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Ridge beam calculation

BAB's picture

I'm planning a very simple structure, and want to use a load bearing ridge beam (to avoid collar ties, etc...).  The size of beam my lumber yard recommended seems rather large to me.  I'd like to get a second opinion.  Are there any on line tools a non-professional could use to do this calculation?

It is a simple gable end structure.  14' long, 8' wide, with an 8:12 roof, and I'm in Minnesota, so snow load is a definite consideration.


(post #69763, reply #1 of 149)

Let me get this right.....

You said you're a "non-pro", right?  Your words not mine.

You went to the lumberyard to have a "pro" size your beam.

Now you want to second guess him cuz "dat don't sound right"?

I don't get it.  What did the lumberyard come up with?  You thinking they're trying to get rich by jacking you on a 14' long beam? 

(post #69763, reply #2 of 149)

I'm not worried about cost, I'm worried about size, and how it will look.  As well as how difficult it might be to install the beam (I'll be building this on my own).  I'd also like to learn how to do this kind of thing myself, because it interests me. 

I'm "second guessing" the pro because he didn't do it himself, he went in the back room to get Joe to do it, before Joe ran off to lunch.  Hurried third party exchanges like that are prone to mistakes, or at least mis-communications.

The "pro" came up with the following options:

- one 11.5" LVL

- two 9.5" LVL's

-three 7.5" LVL's

- one 6 x 12 timber

Would anyone care to answer my original question?


(post #69763, reply #5 of 149)

U ain't gonna do ant better'n'that. I'd have rough guessed a 14'deep LVL.

They did you good offering several options there. They do this all day long so a hurried shush-slush of information was fine. There was nothing hard about it.

There are online calculators for joist or rafter spans, but beam loading is far more complicated than you might imagine, with lots of potential liabiilities, so you don't get that for free



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Oh Well,

We did the best we could...

(post #69763, reply #7 of 149)

A single 1-3/4 x 11-1/2 LVL? That sounds a little light.

My guess would be that a 4x12 DF#1 would work, and if I was building that shed in my backyard with no one watching, that's what I'd use, since I can go pick one up right now. A 6x12 seems like overkill, but when I look outside right now there's no snow.

(post #69763, reply #9 of 149)

Aw - you've got lite snow up there anyways...




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Taunton University of
Knowledge FHB Campus at Breaktime.
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Oh Well,

We did the best we could...

(post #69763, reply #24 of 149)

I wonder if there is any significance to the "pig"?

(post #69763, reply #35 of 149)

There is tremendous significance to the pig. He is one of the finer framers around, a man with great integrity who not only knows what he is talking about, but who regularly shares good advice based on his experience.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #69763, reply #38 of 149)

Great, glad he is a good framer.  I suppose it comes with an additude, naturally.  Like some artists.

(post #69763, reply #46 of 149)

Well, I had the same attitude rise up in me when I read the post. Brian got to write it up first. Maybe he had a worse day than I had...I might be wrong in this case, I don't know but there is a reason for it.

Want to know where it comes from?
No, you probably don't, but I'll say anyhow.

In a 35 year long career in the trades, dealing with all sorts of folks, I have had my instincts refined by experience. And most of the times when a potential client starts early on to questioniong my judgement, it is a sign of problems on the horizon. It means it is time to say see ya later.

Then the contractor who does get that customer usually finds out later that they are the most nit-picking, slow paying, fussbudgetletmehelpgetinthewaysecondguessingshortcutters you can find.

Like I said, this might not apply to the OP here. He might be just wise enough to folow up on his instincts, but he is looking for a smaller beam when the general consensus is that he is more likely to be neediong a larger one, so my instinct still says that he is looking for what he wants instead of learning to design with what he needs.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #69763, reply #64 of 149)

Great post Piffin.  And sure I am interested; however, you must admit that the original question was very basic and so obviously innocent, unless you are a very very high-strung individual looking to cut someone a new one.  BIngo!

I completely understand your comments about potential clients.  I met people in dozens of different situations and am pretty good at sizing them up in thirty seconds.  Ususally correctly, not always.  Some people are great people with zero people skills.  But, in either case you don't rip their head off for one question.  Not very professional.

As for the original post.  You say you believe he was looking for a SMALLER beam.  I did not read that into the post at all.  To me it seemed that he wanted to know the right answer, larger or smaller?

(post #69763, reply #28 of 149)


It seems to me like these guys have all gotten off on a tangent and noone is bringing up how much deflection you'll get fully loaded. I would ask what the deflection is. I'll specify that I want minimum deflection- not what the code says is okay because I don't want to find the ceiling cracking down the road. Those LVL ridge poles can be wrapped with trim and add alot of detail to the room, so I'm not worried about stepping up the hieght of them. It's not like headroom is an issue with a ridge pole.

(post #69763, reply #43 of 149)

Yes, my innocent question seems to have stirred up some people's private issues.  Maybe decaf is the solution there?  Back to the original issue...

You raise a valid point about the deflection.  But the structure is just a screened  porch, with a plywood beadboard ceiling, so I have no worries about cracking sheetrock joints etc...

However there isn't going to be any too much head room in the structure.  Hemmed in on the high end  by the need to mate its roof to an existing roof, and hemmed in on the low end by needing to build up deck pallets to form a floor (on top of another existing roof).  That is why I wanted a second opinion on the beam size.  I'm 6'4" and don't want to be thinking "my that's a massive beam just above my head" every time I walk into the thing.  I had been worrying that the beam would be even lower, because I was picturing the rafters resting on top of it (as was done on a similar feature elsewhere on the house, where head room isn't an issue), but then I realized I can probably hang the rafters off the side of the ridge beam.  That is OK on a load bearing ridge, right?

While I can see that in some cases a beam calculation has the potential to get much more complicated than some other structural members, I don't think it is possible to have a simpler case than mine.  Each wall carries 1/4 of the load.  The ridge carries the rest.  It is a simple span supported on either end.  There are no intersecting or oddly shaped roof sections that mate with it, etc...  The TrusJoist link was helpful.  Thanks.  That was just the kind of thing I was looking for. 

(post #69763, reply #49 of 149)

Since you can see how simple it is relative to other beam designs, why in the world would you assume they were wrong about it at the yard, or that you are capable of doing better with minimal knowledge of how to figure it.

I had never considered the beam underslung. a ridge beam is normally flushed up in unless there is adesign reason for wanting it to project into the living space, say for appearance it something like a timberframe or log home.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #69763, reply #66 of 149)

I assumed a mistake could have been made because...

- I have an almost identical structure on the front of the house (but with only a 5' length).  It uses a 4x4 for the ridge.  The increase in beam size for going from 5' to 14' length seemed excessive to me, and worth double checking.

- I told the guy "fourteen feet", he could easilly have thought I said "forty" feet.  He could easilly have mistyped the value when entering it in his computer.  He's human.  Humans can make mistakes.  Buying and installing a wrong sized ridge beam would be a big mistake for me to have to correct. 

- Also, as I've already said, the process of determining beam sizes intrigues me.  I wanted to find out if I could learn how to do it.  This seemed like a good excuse to ask for advice in learning how to do that.  I find internet forums like this one are usually an excellent place to get such advice.  I've had very good luck in the past learning from experienced people on other forums, and have tried to help others in return when my knowledge would help them. 

Maybe I've somehow stumbled into a touchy area with this beam question, I hope so, because if this is how you guys respond to the typical question on this forum then I have to say you need to lighten up.  The attitude on other forums is much friendlier.  I've learned much from the (almost universally gracious) people on gun forums, for example.  Who'd have thought the shooting community would be less confrontational than carpenters?

Why in the world is this such a big deal to some of you?  Why do some of you get so bent out of shape when I ask a simple civil question?  If you think it is a frivolous question then feel free to ignore it.  If you have some "personal issues" about civilians second guessing your God like judgement then... please start your own thread rather than using mine to vent.

To those of you who've responded to my original question, thank you, your responses have been helpful.

The beam will be attached to the existing gable end of he house.  It will rest above a door, so I assume a properly sized header, supported by posts that trace their support to the foundation, is what will be happening there.

I'm inclined to go with the two 9.5" beams rather than the one or three beam options, as that will be the best balance of hieght (to maximize headroom) and width (to avoid being wider than the 4x4 post).  The structure will remain exposed, so I don't want it to look clunky.

(post #69763, reply #73 of 149)

Don't take any of this personally.

Some thread just take off like this. There are personality conflicts, disagreements, ignorance about issues, and stuff like that.

There's also some inherent suspicion sometimes when someone unknown questions something. Sometimes it looks like they're trying to get away with something cheap or something along those lines. That tends to get everyone's blood going a bit.

You basically got the info you wanted. So let everyone slug it out, and the thread will die off eventually.

Better over the hill than under the hill

(post #69763, reply #74 of 149)

Good advice Boss.

(post #69763, reply #82 of 149)

Quick thoughts,

Joe in back is probably costing the lumberyard twice as much as Bob the salesman up front. So the yard doesn't want Joe talking with customers all day.

For your beam weight, I usually round up to 6lbs per sqr ft for 1 3/4" LVL. So your 11 7/8 is about 6lbs per foot. 14 feet is 84lbs.  Piece o' cake.

If you go the double 9.5", place them seperately, and spike them together when they are in place. 



(post #69763, reply #89 of 149)

Let me start by apologizing. Looks like I read too much into that first post request. You do think things thru well enough to do fine, especially as small as this adition is.

But5 to answer your query why we can get so wound up over something so simple ...

One is that we way too often see a situation where the guy asking the question leaks information out a dro ata time until we find out that this is really a whole diffeerent project than what he first described so things are rarely at face =value

Two - this forum really is quite congenial, more so than all the opther buiolding forums I know of put together, but it is made up of some top guns. If you know how high strung top performers can be - don't let your daughter date one of them...

Three - if you visit most of those building forums, you will find that it is a stock answer for structural advice to seek a professional, mainly because all the facts cannot be assessed from an owners descripotion because most of the time, they don't begin to know what to look for and don't know the difference between a joist and a stud. The liability of giving wrong advice or having it wrongly interpreted and applied rests heavy
So- you had already gotten the advice of a pro on the issue, suggesting in the lack of trust in that opinion that there could as likely be a problem with you as with the pro. People who lack trust are often not trustworthy in my experience ( No offense nmeant to Gene) so the radar was buzzing.

You mention gun forums. This subject here could be compared perhaps to someone coming on such aforum with the following;

> "I have overheard some guys at Walmart talking about bumping up the velocity of their bullets. My barrel is 30" long. Can I add some powder to each cartridge and make sure my bullets kill the deer? They are thirty calibre. Aslo if I can make my gun shoot faster like on TV that would be good too."<

See how many questions come up in your mind when you read that? How many things are implied to the seasoned shooter by the choice of words from the novice? Doesn't his last line sound scary?

I have used some hyperbole` in that made up request to get tthe point across. Yours was far less inexperienced than that, but there were still hints that you are a novice, possibly one alreaduy in over your head on this. i think you can surely handle it now that I know you better, but that's where the first impression comes from that you asked about. Sorry to have been too brutal.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #69763, reply #50 of 149)

"but then I realized I can probably hang the rafters off the side of the ridge beam. That is OK on a load bearing ridge, right?"

Definetely. I have no way to quantify my guess, but I would be inclined to think that rafters connected to the side of the ridge would give you a better connection than the beam supporting the underside of the rafters.

How are you supporting this beam where it meets the house?


Jon Blakemore

Fredericksburg, VA


Jon Blakemore

Fredericksburg, VA

(post #69763, reply #51 of 149)

Guys, I've watched this thread develop, and I haven't heard anyone give this beam "requirement" the gut test. 

Fact - NO 80-150 year old houses in my county in Georgia (and I've worked on a lot of them, and been in more) have ANY RIDGE BEAM AT ALL, even with 18' rafters, merely the 1x3 strapping purlins at 10-12" OC that the wood shingles sit / sat on.  The rafter ends simply abut. 

Granted, we don't get snow, but a 6x12 ridge beam equivalent for this tiny shed? 

A decent skin of 1/2" ply on an 8/12 pitch would be measurably stiffer in plane than any ridge beam would be capable of resisting in sideways deflection, that is, resisting the asymmetric loading from high wind on one side of the structure.  Further, evenly distributed snow load places NO LOAD on the ridge beam at all, if the rafter ends line up.  the ridge beam is merely a place holder.

Collar ties, or "wind braces" are also foolish and only useful for whacking your head.

Forrest McCanless - BSME 10 years; builder / general contractor for 10 years

(post #69763, reply #52 of 149)

Those houses you're referring to.... the ones with no ridge board or beam.  I'll bet my last dollar that they have ceiling joists/attic floor joists keeping the walls from spreading. 

In the OP situation he is trying to build a structure that won't have any ceiling joists or walls ties to prevent the walls from spreading.  Hence the need for a ridge beam.  One way or another you've got to have something to stop the walls from bowing out.  That's not technology.... it's just physics.

I think maybe you might be getting ridge board and ridge beam confused.  Correct me if I'm wrong.  Wouldn't be the first time. ;)

(post #69763, reply #56 of 149)

Okay - so there is no "attic" and flat ceiling below?  This structure has a 5-sided open cross section (floor, two vertical walls, and two sloped ceiling halves?

Then I stand corrected - the ridge beam is supporting half the vertical roof load in this case.  Maybe he wants to make sure any gable end door or window is not centered, or darned well headered!

Forrest .

(post #69763, reply #57 of 149)

Yes... that's the way I've understood the project anyway.

I agree with some of your original post though, just not in this particular instance. In many cases a ridge board isn't necessary and just serves as a nailer for the rafters.  Roof framing seems to have become more complicated on the whole compared to the houses you were talking about.  Tough to frame a true hanging valley without a ridge to nail the major off to.  Same goes for hip roofs.  In all, I think a ridgeboard often makes the roof logistically easier to frame.

But I also don't really agree with your take on collar ties either.  The way I've always understood them, their purpose is to prevent a roof from opening up (like a clam) at the ridge under severe winds like a tornado or hurricane.  Why is it that you think they're nothing more than headknockers?  I'm not trying to pick a fight, I'm just curious what your line of thinking might be behind that.

(post #69763, reply #58 of 149)

I got up in my attic last night and checked - my 1886 house has four "hanging" valleys with no ridge boards; rough cut 2x4s for all rafters.  The plan view of the roof is a squared "U" with the base facing the street, hipped all around.  There is a gable facing the street.

I'll see if I can dig out the FH article from some years ago that discussed the needlessness of collar ties / wind braces, from some structural engineer.


(post #69763, reply #53 of 149)

Now there's a line of pure BS.

"A decent skin of 1/2" ply on an 8/12 pitch would be measurably stiffer in plane than any ridge beam would be capable of resisting in sideways deflection, that is, resisting the asymmetric loading from high wind on one side of the structure. Further, evenly distributed snow load places NO LOAD on the ridge beam at all, if the rafter ends line up. the ridge beam is merely a place holder."

If you designed up here, it would keep a crew busy rebuilding your stuff

you are totally ignoring that the cieling joists you use act as rafter ties to prevent wall spread and ridge sag by creating a compleete triangle. This thread example has no cieling ties resisting those forces. A roof design must have one or the other to resist the loads.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #69763, reply #59 of 149)

"A decent skin of 1/2" ply on an 8/12 pitch would be measurably stiffer in plane than any ridge beam would be capable of resisting in sideways.."

A plywood skin can definitely collect a lot of shear forces. But that force has to be tied off to something that will RESIST that force.

That requires some more design work, and a specific naiing schedule. You might end up blocking the edges of the plywood or something like that.

Dieselpig is right about the function of collar ties.

I decided to get in shape, and the shape I chose was a sphere.

(post #69763, reply #54 of 149)

It is normal to attach rafters to the side of the the structural ridge pole- in an insulated roof I usually let the rafters sit about 3/4'' above the beam to complete the airway to the ridge vent.

However, as for your situation being "simple"- I guess I'm not getting the whole picture. You talk about built- up deck pallet floor and attachment to adjacent roof,etc.- this is why it's important that the beam designer understands the exact circumstances of the beam's layout.  Also, you say you assume each wall takes 1/4 and the ridge takes the rest but that's where an eingineer comes in. If you talk to an eingineer you may find out that in a wind loaded situation the ridge may be required to take on the entire load. Engineers, often times, are accussed of not having common sense, I've learned from using them that there can be important reasons for it. This is why I prefer to pay a certified stuctural engineer to size a beam I feel is getting past the "simple" layout.

(post #69763, reply #90 of 149)

Hopefully you got an answer after all. Sometimes you've got to wade through the crap that floats around here, end of the day this is a public forum, nobody owns it except the Taunton Press, and you can ask any question you like, novice, professional or percieved professional alike. Good luck with your project.

(post #69763, reply #4 of 149)

I wouldn't trust anybody at any of the local yards to size me a roof beam.  Period.

Maybe you have pros there you'll trust, but where I am, nobody knows nuthin'.

Give the guy a break.  Maybe the "pro" he asked at his local yard didn't seem too bright, and he came here to talk to some more "pros."

(post #69763, reply #6 of 149)

Maybe I'm jaded, but maybe I'm not.

Just sounded like he's second guessing what he was told would work using nothing more than "Kentucky windage".  See too much of that.  If it were easy, everyone would be doing it.  And lately everyone is.

And yes, around here the larger lumber yards have architects and engineers on staff along with a huge selection of manufacturer's engineering software to work from.  You bet I trust them.