To birdsmouth, or not to birdsmouth, I-rafters, and more...

[hnash53]

As I look around at construction in my area, I see less and less where birdsmouths are being used when installing rafters using 2X material.

Are birdsmouths not necessary now because of the use of rafter ties?  By the way, If one is using 2X material, one installs blocking, right, between the rafters once they are installed? 

How does one install blocking between installed I-rafters?  Or is something else done when using I-rafters?

What if one is using engineered I-rafters?  Birdsmouths certainly aren't used there are they?  And by the way, in my project, where I am adding on an addition with a shed roof, I attach the rafters to the old structure using joist hangers, right?  And if I am using I-rafters, there are special hangers made for using I-rafters, right?

Yeah, I know...lots of questions.  But my framing book by Wagner doesn't have much on actually using engineered joists and rafters, which is what I see being used mostly.

Any help/comments/suggestions/links would be appreciated.  Thanks.

Hal

[MountainDon]

Myself I would never build a roof with standard rafters and NOT have a birdsmouth where the rafter sits on the upper plate. The birdsmouth gives a solid flat surface where the downward vector is transferred to the wall. Without a birdsmouth the downward force will be transferred across a point to point contact.

The birdsmouth also makes it easier to position the rafter, hold it in place, and nail the upper end to the ridge board, especially when working alone.

Yes blocking material is installed at the wall, in between the rafters. This is mandatory in many places, good practice at any time. It's much easier to install them before the roof is sheathed.


I am not familiar with using "I-rafters". I have used conventional built up 2x trusses. They have had a flat section that sits on the wall plate.

[MountainDon]

... by the way, in my project, where I am adding on an addition with a shed roof, I attach the rafters to the old structure using joist hangers, right? 

I'm not sure what you mean by that... joist hangers?

[hnash53]

I am adding on to my cabin.  The addition will have a shed roof which will attach to the end of my existing cabin.  To attach the shed roof to the existing structure, I will install a ledger board to the existing structure, then attach the rafters to the ledger board.

By the way, as I try to research about using engineered I-rafters, I can't find much on I-rafter span tables.  There's lots of info on I-joists, but not much on using I-rafters, although you see them used on all new construction.

Question:  Rafters are sized smaller than floor joists, aren't they, because of the heavier loads joists support?

[MountainDon]

Ah, a shed roof. Yes you would use a ledger and hangers on the existing wall. IIRC there are some special Simpson products for this.

You likely won't find tables on I-joists because they are engineered solutions. Normal practice is to go to the manufacturer/sales desk and give them the basic dimensions and they go from there.

Answer to Question: Rafters are usually smaller in the cross section than a floor joist when the roof is a gable or hip type. When it's a shed roof, and with heavy snow loads they could be larger than the floor joists though. [span for a shed roof is twice that for a gable roof] Run some numbers through the AWC calculator with 55 lbs and up snow loads and see what comes out.

http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

[hnash53]

IIRC?  What is that?

[John_C]

IIRC  =  If I Remember Correctly

[MountainDon]

Simpson  LSU  series




http://www.strongtie.com/products/connectors/lsu-lssu.asp

[firefox]

http://www.strongtie.com/products/connectors/VPA.asp

Hi Don,
I used these on my garage and they worked pretty well. They were reasonably easy to use. They look like they will handle the loads, but then I didn't research it that well since this was just for a 16' x 10' garage.

I am thinking that they would transfer the loads properly and you wouldn't be weakining the rafters by notching them. But then I really don't know that much.

Bruce

[MountainDon]

Firefox's connector



I can see how they would work; I've never seen them around here.

This is maybe just me... but I look at them in the same way I look at the change made years ago to ham radio licenses; now you can get a basic one w/o knowing Morse code. It just doesn't seem right.    And it isn't all that hard to cut birdsmouths; but that's just sometimes cantankerous me.   


The cuts don't weaken the rafters if done correctly. It's only the tail load anyways. Between the ridge and the wall there's no difference.

Anyhow, something learned every day. Simpson does seem to have something for everything.

[glenn kangiser]

....including lots of peoples money....

[rwanders]

Go to the APA website at APAWOOD.ORG and download D710, I-beam Framing Details and  E30, Engineered Wood Construction Guide. They will refer you to many other APA documents on use of I-Beams and similar wood products.

[Native_NM]

The major mfgs. of I-Joists have detailed technical documents on their websites.  GP, for example, has a 52 page technical manual available for free download.  The documents have detailed pictures and installation instructions.

As mentioned earlier, I have a friend who is a framer.  He primarily built high-end  homes in California.  His experience there was vastly different than New Mexico due to the earthquake requirements.  In any event, we were actually just discussing birdsmouth on I-joists, and he referred me to the GP website.  They do not recommend cutting a birdsmouth without beefing up the web.  In a non-cut installation, they have a diagram of the i-joist laying flat over a second top plate that has been beveled to the angle of the roof.  This provides more surface area for the joist to rest on, provides a larger nail surface, and another surface for the Simpson fastner.  

In talking with my friend, he indicated that for a simple gable roof one could use the same technique for traditional 2X framing and save a whole bunch of time. The second top plate would provide enough surface area to absorb transfer the load downward.  He indicated he had seen the technique used years ago.

I'll defer to the experts here for additional guidance, as I am not a framer, and my friend is mostly retired.

[hnash53]

The simpson connector pictured would work well for 2X material.  However because of the I-beam construction there's nothing to nail/screw to except the web ... and I don't think one wants to do that.

Seems I've hit something here that either people don't know about, or I am very confused...which is possible.  This I-beam stuff is used all over but I still don't see how one attaches them to a ridge board or ledger board....  same goes for where they attach to the top plate of a wall.

Seems to me that there needs to be 2X material at both places where the I-beams are attached to the building.

What am I missing?

[hnash53]

I took a look at the apawood.org site and viewed some of the documents there.

I guess I understand a bit more, but with all of the stiffening, filler blocks, squash blocks and such required for this engineered lumber, I'm wondering if I shouldn't just go with conventional 2X material.

Do they make 18' 2x8s?  Would simplify my project.

Thanks for all the input guys.

Hal

[John_C]

Do they make 18' 2x8s?  Would simplify my project.

I can't speak for your area but I can get lengths to 24' locally.  You might have to go to a 2x10, but that would still probably be cheaper than I joists.  They charge extra when they call it engineered lumber

[rwanders]

I-joists are quite good for long spans and stiffer floors----I see less value when used for rafters except they are much lighter and somewhat easier to handle up high for small crews. There other advantage is no warping or shrinkage like you sometimes get from the quality of lumber you see now. Lots of contractors like them and claim labor/cost savings----less important for a DIY builder.

[MountainDon]

Do they make 18' 2x8s?  Would simplify my project.

Likr John said.
Depends on the local dealers. Around here most of the time the boxes stop at 16 ft. Some of the real lumber yards stock 20's and 24's. I've never seen 18's; doesn't mean they don't exist though