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LVL vs sawn lumber for joist sistering


chimpywrench's Avatar
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12-03-12, 09:39 PM   #1  
LVL vs sawn lumber for joist sistering

I own a very old/crooked house and have been doing a lot of straightening and strengthening of the framing, especially joists in the basement. Problems areas are spots where there are loads above and the joist support below is not adequate. What I've mostly been doing is jacking things up, adding joist hangers where needed, sistering here and there, and tearing out junk diagonal bracing and replacing with solid bracing. For soundproofing and cleanliness I've also be gluing up strips of osb against the bottom of the subfloor (which is diagonal 1x10's).

Down to my questions: I have 2x8 joists and have been sistering with the same, but would I be better off using lvl? I only have a tiny bit of experience with LVL but I spent some time searching online and can't find any info on how much stronger it is than dimensional lumber.

The spans are 12-16', 16" centers.

 
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12-04-12, 02:22 AM   #2  
You will absolutely be better off with LVL, but will your pocketbook stand it. LVL will not twist, sag or otherwise cause problems you are encountering. However, it is substantially more expensive. I would recommend it with those limitations.

 
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12-04-12, 11:03 AM   #3  
I can imagine setting a new LVL next to a sawn wood member. What I have difficulty imagining is sistering the two together. How would you do that? Also, what dimension are your existing joists?

tearing out junk diagonal bracing and replacing with solid bracing.
Just for the record, properly installed X-bracing creates a more stable platform that cross-blocks do. Since you're adding to the sides of your joists, though, and don't have access from above the framing, that's probably a moot point.

 
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12-04-12, 04:02 PM   #4  
NashKat stole my thunder regarding X bracing. It isn't junk. It's actually a very simple and cost effective way to add stability to a floor structure. It probably doesn't provide the same amount of support as solid bridging but it makes running plumbing and electrical easier.

I don't know why you would want to spend the extra money on LVL's for floor joists. If the existing 2X joists were adequate when the house was built then sistered 2X should be adequate now. Whatever the case, LVL's are most definitely stronger (deflection and shear characteristics) than dimensional lumber. However, according to this article (written in 2010) LVL's to span 10' would cost $2.40/ft compared to $1.00/ft for SPF.

 
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12-04-12, 05:10 PM   #5  
Just for the record, properly installed X-bracing creates a more stable platform that cross-blocks do.
Really? What kind of X bracing? Mine were toe-nailed 1x4's and seemed useless. Now I have solid 2x held in with lag screws and things are really solid. How can you beat a solid block of wood? Solid block of metal is the only thing I can think of :P

Are you saying that setting the grain in the right direction makes a big difference? The solid blocking grain is horizontal and the line of force (from top of joist to bottom of the next) is about 26 degrees, which isn't ideal, but I still think the solid wood will be stronger.

 
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12-04-12, 05:19 PM   #6  
Thanks for the article link, Wayne. From the chart they posted it looks like LVL is maybe 3x stronger (looking at the bending stress column). I'm guessing that means given a certain load, the LVL joist will show 1/3 of the flex that sawn lumber will. Does that seems like a reasonable estimate?

We're just talking about a few choice spots where there are loads above the joist. I'm not sistering with LVL on every joist. In other areas where there was joist damage etc.. I sistered with the normal 2x8's.

I should have mentioned that my joists are oak, so comparing what they used then to what I should use now is difficult. Their oak joists were much stronger than today's pine. Also, what they used then wasn't quite enough because the house sagged a lot after 120 years, so I need to think stronger.

 
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12-04-12, 10:04 PM   #7  
Mine were toe-nailed 1x4's and seemed useless. Now I have solid 2x held in with lag screws and things are really solid.
A common misconception. "It's a solid piece of wood so it must be stronger, or provide more stability, or something."

The next time you're in a building with a large roof span, and you can see the roof structure, take a look. It's a bunch of pieces of 1/2" rods and rather small angle irons welded together to make trusses. Something that a couple of workers can maneuver into place can support a platform, all of the equipment mounted on that platform, and a bunch of moving people.

Solid blocking is medieval stone-bridge engineering. X-bracing is rethinking the material and the challenge to devise a more effective solution.

 
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12-05-12, 04:25 AM   #8  
Hmm, I don't know what else to say about the bracing - I tore out cross bracing and put in solid blocking and the floor got significantly stiffer. You could easily tell the difference when walking on it - a lot less localized flex. It's much straighter now too because I was adjusting all the joists to line up perfectly as I went. I had no problem making small cutouts when needed for utilities. Since the house is so old there were many joists that were turning and my lag screws into the blocking pulled them back plumb. I don't think this could have been done nearly as effectively with cross bracing. I understand why steel structures are built with triangles but I don't think the comparison to a large steel building is terribly useful here since my solution didn't add noticeable weight or cost to the structure. If we took that light-gauge structure and made it solid it sure would be stronger, right? If I put in wood cross bracing then somehow filled in the space around the braces with more wood until I have a solid brace, I sure didn't make anything weaker, therefor I either made it the same or stronger.

Anyways, my question was about LVL vs sawn and from Wayne's article is seems like the difference might be ~3x so unless someone tells me otherwise I'll keep that figure in mind when doing the rest of my work.

Thanks for the help guys!

 
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12-05-12, 09:08 PM   #9  
How old is your house? The one that convinced me that diagonal bracing has a distinct advantage ov blocking was built in 1908.

Quick story: One day a friend and I reworked two adjacent sections of floor framing on that old house. After we'd inspected, flipped (if needed) and reset the joists, we were to install some bracing. My friend unlimbered his job saw and got set up to cut cross-blocks. I told him I wanted to go back with the diagonal bracing that was there before we started. We got in a pretty good argument, and decided to settle it with a test. We would each brace each of the sections, differently and see which came out stiffer.

When we finished the bracing and walked the framing, the X-braced section was visibly more stable against both horizontal and vertical deflection. As a final test, we replaced the cross-blocks with X-braces, and that section stiffened up too. I laid the new floor over the next week or so. That fall, we hosted one of the largest Halloween partied in town - it was usually held in a club or warehouse - and any number of folks danced and stomped on that floor without creating any give in it.

It sounds like the diagonal bracing in your house may not have been installed properly, or it had become damaged. Certainly it should have kept the joists plumb.

As I said, though, it's moot. Since you're working with the floor in place, you don't have the access you need to install X-braces anyway.

 
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12-11-12, 12:23 AM   #10  
My house was built in 1893. Yes, some of the bracing was in bad shape. I don't really know how to tell if it was installed properly - it was 1x4 with two nails on each end. Even installed very well I don't see it lasting as long as solid blocking just because of the holding power of those nails and because of the small contact area. I observed the following on my 120yr old cross bracing:

- It did nothing to deter cupping of the joist because it only contacts top and bottom, but solid blocking will not let it cup. Once the joist cups some the cross bracing isn't tight anymore.
- The size of the cross bracing and the fashion in which it is nailed makes splitting likely. There is shear force on the nail, yes, but it is along the grain of the brace - this is how a maul splits wood.
- The subfloor could keep the cross brace from shifting up, but only the nails and friction keep the other end of the brace from shifting down. I saw plenty that had worked their way downward over the years - the nails could not stop this.

I can image that if you cut them right and wedge/slam them in really good, cross bracing will be quite stiff, but I wonder how long the fasteners & wood will maintain integrity. In my case I used 3/8 by 5" Spax lag screws with solid 2x blocks that I cut slightly oversize such that I'd have to hammer or jack them into place. Tight as a drum.

 
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12-11-12, 09:42 PM   #11  
Yeah, it sounds like the X-braces in your house weren't adequately installed, or at least what I'd call adequately.

We cut the new braces so that we'd have to wedge them into place, attached them at the bottom first, with screws through shape-drilled holes, and then stood on them while attaching the tops. We checked and corrected for plumb and straightness as we went, and we patterned the installation so the ends of the braces were exactly opposite each other where they attached to each joist.

My friend and I could dance on that framing before I laid the flooring.

I put plenty of solid blocking in that house where that was the only way to do it, or better for some other reason, like creating a bearing point. Just not to stiffen joists at mid-span.

 
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12-12-12, 03:55 AM   #12  
Nashkat1,

Well I can't dance on my floor, but it has little to do with my framing skills :P

 
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08-21-13, 12:23 PM   #13  
To answer your question on sistering, I am doing similar work and my plans from my engineer require bolts every 12" on center to sister the lvl to the 2x10 floor joist. That help at all?

 
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