How many 2x12's for this beam?

[SpiderDave]

I'm building a 16'x48' lean-to. 2x10 rafters would run perpendicular from the shop at a 3:12 pitch, resting on a beam at the top of 6x6 posts, where the eves would be. (I may go with 2x12's, but I'm undecided.) Posts will be planted 32" in ground, spaced 12' o.c. It'd be about a 14+ft rafter span not including the 1.5' eves, etc. Then, plywood sheathing and metal roofing, rated at a snow load of 48psf.

My question is: Would using 4 mated 2x12's create a sufficient beam for the rafters to rest on?

Then I got to wondering how I'd properly alternate the seams of those 2x12's over the posts when they only come in 20' lengths? Should I put the posts at 8' o'c' then? That would be easier and certainly help any beam load issues if there are any, but it sounds cluttered. Most of the time I see people here spacing them at 10'-12' but rarely 8'. I feel like I'm missing something here. And LVL's or LAM beams aren't an option - just not in budget.

 

[rancherbill]

I dunno, but a 14' rafter span in a 16x48 with a 3:12 slope lean-to seems short.

 

[loganb]

Yeah....when you say 16' lean to I take that as 16' clear width from building to close side of post...is that what you're meaning or are you referencing something different?

Span tables for a variety of sources are below, but table 20 has the spans for 50 psf live load/10 psf dead load configurations

Joists & Rafters - Southern Pine

Tables listing Maximum Spans for Southern Pine joists and rafters are detailed below. These 46 tables are based upon design values for visually graded Southern Pine dimension lumber that became effective June 1, 2013. For each application, a PDF download is available. Also see Using These Tables and

www.southernpine.com

 

[Old tool guy]

I dunno, but a 14' rafter span in a 16x48 with a 3:12 slope lean-to seems short.

The roof area is 16x48 but the beam is set back 2 ft from the drip line. Just like eaves on a house …

 

[bb29510]

it depends on the roofing material, if it just light weight tin roof with no snow load, you could almost do anything. instead of stacking on the beam, I would use metal joist hangers

 

[rancherbill]

The roof area is 16x48 but the beam is set back 2 ft from the drip line. Just like eaves on a house …

It will still be short, because it is the hypotenuse.

a2 + b2 = c2
3^2 + 14^2 =c
9 + 196 = 205 and the square root is 14.31

So your 14' lumber is .3 of a foot short BEFORE the overhand of 2 feet.

 

[Old tool guy]

Whatever. I was explaining the concept, not creating a cut list.

It'd be about a 14+ft rafter span

 

[428PI]

Get this. I have a double 2x8 that is 16 ft across that holds up a roof truss that spans 32 ft. Attached to this double 2x8 is a vertical 2x6 that is just nailed in that drops down to a sistered in single 2x10 that holds a lean to roof that is 14 ft across. I didn't trust this and have installed studs holding up the double 2x8s and 2x10. Perhaps it was safe with 50 year old wood. Today's wood is not very strong.

 

[billconner]

Tributary area on a 1' of beam is ~9 sf @ 48 ll + 5 dl = 477 plf. I believe a 2x12 spanning 12' can support 180 plf, so you might be fine with 3 but use 4 and just don't have joints within 3 to 4' if each other.

Not an engineer, just a designer and sometimes builder.

 

[SpiderDave]

Tributary area on a 1' of beam is ~9 sf @ 48 ll + 5 dl = 477 plf. I believe a 2x12 spanning 12' can support 180 plf, so you might be fine with 3 but use 4 and just don't have joints within 3 to 4' if each other.

Not an engineer, just a designer and sometimes builder.

Thank you Bill. I 'think' it was you who suggested 4 2x12's in an earlier thread, which is where I got the idea. I just wasn't sure about the joints. Appreciate it as always.

 

[SpiderDave]

My apologizes for not answering sooner. I tried 3 times yesterday and an error code kept blocking me. Just so people don't think I was ignoring them here's what I tried to reply with:

I dunno, but a 14' rafter span in a 16x48 with a 3:12 slope lean-to seems short.

@oldtoolguy got it right. (example photo below) The covered area will be 16x48, but the rafter span will be about 14' after beams, eves etc. I don't know what that equation was, but it made smoke come out of my ears and my eye's still twitching from reading it. Ha hA! I only wish I was that smart.

@loganb I tried reading that span table, thank you, but I couldn't make sense of it. What should I be seeing? If I recall correctly, the table I went by, was specific set to a 3:12 pitch of unsupported #2 Douglas Fir rafters with a snow load of 48-50psf. For my application it read #2 Doug Fir 2x10's rafters @ 16" o.c. can span 14' 2" to meet the requirements. I can dig around for the site later, but here's a screen shot of the table at the bottom. If you think it's wrong, by all means speak up. My pole barn shop has 11' long 2x6's @ 24" o.c. between trusses. I've done some bracing, but it held up great before I came along. And the city won't let me copy that design on my permit.

@bb29510 Yeah, I'd prefer hangers actually. it'd solve a lot of problems I have and even reclaim some height at the eves. But wouldn't that eliminate the eves, making them almost flush with the posts? Or am I seeing it wrong in my head? I kind of need the eves. For a while I was thinking it would be easier to use trusses post to post top, perpendicular to the wall. And having the top board extend further for adding eves. ( i can draw it if need be?) And then secure the rafters parallel to the shop wall with hangers all the way down @ 16" o.c. But I don't trust the strength of that last rafter that would take all the snow loads in winter? Or weight of gutters full of ice. When all that slides down off the shop and hangs there before it breaks off, that last 11-12' long 2x10 rafter and the extended truss end would take worst of it. I couldn't figure out a practical way to address that structurally.

@428PI Man, I'm no engineer but,... wow. It's impressive that held up all that time, before you got to it. You mentioned today's lumber being weaker, I totally agree! It's not vintage, but I have lumber I bought just 15 yrs ago that's amazing compared to the stuff I buy today. You can feel it when you pick it up. Besides that, the grain in the 15 years ago stuff is so tight and uniform. In some the rings are almost as thick as the white wood, on some pieces. The grains super tight and fine that when you hit it, it rings like a tone wood. The first time I dropped one I thought, well those pieces are going to be cigar box guitar build stack, not the patch my porch pile. I'll put a photo of it on here later, just for giggles.

Thanks everybody!

 

[billspit]

Am I the only person that feels 4 2x12s is overkill? Or is it the snow load factor?

 

[Old tool guy]

Am I the only person that feels 4 2x12s is overkill? Or is it the snow load factor?

I’m thinking 3 would be a lot.

 

[billconner]

so you might be fine with 3

Am I the only person that feels 4 2x12s is overkill? Or is it the snow load factor?

No, not the only person. OP asked if 4 would work. I do think snow load is the factor, at 48 psf, but struggling that number, since usually rounded to 40 or 50. Makes me curious as to source. Where are you Spider Dave?

 

[SpiderDave]

No, not the only person. OP asked if 4 would work. I do think snow load is the factor, at 48 psf, but struggling that number, since usually rounded to 40 or 50. Makes me curious as to source. Where are you Spider Dave?

Sorry for the delay, I was online yesterday answering and our internet went out until this afternoon sometimes. Not even TV on a Saturday night! Not many options out here.

No town, we're Tillamook County Oregon now up over the Mnt Range. Rated by Oregon.gov @ 48psf entering our longitude and latitude (50-60 on some maps). So, 48-50 should be fine. Fairly average winters here, but we've had some bad ones too. Once almost 3ft, was so worried I kicked up 6x's under the middle of my shop trusses. Half way though the day I couldn't budge them from the added down pressure. My neighbor down the road was making fun of me, until he lost his pole barn roof.

We can get a foot over night on occasion, then a couple days of gentle snow, then it rains and sometimes freezes heavy. Goes boom when it slides off too. It's another reason to get that lean-to up, so the snow drops out away from the pole barn wall. It piles up against it creating ice damns. Those walls have taken a beating,.. along with my back having to use a ram rod and shovel to remove it. Average bad winter is only about 1 1/2, maybe 2ft. I don't mind over building it a little bit, if it helps me sleep at night when we get a bad storm.

So, do four 2x12's for a beam still sound about right then, or?

 

[billconner]

You can probably get away with three. Are another 4 - 2 x 12 x 12s a big deal for this project?

I think if it were my project I'd run all the numbers again and see if I'm comfortable with three.

 

[rayra]

4 still seems like way overkill. Make your own laminated beam with 3, stagger the joints about 4-5'. Glue and screw.

3:12 is not the pitch for a snow load concern.

 

[SpiderDave]

You can probably get away with three. Are another 4 - 2 x 12 x 12s a big deal for this project?

I think if it were my project I'd run all the numbers again and see if I'm comfortable with three.

I can build stick frame structure, but this one's a first. I don't know hot to run numbers, or I would gladly and be done with it, doing a little over whatever I ended up with, just to be sure. I have tried, but it got overwhelming as I kept learning more about what I didn't know, than what I did. And I get overwhelmed and freeze up every time. At this point it'd probably be cheaper to toss another 2x12 at it than the time I'm losing. IDK. I've never had to think so hard about building something. I had no idea it was gona be so out of my wheelhouse.

4 still seems like way overkill. Make your own laminated beam with 3, stagger the joints about 4-5'. Glue and screw.

3:12 is not the pitch for a snow load concern.

Thanks rayra, I like a little overkill,.. but not way over.

EDIT: One more thing to add, last night I sat down and decided that a 10' o.c. post spacing actually works much better than 12'. becasue of the built in shed and a few other things. Does shortening that beam span from 12' to 10' change anybody's thinking? Thanks as always.

 

[Skooterj]

Wood Beam Calculator | What size do I need?

Calculate the size needed for a beam, girder, or header made from No. 2 pine or LVL. Covers any span and every load with pin point accuracy.

learnframing.com

You can almost get away with 2. 3 would work fine. 2 with a plywood spacer would work too, but exterior plywood is stupid expensive.

 

[billconner]

I agree that you'll still get a little overkill with 3 at 10' spans.

I did think 12' made sense with a 48' length.

 

[SpiderDave]

I agree that you'll still get a little overkill with 3 at 10' spans.

I did think 12' made sense with a 48' length.

 

[SpiderDave]

I agree that you'll still get a little overkill with 3 at 10' spans.

I did think 12' made sense with a 48' length.

I would like to, but I realized I should probably have the posts alternate to the shop posts because this is a freestanding non structural build - unattached. I have one corner post that I'll repair with a perma column, and would like to make any future access to the rest of them easier on myself as the pole barn gets older.

 

[Shoptime]

Rafter & joist span tables are great but beam sizing info is harder to come by. I've used Medeek for mine, excellent site & stuffed with good info. Easy to use and you don't need to be a PE to navigate it. Go to http://design.medeek.com/resources/beam/beam_calculator.pl I think limited use is free, license for unlimited is very reasonable price. Also keep in mind if you end up using a quad beam, 6x6's won't give full bearing support. Even slight cupping will push the total thickness to around 6-1/2", sitting on top of a 5-1/2" post. Curious, how high is the beam and how are you handling lateral racking since it's a freestanding structure?

 

[SpiderDave]

Rafter & joist span tables are great but beam sizing info is harder to come by. I've used Medeek for mine, excellent site & stuffed with good info. Easy to use and you don't need to be a PE to navigate it. Go to http://design.medeek.com/resources/beam/beam_calculator.pl I think limited use is free, license for unlimited is very reasonable price. Also keep in mind if you end up using a quad beam, 6x6's won't give full bearing support. Even slight cupping will push the total thickness to around 6-1/2", sitting on top of a 5-1/2" post. Curious, how high is the beam and how are you handling lateral racking since it's a freestanding structure?

Sorry for the slow response, I've been away. Thanks Shoptime, I'll check that site out! Appreciate it. I thought about that thickness issue and considered either a metal bracket or jumping up to 8x posts - big price difference though. Height? If I remember right, it's 14' on the high side and somewhere around 10' on the low / eves end. So with a beam on top, ply sheath and roofing I'd say 12' 8"?

So far I'm trying to address racking with a combination of things. 1/4 of sq ft. would be a closed in shed with ply sheathing and metal siding. Below the eves would be trusses and a metal skirt that would also help keep the rain from blowing in. And / or diagonal bracing post to beam. 3-4' deep in ground posts (probably 4'). 3 half trusses built into the design too. And most recently I think I've found a way around things where I can bolt the lean-to to the existing pole barn now.

 

[countrypapa]

What am I missing here? You have 6x6 post 12" o/c that puts about 7 1/2 " of air space between posts. I would think you could use a 2x4.

 

[cgrutt]

It will still be short, because it is the hypotenuse.

a2 + b2 = c2
3^2 + 14^2 =c
9 + 196 = 205 and the square root is 14.31

So your 14' lumber is .3 of a foot short BEFORE the overhand of 2 feet.

I think your math is a bit fuzzy shouldn't your b^2 be 144 with overall result just over 12'?

What am I missing here? You have 6x6 post 12" o/c that puts about 7 1/2 " of air space between posts. I would think you could use a 2x4.

Believe OP means 12' spacing between posts

OP, is notching both sides of post 1.5" × 11.5" an option so you can through bolt the outside of beams through each side of the post? This way you can have 12' boards on each side of beam with ends bearing directly on the post (you can do this on top of post as well but they won't be through bolted). Then solid (i,e. no joint) between each post? Three boards with two pieces of 1/2 plywood filler sandwiched in between will take you to the full width of your posts. I'm not an architect but believe that would be more than enough on a 14' +/- span back to front.

 

[readhead]

Use more wood if it makes you comfortable. Keep in mind that the open section of the lean to will be cold and the snow on the roof will freeze and could cause snow not to shed off of the main building. Keeping that snow removed will be a high priority.

 

[countrypapa]

What am I missing here? You have 6x6 post 12" o/c that puts about 7 1/2 " of air space between posts. I would think you could use a 2x4.

My bad, it does say 12' o/c, these old eyes just ain't like they used to be, thanks.

 

[SpiderDave]

What am I missing here? You have 6x6 post 12" o/c that puts about 7 1/2 " of air space between posts. I would think you could use a 2x4.

@cgrutt is right,... 6x6 posts, 4' in ground @ 12' o.c. (post spacing). That's be about an +/- 11' beam span between posts. Sorry if I caused any confusion there. Diagonal bracing will also assist on that span too.

I think your math is a bit fuzzy shouldn't your b^2 be 144 with overall result just over 12'?



Believe OP means 12' spacing between posts

OP, is notching both sides of post 1.5" × 11.5" an option so you can through bolt the outside of beams through each side of the post? This way you can have 12' boards on each side of beam with ends bearing directly on the post (you can do this on top of post as well but they won't be through bolted). Then solid (i,e. no joint) between each post? Three boards with two pieces of 1/2 plywood filler sandwiched in between will take you to the full width of your posts. I'm not an architect but believe that would be more than enough on a 14' +/- span back to front.

Took the words right out of my mouth, thanks! And I like the idea of notching instead of on top. Just wasn't sure how or if it'd be substantial enough? And if not, then what 'would' work and how to do it? I'm doing this with a permit, so I was hoping to get it right as I can so that any corrections will be minor enough that the city reps would consider me worth advising or helping make adjustments as painlessly as possible. Our city isn't very enthusiastic about helping. I'm not at all afraid of overbuilding it a bit to be safe. Despite a tight budget, I still prefer it.

Use more wood if it makes you comfortable. Keep in mind that the open section of the lean to will be cold and the snow on the roof will freeze and could cause snow not to shed off of the main building. Keeping that snow removed will be a high priority.

I've made this point before but I don't recall whether it was on this thread or not? I'm in a hurry with the wife waiting on me, or I'd double check. But it was so good to hearing you say that because it makes me feel less paranoid. Ha! The snow that'll be moving down that roof's like a glacier. When I turn the heat on it starts to slide off and the possibility of it piling up on the cold lean-to roof, has been a serious concern. It's one reason I'm not afraid to over build it a little. I get slabs that hang, curl and so heavy when it falls off it goes 'boom' and I worry about the impact on the wall sometimes. Especially when rain freezes into it beforehand, it gets real heavy. I often go out there with a huge pole and break those slabs off and stay the he;; back becasue it could take a person out. The I spend hrs shoveling and ice picking it away from the wall at all hours so that when it melts it doesn't ice dam into the wall and cause a flood.

That happened a couple times and I'm too old to be out with a pick & shovel removing all that, with a blown out back too. We had a huge 48' ice berm up against the shop wall one year and learned the hard way. Having this lean-to will get it out and away from this wall where it can melt when it wants to This is another reason I have the eves posts recessed. When that snow hits it spreads out with some force especially when there's ice in it. That'll help keep the posts out of the worst of it. I could even put a guard in front of the posts later if thought it needed it. The other side will be next, but that'll be a piece of cake and be more of a jot-out about 5-6 ft (becasue of property lines). Enough to walk under in the rain and throw the snow out enough not to be a threat.

I wish I could use hangers for the rafters instead of on top, that's my preference. But I don't know how to keep eves in the design doing that? Any ideas? Thanks as always. It's more appreciated that you guys know.

 

[SpiderDave]

My bad, it does say 12' o/c, these old eyes just ain't like they used to be, thanks.

Yeah, in the first first intro it does if that's what you were reading. No big deal. I don't know about you but the writing gets smaller as I get older. HA! I get confused all the time. Sometimes can't tell if it's one or two hyphens. I have glasses and I still squint all the time. Is that an l or and i? E or an o? Semotimos yeu can"t tell.

 

[lynnbilodeau]

Screw the 2x12s.
Use LVL. The smallest part of your investment in this part of the build is the material. Your labor is the big investment. I always go overkill on stuff like this.

Span tables are great. They are designed to help you get by with the LEAST amount of strength. Remember: "Anyone can build a bridge; it takes an engineer to build a bridge that will barely stand."

 

[SpiderDave]

Screw the 2x12s.
Use LVL. The smallest part of your investment in this part of the build is the material. Your labor is the big investment. I always go overkill on stuff like this.

Span tables are great. They are designed to help you get by with the LEAST amount of strength. Remember: "Anyone can build a bridge; it takes an engineer to build a bridge that will barely stand."

Man, I wish I could. The last time I got just rough estimates on LVL's, it was easily 1/3 of my total build cost without delivery. That's why I went back to laminating 2x12's. Plus, I'll be doing it all by myself - no one to help. I'd have a hard time getting those beams in place.

Span tables? Sheesh, I wish I could. But I couldn't make sense of them. When I THINK I've made progress and understand it, I end up with more questions than answers and locked in place again. That's why I came here lookin' for help.

I really liked that bridge quote BTW. Makes yuh think,... that's one hell of a good point.