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long unsupported rafter spans

KirkpatrickFramer's picture

Does anyone have a link to a rafter span table ? I'm framing a house with a 20'-7 1/2" run from 1st floor wall to 2nd floor beam. It's a 6/12 roof and the rafteres are about 26'. No loads other than composition shingles and drywall underneath.
I know I should just google this but I'd like some feedback, too.
I've done a similar house with a slightly shorter run using 2x10 with a couple of rows of bridging blocks and it worked fine, I'm just wondering if I should beef up to 2x12 or use I joists. I'm not even sure if I can get 28' 2x12 in my neck of the woods and most likely the lumberyard will send me fingerjointed 2x10 at this length...
Thanks,

John

(post #101184, reply #1 of 14)

John,


Try this http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/


As long as you know the loading conditions in your area and the material you will use (grade), you can figure it out pretty quick

www.pioneerbuildersonline.com From Lot 30 Muirkirk

http://picasaweb.google.com/TimothyUhler                                     

(post #101184, reply #3 of 14)

Thanks, Tim that's what I was looking for.

(post #101184, reply #4 of 14)

Is the span in that table horizontal or diagonal ?

(post #101184, reply #5 of 14)

Spans are always the horizontal

You said no loads other than the dead load but all structures have other loads applied to them. You might not have snow , but you have wind - and the occasional cat chasing a squirrel

 

 


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(post #101184, reply #2 of 14)

I-joists aren't out of the question, I have considered using them. I usually use them for floor joists and I'm not sure what to do for the birdsmouths. I guess a rafter repair kit would work fine, and these are sold at the yard, too, so it makes sense.

(post #101184, reply #10 of 14)

 I guess a rafter repair kit would work fine,


     Maybe this is a dumb question, but what is this?

(post #101184, reply #11 of 14)

The rafter repair kit is just some OSB or plywood ripped down to fit inside the flanges of the I joist to make it a uniform width.
Sometimes you have to put a roof cut on the joists, and they want a 2' long filler put on each side of the plywood webbing so the rafter and joist can be nailed together better, as well as giving support to the top flange.
BC supplies these when requested with the joist pack.
I know the main strength of the I joists are in the flanges, and to avoid cutting these if possible.


Edited 5/9/2006 11:36 pm ET by KirkpatrickFramer

(post #101184, reply #14 of 14)

"I-joists aren't out of the question.....I'm not sure what to do for the birdsmouths."

The I-joist manufactureres have standard details for that. If you ask your suplier for a design book it will show what is and is not allowable.

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(post #101184, reply #6 of 14)

Others I'm sure will give you a web site that has the tables you want, but I was wondering where you live--will you have snow to worry about? That can add quite a load to a roof. Also, you must have some kind of roof deck--shingles on plywood or OSB or something.


Anyway, I have a table that has max. spans for low or high slope rafters with drywall ceiling and live load of 30 psf.  It shows that you could use 2x10's at 16" o.c., to span up to 21' -10" but the wood has to have the an Fb rating of 2000 (psi) which makes it something like select structural Douglas fir. With a rafter spacing of 13.7 inches, you can get away with wood having an Fb of 1600 (psi) which would be something like dense No. 2 Doug fir. If you go 2x12, 16" o.c., it'll span 21' -5" with boards having an Fb value of 1300, so something like No.2 Douglas fir would work.


I am not an engineer and the book I'm looking at is old, and you don't know me or know whether I know anything (and I don't), so take all of this with a grain of salt. This reply is just to give you some ideas, not for you to use to go out and buy lumber with and build something! I guess I'm telling you you'd be better off going with 2x12's, but they still have to have the right "extreme fiber in bending" (Fb) value. I think I-joists are probably a good idea.


Someone who knows something will probably reply shortly.

(post #101184, reply #7 of 14)

sounds good to me . i'm in houston w/no snow loads (that sux) and we'll be using tech sheild OSB for the decking. I'll be going with 2x12 #2 syp
if available, if not good doug fir.

(post #101184, reply #8 of 14)

If you are in Houston:


Home Lumber in Rosenberg / Rick Rangel / Sales Rep / 281-238-1112


My main supplier, and very builder friendly lumberyard with long lengths and good pricing. They will deliver anywhere in the Greater Houston area. They took good care of me when I built on the coast in the Palacios area.


Also:


Montalbano


Bison


When in doubt, or close to span tolerances, I have doubled every 4' and used blocking @ approx 8' O.C.  No call backs.

(post #101184, reply #9 of 14)

I'm on the north side Woodlands/Spring area and we use Bison most of the time. My lumber rep gave me the numbers but I wanted to hear from some other guys here, maybe somebody has done something on the limits of code and have some other advise to share. I like to use better than minimum requirements.
I'm also wondering about I-joists as a solution and how others cut these as rafters or if they use a beveled plate. I messed up on the original post the span is actually 23-6, a long span for an unsupported rafter.

On another note, what's are some different ways to insulate this ? Around my area I see lots of condensation on humid mornings or cold mornings, I can actually tell where the rafter bays are and would like to avoid this any clues ?

John

(post #101184, reply #12 of 14)

I'm on the north side Woodlands/Spring area


I am in Hungerford, about 30 miles south of SugarLand on 59 


I'm also wondering about I-joists as a solution and how others cut these as rafters or if they use a beveled plate. 


Instructions can be obtained from the manufacturer.


On another note, what's are some different ways to insulate this ? Around my area I see lots of condensation on humid mornings or cold mornings, I can actually tell where the rafter bays are and would like to avoid this any clues ?


I would think continious soffit vents / continious ridge vent / eggcrat baffles against Tech Shield (formerly Kool Ply) decking / baffles keep a space between deck and insulation allowing air flow...not sure if that is the correct term "baffle" / R30 or better under the baffles in the rafter cavity.


 

(post #101184, reply #13 of 14)

John,


We are using I-joists on the next roof.  I'll take lots of pics and let you know what it was like.  Should be interesting, but we have all the tools and machines so it should go really well.


As soon as I get the layout from Weyhauser (TrusJoist) I'll send it to you so you can see what it's like.

www.pioneerbuildersonline.com From Lot 30 Muirkirk

http://picasaweb.google.com/TimothyUhler