Jump to content

Sistering floor joists


Recommended Posts

Many people have an opinion as to the best way to sister floor joists. However, I have never found any reliable information regarding the subject. I have performed calculations, but they usually result in using an excessive number of fasteners. I don't like to over-engineer things if not needed, or fail a contractor's work without good reason.

Last winter when things were slow I set up a somewhat crude test rig and ran some tests on floor joists and sistered floor joists. I did not run many tests, and I am unsure about the accuracy. I suggested running similar tests at a university testing lab, and it seems like I have some interest. I need to develop a testing program and was looking for any input. In general terms, I am thinking about sistering for a joist damaged near mid-span and for a joist damaged near a bearing end. I would like to test nails, lag bolts, Simpson SDS bolts or similar, and bolts with nuts. If time or budget permits, it would be nice to test some improper options, like fewer fasteners than needed, deck screws, and maybe carriage bolts installed at an angle.

Any input?

See attached for some background.

Download Attachment: icon_adobe.gif floor joist testing-TIJ.pdf

530.01 KB

Link to post
Share on other sites

Glue. Mechanical fasteners would only be for compression until the adhesive set.

Sounds interesting. The only time I've ever done it, I used PL200 or one of those type construction adhesives. Damn things were like steel.

Link to post
Share on other sites

I recall a joist I sistered for an inspection client, shh, don't tell anybody.[:-magnify

For good luck, we put one on each side, a 2X10 sandwich.

The joist was not rotten. It was a Douglas fir 2X10 that was crowned correctly but humped up so bad it created a high seam in the plywood under the carpet.

My client had been upset for about 6 months, and to end the irritation, I got a helper and we did surgery on the hump, pulled it down and double sistered it.

For leverage, we attached a 2X4 prybar to the humped joist with a long joist hanger for a strap. Then I held that down and buddy applied glue and drove in some screws.

I jumped up and down on that floor after that and could not shake a model sailboat on a sidetable 6 feet away. [:)]

Link to post
Share on other sites

Thanks, Mark, for posting this.

On a recent crawl with a client who wore the paper suit I issued him, we found more than several SPF (I know, I know) floor joists with extended longitudinal splits. (Sorry no pix)

Most showed no deflection to the eye. I suggested sistering, but had no real references. Because I could see no deflection I opined that the splits were from desiccation, not stress from load, and I suggested even 3/4 plywood screwed on.

I don't remember if it was this forum or another, but an architect on one posted that no fastening procedure that depended on glue, or any other kind of "goop", had any science or reason to support it.

Link to post
Share on other sites

I will suggest that the architect that indicated there was no science or reason behind why a glue joint was stronger than a mechanically fastened joint is a fool.

Every first year wood shop student learns that a glue joint is stronger than the wood surrounding it. There is a reason they make glulams and LVLS with glue instead of a mountain of fasteners. Anyone that's ever actually built something with adhesives knows the immense strength created by a glue joint as opposed to a nailed or mechanically fastened joint.

If you don't believe me, do a very simple test. Glue together any combination you want and then mechanically fasten the same combination. Then load it to the max let it sit around for a while. Take a few measurements.

This was the thing my shop teacher did back in the '60's to show us the amazing wonder of new adhesives. It impressed us. It seems we've forgotten things we ought not be forgetting.

Now, I'm sure there is someone out there that can devise a test that contradicts what I just said. Everyone gets to do what they want.

I'm with adhesives.

Regarding Les' question about carriage bolts at an angle....they make a joint a whole lot stronger than simple screws. We did some tests and determined carriage bolts make a difference. But, they're not as strong as adhesive.

Link to post
Share on other sites

I'm building a gull-wing cover for my compact pickup to make service calls. It's 95% plywood and Titebond Premium. Amazingly strong and light.

That's good work, Marc. But how do you teach it to make service calls? [:)]

I think fasteners on an angle are strong, but could pull thru due to the angle of the head, just as an angled roofing nail will tear the shingle. The head of a carriage bolt won't sit flush if the hole is on an angle.

Link to post
Share on other sites

I will try to respond to several posts. Glue can be strong, but quality control is a concern. Also, the type of glue is important. Most glues are not approved for structural use. Construction adhesive and typical wood glue can creep under load. Polyurethane glue does not creep and can be used for structural purposes, but it is a mess to work with.

Carriage bolts are not approved for structural use, and my concern with the angled installation is that there may be more movementbetween boards.

Plywood is a poor choice when bending is involved. Half the thickness is not effective because it is oriented the wrong way. Also, it is not graded or rated for bending.

Full length sisters are always best, and the fastening pattern does not really matter if the sister joists is in contact with the subflooring. However, it is often not practical. When the damage to a joist effects and significant length of the joist it can be the only way to go.

Link to post
Share on other sites

Having been to a few training classes at Simpson Strong-Tie in McKinney, TX I seem to recall some tests they showed the crew of inspectors.

I did not take pictures and I can't precisely recall, but I saw lots of timbers and many with glue used in their testing of fasteners and brackets, etc..

I'm sure they have tested many combinations of framing for their products.

Link to post
Share on other sites

Glue can be strong, but quality control is a concern. Also, the type of glue is important. Most glues are not approved for structural use. Construction adhesive and typical wood glue can creep under load. Polyurethane glue does not creep and can be used for structural purposes, but it is a mess to work with.

Quality control and adhesive use are part of the same equation.

Construction adhesive works fine for sistering. It's just a house, with minimal loads. And it's sistered fer chrissakes, which is about as anemic a repair as I've ever done and it works fine.

We've built boxed beams and boxed damaged truss with plywood and construction adhesive. The repairs were designed by us, and approved by a structural engineer. It works fine. Some of this gets a little rarified when the engineers get hold of it. It's just a house.

"Typical" wood glue isn't anything I recognize as a material. I don't know what typical wood glue is. Elmers? Titebond? Gorilla?

The builders of cold molded boats...that rely completely on adhesives to hold together wildly complex compound curves and structural components that experience shattering loads and perform for decades... would be amused at the idea of adhesives creeping. Resorcinol and epoxies are the primary adhesives. They don't creep.

Carriage bolts are not approved for structural use, and my concern with the angled installation is that there may be more movement between boards.

Angled bolts move, no doubt.

There's a local architectural firm that only builds institutional projects (schools, hospitals), specifies carriage bolts for a lot of structural applications. Where's it say carriage bolts aren't approved for structural use?

Link to post
Share on other sites

Glue can be strong, but quality control is a concern. Also, the type of glue is important. Most glues are not approved for structural use. Construction adhesive and typical wood glue can creep under load. Polyurethane glue does not creep and can be used for structural purposes, but it is a mess to work with.

Quality control and adhesive use are part of the same equation.

Construction adhesive works fine for sistering. It's just a house, with minimal loads. And it's sistered fer chrissakes, which is about as anemic a repair as I've ever done and it works fine.

We've built boxed beams and boxed damaged truss with plywood and construction adhesive. The repairs were designed by us, and approved by a structural engineer. It works fine. Some of this gets a little rarified when the engineers get hold of it. It's just a house.

"Typical" wood glue isn't anything I recognize as a material. I don't know what typical wood glue is. Elmers? Titebond? Gorilla?

The builders of cold molded boats...that rely completely on adhesives to hold together wildly complex compound curves and structural components that experience shattering loads and perform for decades... would be amused at the idea of adhesives creeping. Resorcinol and epoxies are the primary adhesives. They don't creep.

Carriage bolts are not approved for structural use, and my concern with the angled installation is that there may be more movement between boards.

Angled bolts move, no doubt.

There's a local architectural firm that only builds institutional projects (schools, hospitals), specifies carriage bolts for a lot of structural applications. Where's it say carriage bolts aren't approved for structural use?

Kurt, Some engineers approve things that maybe should not be approved. I agree that loads are generally small, etc., but when I approve something I am concerned about liability. If a product is not rated for structural use I would not use it. I recently saw a structural repair spec by an engineer that called for deck type, or general purpose, screws. They may work, but again, are not approved.

The typical wood glue I was referring to is polyvinyl acetate (PVA), Titebond, Elmers, etc. I can't picture this creeping, but they say it can. Remember, unless a manufacturer publishes reliable strength specs and does not limit use in structural applications an engineer should not be specifying it (unless they want to do their own testing to show that the product will work).

The carriage bolt statement was from a Simpson seminar. I have not researched that. but I believe that the issues are crushing of wood at the bolt head (maybe smaller than a washer?). Also, I believe that there are not strength specs that apply to typical carriage bolts, so they could be made out of any kind of steel. That may not seem like much of a factor, and it probably is not with sistering, but could be at times. Bolts can be over-stressed by over-tightening.

Link to post
Share on other sites

Some engineers approving things that shouldn't be approved.....is a fundamental fallacy argument. Meaning, it has no meaning.

I keep my nose out of stuff I don't know anything about. That's why we have a structural engineer look at stuff for us. But we're talking about houses, and not just houses, but sistering joists in a simple floor platform.

Sistering is a *lousy* repair, and it works just fine even without adhesives and even if it's not pressure fit. Add any kind of adhesive short of bubble gum and it's fine. More than fine.

I just did a quick google search for "structural construction adhesive". Seems there's a small mountain of adhesives rated for structural applications, of several different types and formulations, with several being tubed and gunnable. Some of them cop to the "must be used with mechanical fasteners" disclaimer, which is understandable, but how much of that is actuality and how much is corporate legal covering asses? I tend toward the ass covering hypothesis.

The real structural wood miracles of today are wood boats, especially cold molded. There's nothing to them but wood and glue, they're subjected to stress that is orders of magnitude greater than anything ever would be in a house, and they have to endure thousands of pounds of force tens of thousands of times over decades. You'll find they hold up pretty good. Mechanically fastened boats...they get loose and floppy and they have to be refastened every few years. Gimme the glue.

I think it's cool you're doing the research, but sistered joists are kinda basic stuff. Glue out of a tube is fine. Honest. People that don't build stuff much think there's some kind of atomic entropy happens, and shit just flies apart if there's some stress. No, it doesn't.

It's a house. We can work on them and not worry too much.

Link to post
Share on other sites

The engineers that drive me nuts are the one's on historic homes. Old joints, saggy like they always are for all the reasons we see all the time, but they're still there and there's really nothing significantly wrong with them other than folks have totally forgotten how to think and work on them.

Engineers always come in, and after long hours of brow furrowing concentration, determine the old dump needs a gigantic steel I beam that would hold up a 10 story building. For a 150 year old saggy frame house. That's been supported by a termite eaten beam and a couple beer cans for 75 years. Drives me nuts.

If the whole joint is getting stripped to the bones, sure, why not? Stick in whatever steel you want.

Link to post
Share on other sites

When I had my oyster ranch, I bought a second hand 22 ft dory boat. It was glued plywood on a wood frame. Painted plywood, no glass, just good marine glue.

It never leaked a drop, and I used it to haul coffee sacks full of oysters for about 5 years. After which I sold it, sans the 50 hp Merc, to a guy that used it to sleep in sometimes when he left the pub late. Which may have been fairly often.

One night he drifted away in the fog and it took him the best part of a week to get back. He had a little 6 hp with bad ignition. Good boat, tho. [:)]

I recently glued a wood transition strip to vinyl glued on concrete. No nails or screws needed. I used two long bent strips of door casing to put pressure on the wood for 24 hrs. Construction glue.

Link to post
Share on other sites

Some engineers approving things that shouldn't be approved.....is a fundamental fallacy argument. Meaning, it has no meaning.

I keep my nose out of stuff I don't know anything about. That's why we have a structural engineer look at stuff for us. But we're talking about houses, and not just houses, but sistering joists in a simple floor platform.

Sistering is a *lousy* repair, and it works just fine even without adhesives and even if it's not pressure fit. Add any kind of adhesive short of bubble gum and it's fine. More than fine.

I just did a quick google search for "structural construction adhesive". Seems there's a small mountain of adhesives rated for structural applications, of several different types and formulations, with several being tubed and gunnable. Some of them cop to the "must be used with mechanical fasteners" disclaimer, which is understandable, but how much of that is actuality and how much is corporate legal covering asses? I tend toward the ass covering hypothesis.

The real structural wood miracles of today are wood boats, especially cold molded. There's nothing to them but wood and glue, they're subjected to stress that is orders of magnitude greater than anything ever would be in a house, and they have to endure thousands of pounds of force tens of thousands of times over decades. You'll find they hold up pretty good. Mechanically fastened boats...they get loose and floppy and they have to be refastened every few years. Gimme the glue.

I think it's cool you're doing the research, but sistered joists are kinda basic stuff. Glue out of a tube is fine. Honest. People that don't build stuff much think there's some kind of atomic entropy happens, and shit just flies apart if there's some stress. No, it doesn't.

It's a house. We can work on them and not worry too much.

I don't disagree with much of what you posted, but when you look into it, most of the "structural adhesives" are not rated for true structural use (as in load carrying applications as specified in codes) or do not have code-approved load ratings. As an engineer, I have to deal with numbers even in cases where common sense may be enough.

My point about engineers approving things that they should not approve relates to engineers approving things because they are not familiar with what is required.

Your point about engineers and old buildings is right on. I only understand old buildings because of performing thousands of home inspections. You do not learn that stuff in school or sitting in a office designing new structures.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...