Looking For Quick Easy Answer Of Rafter Sizing

IHI

Been awhile since we hand framed and looking for quick a quick yea/nea answer I guess, and opinions if you'd like.

27x16 addition coming up, running into problems with engineered scissor trusses and barely enough cathedral to call it that due to existing homes narrow design and 4/12 roof.....I do not want new additions peak to go above existing homes peak for obvious ashtetics reasons. Only able to do half pitch on cathdral of outside pitch, so for this 3/12 pitch scissor truss the interior cathedral can only be a 1/12 pitch which only puts peak of interior ceiling approx 13" higher than outside top plate...so not really able to get the cathedral feel if ya know what I mean.

So now we're thinking of just hand framing rafters with the 3/12 pitch to stay below existing house roof ridge....will actually be able to tie in exactly if hand framed, but given the lower pitch of the 3/12 and basically spanning 14' if you figure rafter length per side. What nominal lumber would you use? I'm thinking 2x12's 16" oc since snow load will be a factor with that low of a pitch, but cant decide if I'm overkilling, not enough warranting engineered lumbers LVL's, or able to scale back to 2x8's/2x10's

thanks guys, got some major brain frying right now with everything going on.

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karma_carpentry

I went to an online maximum span calculator, put in the figures:

2x10 Hem-Fir (North)
Snow load: 30 (Not sure if that is right for Iowa, it's a guess)
Dead load: 10
Spacing: 16" OC
Deflection: 1/240

And I got a max span of 19'9 -- so according to that calculator, 2x10's would be adequate on 16" centers, with a comfortable margin. You could tweak it more to find your own comfortable margin. Spaced 24"OC, it gives 16'2 max span. Still over 14. Maybe go for 19.2"OC spacing?

But wait - these are ceiling joists, right? You have a structural ridge, because you have no ties due to the cathedral ceiling. In that case, this is a ceiling joist, not a rafter. In that case, a ceiling joist 2x10, 24"OC has a max span of 17'4 -- so it's an even more comfortable margin. Maybe 2x10's at 24OC is the way to go?

edit: added link for max span calculator

IHI

Cool link, never thought to try searching for something like that

I guess if I wanted to push the envelope I could get by with 2x8's 16oc, but I'd sleep better at night doing this with 2x10's based off calculations. I dont want to encroch on interior too much so that extra 2" I'd save from 2x10 over 2x12 will be great. Not going to be too much of a load interior other than the sheetrock, we're doing the poly shoots upto the peak and then spray foaming all the ext wall and under roof sheeting, so that should help keep the overall weight down to prevent deflection if it were to try.

thanks
Josh

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Joe Carola

What are you using for a structural ridge? You can't just frame it without one unless you put collar ties in at the proper height but those would defeat the purpose of your higher ceiling. If you want a full cathedral ceiling with a low 3/12 pitch you NEED a structural ridge.

You don't need Architects where your from? It doesn't sound like it's on your own house.

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snapper21

I don't know about your area, and I'm not completely sure about mine, but I thought that anything under 4.5/12 was not permitted anymore without engineer's sig., and was controversial even with sig..
Also you may want to consider that the formula used in the "calculator" may be dealing with maximum bow tolerances under load. I don't know how you're finishing between the ceiling joists but if you allow it to bow to maximum tolerance you may have problems keeping a sealed finish (ie mud cracks etc.). If you're planning some sort of floating ceiling it will rule that out, but you may want to take that into consideration. Those are some low pitches that I don't think I would consider doing this to. Not to say it can't be done, or shouldn't, just feels like a problem. Your minimal span should help you with this one.

maj

Josh, I agree with Joe & Snapper. Take your plans to the lumberyard or Home Depot(if you prefer). They will be able to have an engineer figure what you need for the ridge beam & rafters. That's why I do all my business with the local lumberyards, they will call the appropriate engineers and have them spec what I need....... for free!

I would guess you'll need a LVL ridge. I would check into using I-joist for the rafters. I have done it several times. They can spec the correct size for you. I have worked mostly with 14" or 16" for rafters, but those were 26' long. Nice thing with I-joists is the top & bottom flanges. I cut 1" rigid foam to fit tight between them, push it up against the top flange, then insulate with fiberglass. The rigid foam up tight to the top flange provides the air space needed from soffit to ridge.

IHI

Hey Maj, I let Menard's do all my "dirty work" so to speak and can honestly say this is the first job I've ordered scissor trusses so I did not know about the reduced pitch on the interior side, but after hearing about it, I can totally relate why due to strength.

Anyways, told menards to really push for a 2/12 interior with 3/12 exterior if it's remotely possible, if not homie just called and said not to worry about the 1/12 pitch if it has to be that, guess he talked to his wife and she was'nt too concerned so long as it is not a flat ceiling. I did explain the rafter/collar tie issue also, even though we'd be able to match peaks and get the steepest pitch allowed, by the time we added collar ties internally to lock it all together he'd have a flat spot across the ceiling at the peak and he did'nt like that idea. I'me sure pressing the issue we could get a deisgn to work out to keep a pointed peak on the interior side, but right now waiting for truss company to get back to see if they can get the internal pitch we're after....sure be glad once this is done...I just dont think this addition was meant to be. 1 week figuring out trusses and the most gawd awful footing/poured walls I'm sure anybody's ever seen I have to work with since homie did'nt want my mud guy and found his own "cheaper" version This is the one in my other post I mentioned about the trench footings being 6" out of square in 16', and poured interior wall startes out 7" wide at house and then bows out to approx 12" by exterior wall it ties into....AND...AND the exposed side of that poured interior wall also tilts/angles into the proposed mudroom area. Bascially the guy lost 6" of floor width due to forms blowing out/bowing out at base when they poured them and stakes/bracing not holding.

Just glad homie is doing all the interior work!!! Funniest part, a very good freind of mine is a rep for a large concrete supplier and I mentioned I'd never have *** construction ever do any mud work for me, explained how bad this deal was and how it was going to suck for use to try and make it right. Find out he's good frined with the owner of *** construction and told him I was not very happy and muthafocking his work.....now I'm the bad guy???? When I stopped by the homies place to discuss some things, homie said I should probably call *** and apologize for muthafocking his work, I laughed and just told him whom ever is thinking I'm the azzhole here, needs to come over and see what he left behind before they start sticking up for him, then they'd all change their tune. Supposedly this *** guy is very anal about perfection and I was told I could be driven out to any of his other jobsites to confirm it....how does this make a fock to me I ask since I'm not building on his "other anal jobsites"...oh well, guess I'll be the azzhole since I'll be the one forced to clean up the mess left in front of me....guess I'm done venting

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Double-A

This is a good example of why we have a policy of never discussing or running down another contractor or professional. The most inflamatory things you will hear us say are, "The work is not to our company's standards" and "The work does not seems to match what the plans call for."

My reason is this. When I have done so in the past it has ALWAYS come back to haunt me in one way or another.

Having said that, I feel your pain. His work on this project tells the story. Nuff said.

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VitoFromNJ

Got a question.. What is the difference between a structual ridge and a non structual?

I allways thought that the walls pretty much held up the roof, or kept it from falling into itself, anyway.

Heck, now that I am asking this question, I think I am getting my own answer. I guess the lower the roof pitch, the more the ridge needs bearing, especially without collar ties.

Am I close??

jmic

You're boiling!

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VitoFromNJ

in ten to twelve minutes the pasta will be ready, Lets Eat.

Hey Thanks

reveivl

"Got a question.. What is the difference between a structual ridge and a non structual?"


You might want to figure that out before you stick frame someones roof.

Collar ties are irrelevant, they do not hold the walls together, they decrease the span of the rafters.

With a structural ridge 1/2 the weight of the roof is borne by the posts supporting it, which must be supported all the way to the ground. In a non-structural ridge, the walls bear the entire load.

With a structural ridge, spreading of the walls is not an issue, with a non-structural ridge (at ANY pitch), it is THE issue.

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Teetorbilt

Collar ties are irrelevant, they do not hold the walls together, they decrease the span of the rafters.


How do you quantify this?

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ApgarNJ

teetor, i was thinking the same thing.
Most of the jobs I've done are all structural ridges because the people all want cathedral ceilings. I still don't buy that all the weight is transfered completely to the gable end posts that run to the foundation. If anything, sometimes these MASSIVE beams that these engineers size for the job are putting MORE weight than needs to be on the whole roof structure.

Back to the roof issue, I'd use 2x12 just to be safe on that low of a pitch. Gives you more room for insulation too. even if we don't need to use 10s or 12s we do anyways just to get the added insulation value for the customer.
I looked a set of plans for an addition a few months ago, for an architect I've never worked with before and he had cathedral ceilings with 2x6 rafters and a 2x8 for a ridge, no collar ties. I didn't even want to deal with that job, I bid it using the correct lumber and stated it to the customer. Not even sure how they got the plans approved using that lumber speced. So many townships will just pass it if an architect has stamped it, not matter what the specs are.

Joe Carola

What!!!?

If you use a structural ridge, you will have a full cathedral ceiling with no need for collar ties because the structural ridge will hold the walls in. He will just have to post down under that where it sits on the existing wall. If there’s a doorway there, then he will have to put a microlam header in there or something else that can support the structural ridge.

If you don't want to use a structural ridge and use a regular single 2x ridge you can use collar ties designed at a specific height from an Architect or Engineer and they will hold the walls in from spreading out. It's just that you will have a level ceiling at the bottom of the collar tie height. We call them clipped ceilings.

You do not need structural ridges for this. This type of framing is done every day. In his case if he wants any height he should use a structural ridge to get the most height and then he can sheetrock the bottom of the rafters because the pitch is so low. Scissor trusses are a waste of time for his situation because of the two different pitches.

I've framed many roofs with collar ties, sometimes using bolts on every one. Sometimes using a collar tie on each side of the rafters is done Sometimes we've used multiple 2x10's or 2x12's every 32” - 4' and then they were boxed out with oak or mahogany or anything else later on and then you can still sheetrock the bottom of the rafters.

He can probably do that in his situation and just have a built up structural exposed collar ties or even nice big pieces of old 6x10 beams from somewhere that can be stained and exposed with a full cathedral ceiling above them.

Some situations you can't just use collar ties and there is a need for a structural ridge also because of a roof above that sits on top of this roof. I had that situation once about seven years ago

He has many options here.

1) Structural ridge with no collar ties.

2) Use a 2x ridge and collar ties @ 16” centers set at a specific height.

3) Use a 2x ridge and built up structural collar ties every 32” – 4’ and sheetrocked or trimmed out collar ties along with sheetrocking the bottom of the rafters in their full length.

4) He can also use cables designed by an Architect or Engineer.

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Joe Carola

Joe Carola

I'm in NJ also and we can't use 2x6's here for any section that is cathedral. We have to use 2x8 minimum. The job I'm on now I'm putting a 36' dormer on the back of the house and renovating the rest of the house and raising ceilings to 9' where the existing ceilings are 8', so the sections of the existing rafters that are exposed are all 2x6's, so where furring the bottoms down to get at least a 2x8 thickness for insulation.

He also has to be careful with the size of the rafters because the bigger the rafter he uses, it will lower the pitch. If he wants to increase the pitch so he can use shingles instead of a rolled roof, he can shorten the outside walls a little and that will increase the pitch.

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karma_carpentry

That sounds like a great idea, to me - if the walls are high enough to beging with, and if there will be skylights or enough openings to avoid any claustrophobic feel. Let's see, over 14' a change from 3/12 to 4/12 would lower the walls by 14". Josh - what would the wall height be if you match the main roof pitch, and have the ridge as high as you can put it?

Then another design idea - still keeping the 4/12 pitch for the addition, what if you offset the ridge. Put it, say, 11 or 12' from one wall and 16 or 17' from the other wall. If you have a north side, a reduced height wall could be a nice feature. Still get great wall height - maybe even more - on the south side, let in lots of light. Less heat-absorbing roof surface on the south side - put in a decent overhang to provide seasonal shade that could partly shade the windows in summer. Maybe put in a good skylight or 2 on the north roof, so that it won't get direct sun but still give that good airy, open feeling. Spray in icynene other spray foam and get great R-value and not have to provide the ridge ventilation.

That's all assuming you have a north-south orientation to play with, but still worth thinking about in an east-west orientation if there is a great view on one side and a poor view on the other side.

edit: added sketches

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ApgarNJ

The engineer I use, he won't spec a non structural ridge even if the collar ties are up at the top of the ceiling, making it clipped, he said the walls will still eventually want to push out. I don't know if I truely believe that or not, but he's the engineer, not me.
better to just spend the money and do the structural ridge and not have to worry about collar ties.

Joe Carola

I'm not an Engineer either but I can tell you that he's 100% wrong because I've framed many houses and additions without structural ridges before just using collar ties in the past 23 years and theses houses and additions are still standing. I’ve built friends houses and additions this way and the walls are still standing straight.

They just have to be designed right and they will work

He has to spend the money either way for someone to tell him what size structural ridge to use or collar ties or any of the ways I've described above.

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reveivl

My engineer agrees with Apgar's, collar ties are used to decrease the size (depth) of the rafters necessary for the span. I agree that collar ties can, and sometimes do, work, but I've also dealt with older buildings where the walls have spread and the roof sagged with collar ties in lieu of ceiling joists. With sufficient fastening in the tension member (collar) and bumping up the rafter size to withstand the moment, it can work, but you'll need an engineer to prove it, and my engineer says they merely reduce the span of the rafter.

Of course engineers also say that knee braces in timber framing do not provide resistance to racking, they merely reduce the span of the beam above, and I know that to be not the case. The oldest wooden buildings in the world are timber framed, we're talking 400+ yrs, so there must be something going on. However try to build a timber building almost anywhere w/o a p.eng. stamp. Rich.

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