Engineer spec’ed 9-1/2″ LVL valley rafters for a roof with 2×10 rafters. Is this common? It goes against common practice and common sense amoung the framers I’ve worked with in the past.
This is for a doormer on a 10/12 with valley rafter length of less than 10′ so there is no reason doubled 2x wouldn’t work. Cathedral ceiling beneith so it needs to plane correctly one way or the other.
The 9-1/2″ lvls aren’t deep enough to cover the rafter ends, let alone allow beveling for full sheathing/sheetrock contact on top or bottom. When something like this is spec’ed is it assumed that a deeper LVL will be used, but the thinner one is all that is required structurally? I’d hate to have to use 40 additional sloped and skewed hangers simply because the valley isn’t deep enough.
My guess is this size was spit out of a calculation model, rather than a good design. The arch. recommends against asking the engineer if there is anyway to work with what was given…for various reasons I can’t go into, other than to say the design may get worse rather than better.
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The architect should go and ask the engineer and point out the things you mention. Or just up the size of the LVLs to 11 7/8", problem solved. I don't want to speculate what reasons there are for not wanting to go to the engineer.
If your not willing to call the Engineer, and the Architect sure doesn't sound as if he will then it sounds to me as if You could:
1) Simply go for a taller LVL as one solution .
2) Use the spec'd 9 1/2 , beveled and planed for the intersecting ceiling lines and simply drop any 2x (ripped to fit)stock on top of it for the roof plane
3) Rip to fit 2x stock for the bevel and rip to fit stock for the top of the 9 1/2' LVL
Me, I would take the risk and politely call the engineer, explain that I am calling because I am trying to learn to work with this new "LVL" stuff and could he please give me a few minutes of his time while I try to understand the what and why 's of his design?
I could always back off, thank him for his input and say that I will go with his original design , now that he has explained it so well to me .
'hoDon:
I suspect your guess that this is just something spit out of a computer without any thought is likely correct. I've run into similar situations when I came in after the design was locked and/or materials ordered. I just furr-up with scrap dimensional lumber and call it good. Too bad that drafting efficiency and building efficiency so often diverge.
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The plumbcut on your 10/12 jack rafters will measure 12.36623" or 12-3/8" using a 2x10 9-1/2" rafter.
Your valley pitch is 30.51° or 10/17. Using a 9-1/2" lvl for that will measure 11" plumb to the bottom of the valley without the 26.91° bevel on the bottom for sheetrock.
If you use a 11-7/8" lvl, the plumbcut on that would be 13.78345" or 13-13/16" to the bottom of the valley without the 26.91° bevel.
Once you bevel the bottom of the valley at 26.91° from that cut measuring up plumb 10/17, you come up 12-3/8" and that's the plumbcut height of the 10/12 common jack from bottom to top of jack and from bottom of beveled valley to the top of beveled valley in the same direction as the bottom bevel. This is where the valley has to be ripped.
That means when you nail the bottom of the jack to the bevel cut, the top of the jack will be flush with the top of the valley and the top of the valley after being beveled will have a V in it so the sheathing hits the center.
Call the Engineer up and ask him. He's not God! You’re trying to do the right thing and get the job done right. If he says 9-1/2" is alright, and then just tell him you want to use an 11-7/8" lvl and just rip the top and bottom to fit and you’re done with it, instead of having to use a smaller lvl and adding to the top and wasting time.
Joe Carola
Edited 12/19/2006 11:35 am ET by Framer
If engineered drawings are required, follow them. While we all know more than the engineer, he has the liability.
If I did the engineering, I might come up with using a 12" LVL, but maybe not.
Only the engineere knows what he was thinking, but I would start from assuming that he merely sized the minimum LVL size that would handle the loads, without thinking of th epractical concern of how it would ned to be attached in conjunction with everything else, and call to confirm.
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I've seen this many times. The engineer specifies the size that is required to handle the load. You can go bigger, but not smaller. If you ask if you can use 2 x 10's his reacton will be something along the lines of "why do I spend the clients $ to figure this stuff out if you are going to second guess my work?" If he has put his stamp on the plan, you can leave those rafters flying loose on the top or bottom as per his drawing or instructions. It's really another subject and I'm sure one that's been bantered about many times here, but I stll can't understand why engineers can't design hip and valley roofs as diaphrams. They had been built that way for most of the last century. With the advent of LVL's all of the sudden we're using two and three member LVL's for these framing members when we used to use a single 2 x 8 or 10. I've seen plenty of sagging undersized rafters in these roofs, but never a sagging hip. (with the exception of some slight movement associated with the deflecting rafters). Seems engineers no longer will engineer for the opposing forces that made these old roofs structurally sound.
I engineer roofs as diaphrams. Doing so allows the use of smaller members.
GHR,Does "engineering a roof as a diaphragm" mean that you take into account the composite action of the sheathing working with the framing members? Or is it something else? Would you explain how this works? I don't do many roof frames, so it isn't obvious. I'm appreciative for any light you can shed for me on this, as I do not know what that language means in this context. Thanks.Bill
My reference to hip roofs being engineered as diaphrams included the whole assembly. Rafters, wall ties, hips, valleys and sheathing. Many engineers will not engineer a roof this way any more because... well I'm not totally sure. Probably because of issues with fasteners, or rather the quality of the nailing job. So, they don't take into account the opposing forces that occur at a hip, they simply calculate the load on a hip as if it were a simple beam at a roof ridge or in a wall or floor assembly. Every time I put up a 3 member 11-7/8" LVL hip I think "What a waste of material and effort" (especially effort, and no I'm not stupid, I put them up individually and then nail them together) When I started building (about 1978) I was using 2x6 rafters with 2x8 hips, Now it's comparitively massive beams replacing these once easy to manuver 2x's
Thanks to everyone for your insites. The options are now much more clear.
Beer was created so carpenters wouldn't rule the world.
GHR, perhaps you can give a more complete explanition. I don't think I can give more of an explanition without looking like a complete idiot. In fact, if you really know your stuff, why not write an article for the magazine. I would love to have one to gently put in the hands of every engineer that looks at our prints.
Cathedral ceiling beneith so it needs to plane correctly one way or the other.
Does that mean that somehow, the ceilings need to plane...or that they need to plane in one way, or the other?
I'm thinking they all need to plane, so something's either got to be ripped or furred...since the architect is not offering, I'd call the engineer and act like a dumazz who's looking for the light.
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God say, "No." Abe say, "What?"
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