathing is resisting lateral loads perpendicular to the rafter because these loads are actually trying to move the top edge of the rafter sideways. Preventing rotation is typically accomplished by installing full depth blocking along wall plates between rafters and ceiling joists."


As for what code it is, not sure. But I am sure a inspector around here would have a fit if he saw us trying to get away without blocking.
 
#7 ·
I was not trained to do rafter blocks at top plate.

But about 15 years ago we were framing a place and a experienced carpenter that I had not worked with before started putting them in because that's how he always did it.

I really liked how strong it made that all feel. sort of plums the rafter. I can't stand rafters that aren't plum. makes a great insulation block and leaving the gap for the very important vent.

Come to think of it seems under-built without them.
 
#8 ·
I don't have any code books where I am at right now, but am still searching.

This is from the UBC in 1994.

Sec. 2326.12.8. Blocking. Roof rafters and ceiling joists shall be supported laterally to prevent rotations and lateral displacement when required by Section 2306.7. Roof trusses shall be supported laterally at points of bearing by solid blocking to prevent rotation and lateral displacement.



I'll find some more, but in 30+ years we have always done it and it was required per plan. Maybe it has to do with the seismic zone we are in.
 
#9 ·
I have never installed rafter blocking but I read about it in Larry Haun's book. It seems like a good idea to prevent rafters from twisting. I think it looks better with the blocking.



Rafters and ceiling joists having a nominal depth-to-thickness ratio exceeding 5:1 (e.g.,2x10) need blocking at their points of bearing to prevent them from rotating or displacing laterally from their intended position. Rotation loads on rafters occur when the roof sheathing is resisting lateral loads perpendicular to the rafter because these loads are actually trying to move the top edge of the rafter sideways. Preventing rotation is typically accomplished by installing full depth blocking along wall plates between rafters and ceiling joists."
Click to expand...
Does that take into account the birds mouth cut and the pitch? Or is it just related to nominal rafter size?
 
#10 · (Edited)
Found it.

2006 IRC
R802.8
Lateral support. Rafters and ceiling joists having a depth-to-thickness ratio exceeding 5 to 1 based on nominal dimensions shall be provided with lateral support at points of bearing to prevent rotation.


We have alway used 2 x 12's, so that is why it was required. And if I remember right Washington and Oregon did not adopt the IRC until 2001 or 2003, we used the UBC before that and as stated in that UBC code ref I posted earlier it was required period regardless of rafter size.



Edit.

From the 2009 IRC

R602.10.6.2

1. For limited height rafters or trusses (i.e., =9.25-in. heel height), blocking is not required in low wind and seismic regions. This requirement establishes a reasonable limit on the generally accepted past practice of not providing eave blocking.
2. For conditions beyond this limitation, e.g., taller heel heights or in higher seismic and wind regions, “partial height” eave blocking is required to comply with one of four options. Three options are prescriptive framing details. The fourth option is for the connection to be designed in accordance with accepted engineering practice.
 
#11 ·
Here in CA. we have always done it, (included rafter blocking) and I spec this out on my conventional light frame construction details. I refer to them as "pressure blocking" and I spec out 'A-35 nailed all holes' as well 16" O.C.

It is just something that I have always done and always been expected for the conventional stuff.

Andy.
 
#12 ·
ScipioAfricanus said:
Here in CA. we have always done it, (included rafter blocking) and I spec this out on my conventional light frame construction details. I refer to them as "pressure blocking" and I spec out 'A-35 nailed all holes' as well 16" O.C.

It is just something that I have always done and always been expected for the conventional stuff.

Andy.
Click to expand...
We have to do those pesky A35's now also. Must be because we are both in higher siesmic regions. Damn engineers have gone nuts with these, we have them all over the house.
 
#13 ·
Here in California every residential or commercial job I have been on has blocking at top plate where rafter is connected. It is not unusual to have the roof sheathing blocked every 4 feet & at the ridge for shear nailing. 6"oc nailing is often called out at the rafter/top plate blocking without the other blocking.
 
#15 ·
ScipioAfricanus said:
Here in CA. we have always done it, (included rafter blocking) and I spec this out on my conventional light frame construction details. I refer to them as "pressure blocking" and I spec out 'A-35 nailed all holes' as well 16" O.C.

It is just something that I have always done and always been expected for the conventional stuff.

Andy.
Click to expand...

Yep. Provides boundary nailing for the roof sheathing. It's always on the plans. Usually A35's on every block too at the top plate.
 
Discussion starter · #19 ·
KennMacMoragh said:
Yes, but not like that. What's to stop critters from getting in there?
Click to expand...
Closed soffits. You mat be thinking about open soffits out there in Seattle. That's all we did when I lived out there was vented bird blocks. (3) 2" holes every 24". Here we have vent strips or vinyl in our soffits as the primary critter stopper.

I just found this unusual, especially on gable roofs. I have 3 primary reasons why I do this.

Shear transferance
Take the twist out of rafters at plate
Insulation block
 
#20 ·
We do not do it and I have never seen it done or heard of it. I have built many houses that were either stick framed or trussed, and worked with several different engineers and architects. We are required to install the simpson rafter/truss ties at the end of all rafters/trusses. Not sure why the block does anything for the insulation. What do you mean by "insulation block"? I can see how there would be some structural help provided but it does seem like overkill.
 
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