Sistering I-beams [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-03-2007, 06:56 PM
Question- what is the procedure for sistering I beams?

I was thinking of ripping corresponding plywood and glueing/screwing it in place along the whole length of the beam as they recommend here,17071,451111,00.html

...the question really is, should I screw or nail and what glue should I use? Nailing vs. screwing...pattern?

The I-beam web is 4.5 inches deep with 1/2 thickness to the flange and the beam span is 13.6 feet long on 24" centers.

I also want to put in some mid-span should this be done



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03-04-2007, 12:54 AM
These PDF's ( should illustrate everything. Might want to search for your particular brand the idea is usually the same.

03-05-2007, 03:21 PM
Hi Andrew!

I'm not a pro....

1) The glue will most likely be a solvent based construction adhesive and not a urethane glue. Solvent based does not expand and will remain a bit more flexible.

2) I'd go with screws. If you had a lot of shear forces you'd have to go with nails (not the gun type) because their cores are thicker (like joining headers and beams). Using screws will eliminate all the violent banging on the joists.

Something I don't get..Do you want to place the plywood on the "inside" of the I joist or apply it to the entire height? You need to get an exact answer from a structural enginnnnnnneer.


03-05-2007, 07:18 PM
Why are you doing this? Recommended by someone? You need an engineer to determine how much stiffness you are adding. Or call the manufacturer of your joists for recommendations. Sometimes an 8 or 10 foot section to the middle of the joists is all thats needed.
Never screw, nail or drill the plywood web. Any plywood would be attached flange to flange. I wouild add some camber before you attach the plywood.

Pete- gun nails are fine and just need to meet specs. of the engineeer. I think your statement is a bit misleading. A gun would be preferable to hand nailing.

03-05-2007, 09:10 PM
Now I'm sorry I asked...I think I will stick with tile questions in this forum. Everything I have read has said that pieces should be added to the web, never to the flange, and to glue and nail. It does not need to be checked by an engineer because I am adding to the structure, not taking away.

Many of the manufacturers allow holes to be drilled in their web spaces so I am not worried about weakening the joist. Some of their joists actually require adding stiffness in this manner to meet certain codes and design needs and even recommend this activity.

thanks eveyone


03-05-2007, 11:14 PM
Anything attached to the web is just a filler. I think you are confused about what the pieces attached to the webs are for. Take a good look at the spec guides for the I joists, most of it deals with the correct ways to block for attachments, carry point loads past the joists, add stiffness to specific area, and the proper attachment of doubled members. To strengthen the joist itself the only two ways that I would be comfortable with/have seen speced are doubling the member- which entails a spacer and second I joist, or boxing it with plywood which attaches to the flanges- which is what I thought you were wanting to do. Blocking put in the wrong place on that web will pop the flange in some cases , its important to talk to the mfr.

03-06-2007, 06:09 AM
I am sorry I asked because everything I have read at every manufacturer's website has been contradicted by the advice here, other than to check with the manufacturer.

I have not read a single article about reinforcing EW I-Joists by nailing into the flanges on the sides, I have seen articles to reinforce by nailing plywood to the bottom across several joists, but never to the sides of the flanges.

What I plan to do is called adding web stiffeners. I would be afraid of splitting the flanges by nailing or screwing into them along the grain, which is what I believe has been suggested here.

I believe the flanges are really there only as a nailer and to give the web a larger surface to bear on and to resist twisting the joist. I also plan to add between-joist blocking before I finish b/c there are only blocks at one end of the joist, the other is in a hanger.

Tim P.
03-06-2007, 06:26 AM

Good job on looking up the instructions! SO many problems can be avoided by just reading the instructions. Too bad it's genetically coded in males to immediately discard all paperwork!

If I remember correctly, TrusJoist had a detail wherein you ripped 1/2" plywood and inserted it into the web of the I-joist. However, I think this was for when you needed to attach joists perpendicular to that particular member; it gave a place for the joist hanger to be nailed.

You would not need an engineer to do this as it is a prescriptive method. However, what I think Steve was saying is that you really don't know if it is reducing deflection without an engineer.

I don't believe I've ever seen an I-joist sistered, other than 2 of them placed next to each other.

Mid-span blocking is usually a mandatory detail. Can you tell us the manufacturer and series of your joists?

You are absolutely correct about avoiding the top/bottom flanges unless directed to do so by a design professional or manufacturers instructions.

Are you trying to reduce deflection?

Good luck,


03-06-2007, 07:05 AM
Yes- I am trying to reduce deflection. My joists are marginal at best for ceramic tile and they are 24"oc, 13.5' span with 23/32 OSB subfloor and will also have 15/32 BC plywood over the tiled area with Ditra (I think) Area to be tiled is 8'x3' half-bath with 12"x12" tile.

I have looked extensively at the joists to find manufacturer markings and only find long production numbers, no names or model. The house is only 4 years old.

I have also looked at building a small wall in the crawl space to reduce the span, but that would require pouring a footing and I am afraid as the house settles, it may not settle evenly and may cause a bow in the floor. Plus I want to keep it as "stock" looking as possible for resale.

If web-stiffening does not help with floor bounce, I will sister 2x10 joists with the I-Beams- by adding web-stiffeners now I can use them as filler/spacers for sistering with dimensional lumber if needed in the future.


03-06-2007, 08:03 AM
Tim is correct about my suggesting an engineer to determine what the outcome will be. I dont think what you are proposing to do is going to be effective at reducing the deflection.

The detail you reference is just a blocking, to carry concentrated loads past the joist, basically keep the web from rotating or buckling, I dont think it is applicable to reducing deflection. Take a look at this:

go to the floor and roof systems guide and scroll down to page 27, cantilever details. It shows a nailing schedule for plywood attached to one or both sides of the joist.
I am not an engineer,but Ive done structural repairs for 15 or so years and just about every engineered fix for an I joist that I have seen has been similar to the cantilever detail plywood sandwich. Other details involve adding another joist, or a new or wider flange to the existing flange.

Your understanding of the I-joist is backwards, the web supports the flanges which do most of the work in a beam. Compression loads in the top flange and tension in the lower, not much in the middle so the web is not as substancial as the flanges. Since the I joist strength comes form the flanges, to stiffen the joist you support the flanges better. The web is basically a spacer(which is why you can knock big holes in them with little effect on its carrying capacity- with in ddesign limits) so beefing up the web isnt going to do much. The blocking and spacer details are needed to support point loads which could deform the web.
Dont mean to be discouraging, just helpful.

03-06-2007, 08:19 AM
Afte rereading this I have a question about the deflection you are trying to correct.

What exactly are the joists you have?

The description you give is ;

"The I-beam web is 4.5 inches deep with 1/2 thickness to the flange and the beam span is 13.6 feet long on 24" centers"
-and later...
"I have looked extensively at the joists to find manufacturer markings and only find long production numbers, no names or model. The house is only 4 years old."

How have you determined the deflection is out of spec?

It seems that you think the web is the critical part, since you describe it uin the first post, and later you state the flanges are basically for nailing.

What is the joist total depth? , and what is the width of the flange? - these are the two main criteria to determine the carrying capacity of the joist. Post those and maybe we can determine what you have.

03-06-2007, 08:45 AM

Thanks for the good insights! The doc from Georgia-Pacific is very good and quite similar to what I saw from Weyerhauser. I have a strong tendency to go with the "box" technique. Thanks too for underlining the principles of how the joists work :-)

Andrew I like the way you mentioned the word "stock" For that reason I'd hate to see any dimensional lumber used in your project, keep with plywood and engineered products!

__ pete

03-06-2007, 11:44 AM
The total depth is 9 1/4 inches. Width of flange is 1 1/2", depth of flange is 2 3/8". The flanges have fingered joints which is why I do not think they carry that much of the load.

No idea who makes the I-joists. I have a GP brand EW support beam though.

In the GP paper, Image F18 on page 25 is what I plan to do. I believe the place where the reinforcing piece touches the flanges has an impact on the stiffness of the flange/web connection. In my case, the reinforcing piece would be tight against the top surface of the bottom flange and there would be an 1/8 of an inch between the bottom surface of the top flange and the reinforcing piece.

I hesitate to nail into the grain of the flanges because I am afraid they will split. They are fairly small and I think I would have to use 6-8d nails, likely spliting them even with a nailer.

Not that I think Tommy from "This Old House" is the best carpenter but he is experienced. What I detail is based on what he recommends at the bottom of the link in the start of the thread.

Thanks for making me think


03-06-2007, 12:15 PM
I was able to decipher a name- my joists are made by NASCOR. I will give them a call and see what they say. I believe these joists are guaranteed for the life of the home so I need to figure out if this will void the warranty. Probably just against workmanship anyway.


03-06-2007, 12:48 PM
Engineer from NASCOR recommended cross bridging at mid span or continuous mid span blocking. Not sure how I will toe-nail to the top flange- I guess in through the side of the cross bridge.


04-12-2007, 06:42 PM
Weesa20 - Have you completed the cross-bridging? I was wondering if it helped the deflection problem you were having. I also have a deflection problem and was thinking about bridging also.

04-12-2007, 07:11 PM
Maybe this will help....

04-12-2007, 07:11 PM
Bridging helps to spread the load, but does NOT decrease deflection overall of the floor, only for a point. Put dead load on the floor, and it deflects the same amount, regardless of whether you have blocking in or not, especially if the subflooring is glued and well attached to the trusses. Your best bet would be to make the spacing 12", as getting the subflooring stiff enough to meet the deflection specs between the 24" centers is not trivial, either.

04-13-2007, 10:40 AM
When bridging and/or sistering are the tops of the added supports glued and nailed to the overhead subfloor? I think this would be the case to prevent squeaks.