(Massively?) Bored Joists need re-inforcing

illadvisedApril 13, 2009

Sorry for re-posting ... in hindsight, I felt this topic was distinct from "bouncy floors"

I was forced into renovating a second floor bathroom for numerous reasons. Never having liked the layout, I thought this was a perfect time to change it.

The room is 70" wide by 126" deep. I wanted to put the vanity on the left wall, the toilet next to the vanity and then the tub on the back wall, drain to the left, so the left wall would become a wet wall.

Problem was, the joists ran across the room, not down. Putting the toilet on the left wall would force me to bore 4" holes through 2"x8" joists. The two joists to be bored would sit directly under the tub. A real no-no.

I did a little (clearly, not enough) research on the internet & mostly talked to some local building supply stores. I had an idea that laminating (glued & screwed) 7 1/2" X 30" strips of 3/4" plywood to both sides of the joists might provide enough support that the joists could then be bored.

And this is where I was ill-advised. I was assured this would provide enough support, and that with 3/4" ply subfloor followed by 1/2" cement board the floor would be solid enough to support the tile I wanted to install.

After completing this portion of the renovation and replacing the subfloor, another trusted advisor spoke up and said that I can't do that. I've since stopped the renovation in order to take the time to figure out what to do so that the plumbing and floor support will be safe and to code; hence, my now having found this thread.

As an FYI, in doing my second round of research, I stumbled across these metal joist reinforcers - http://www.metwood.com/products/reinforcer . They look like just the thing I need to solve my support issues, assuming I leave the bathroom in it's current configuration. The problem with them for me is that they cradle the bottom of the joist and I don't want to have to tear up the first floor ceiling in order to install them.

So, one of my questions for all you engineering gurus with your mathematical wizardry is, if I were to attach 30" long strips of 1/8" or 3/16" steel strap to both sides of the joists, both above and below the holes, do you think this would provide enough support for a full tub of water and a tile floor?

If so, how would you suggest fastening them? Should they be screwed independently to each side, or bolted so that they sandwich the existing laminated joists? Further, should I be using some form of glue or cement to 'laminate' the strapping to the bulked up joists (not that I'm aware of any metal-to-wood adhesive)?

Another option I have is to remove one side of the plywood lamination I've done. Then sister both existing joists with a new (at least 6', but as long as possible) joist that has one of the joist reinforcers listed above attached. Due to the construction, the joists are fully supported by a bearing wall on the left side of the washroom so these new sistered joists would have the benefit of being supported on one end.

Any thoughts on any of these potential solutions?

Thanks in advance for anything shared.

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metal straps of that size ain't gonna do a thing. they will still eventually bend. you are talking about removing over 1/2 the joists, thus losing all structural integrity.

is it possible to install a bulkhead on the 1st floor so that the pipes can run UNDER the joists without going thru them?

    Bookmark   April 13, 2009 at 4:44PM
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What is the span and spacing of the joists? Where in that span are the holes and what size are they? How many of the joists are affected? Are any of them intact?

    Bookmark   April 13, 2009 at 5:31PM
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The joists are about 16" centres; they weren't spaced very precisely; but, I'm going by memory on this as the floor isn't currently open.

The holes are 4" in diameter approximately 33" from one end of the joists

Two joists are affected.

By way of an update, I contacted the people at Metwood (URL listed in my original post) and they assured me that installing their joist reinforcers would return the joists to their original strength. Does anyone have any experience with these devices?

    Bookmark   April 15, 2009 at 3:15PM
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If you use reinforcers be sure to follow the fastening schedule exactly.

Size and placement really matter.

    Bookmark   April 15, 2009 at 3:25PM
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Any reinforcing system is only as strong as the nails used to secure them so I don't know why the plywood solution wouldn't be as effective as the manufactured system if both are properly nailed. Without knowing the total span of the joists and exactly where the tub is located on them I can't give you further advice. Everything depends on the tension stress on the bottom third of the joists and that is dependent on the span and the loading condition.

    Bookmark   April 16, 2009 at 3:28PM
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"Any reinforcing system is only as strong as the nails used to secure them so I don't know why the plywood solution wouldn't be as effective as the manufactured system if both are properly nailed."

Note that the system in question uses screws, and they supply them.

Plywood is not all its cracked up to be for major structural repair.

And then you really need to do a shear calculation and a nailing schedule.

    Bookmark   April 16, 2009 at 3:50PM
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Would sistering, or using a manufactured beam (the OSB fin, wood top/bottom I-beam sort of thing) be better?

Unfortunately, the question is, how the op will pass the pipes through. I suspect they will have to cut a section from the pipes, slip sistering joists in, re-glue pipes. Cutting a wedge out of new joist would weaken them too much - rinse, repeat, unless they were perhaps tripled, with the cutout opposed. (one top, one bottom?)

I think the length of the sister material will have a large bearing on it (pardon the pun) - the longer the better.

To some extent I guess the 'sistering' effect works only to the next support point, am I right? Maybe if they are VERY well attached to each other, it would carry the load further?

I don't know if this would raise more issues since it involves more penetration of the joist, but I'd use very snug-fitting bolts at the ends of the sisters, and perhaps one each side of the weakened section.

When drilling for alarm wires, and we sometimes use a very small bit (3/8") I was told, never drill a manufactured beam, but the OSB I-beam type was ok, but not within 20" of the main support point.

Often in practice, though, I would drill next to where the electricians went - since they always came in first, I guess the strategy was "I assumed it was ok there because the sparkies did it ;)

    Bookmark   April 17, 2009 at 2:10PM
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It is my understanding that laminating reinforcement to the sides of the joists is the only way to avoid removing the ceiling below so it should be carefully considered first.

If the span is known the stress at the bottom of the joist can be determined and so can the amount of reinforcement needed. A piece of plywood would needed to be larger than some kind of steel reinforcement to accommodate the necessary number of fasteners. But we don't know the span so this is a pointless discussion.

    Bookmark   April 18, 2009 at 6:59AM
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We don't know what the span of your compromised joists is. We therefore don't know if an uncompromised 2x8 was adequate to begin with. Not knowing that, we can't give best advice. The one hopeful fact is that the 4" hole is quite near the end. That is much better than if it had been at mid-span. Are the 4" holes still needed, or will the repair remain unbored? IMO, best case scenarios all around, the gluing and bolting of 3/4" plywood on both sides of the cut joists would be worth a try. Making the sisters as long as possible will increase the potential for success, as will moving the 4" drain. If the drain could be reduced to 3", that would help a bit.

    Bookmark   April 18, 2009 at 8:54AM
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Use the commercial fix within its listing parameters (size of hole, location of hole, size of joist, etc.).

    Bookmark   April 18, 2009 at 10:09AM
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The Metwood product is limited to use where the joist is #2 Southern Pine with a max. span of 12 ft. and a max. hole diameter of 4.5" centered in the height of the joist. Apparently it can be installed without removing the ceiling below or removing the pipe. I still don't feel comfortable making assumptions about the strength and span of the joists.

Here is a link that might be useful: Metwood instructions

    Bookmark   April 19, 2009 at 6:59AM
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Analyzing a structure before making a repair is not going to happen in many cases.

No one is going to rebuild a house (and you are not required to) if the standards for span have changed (and they have changed over the years).

    Bookmark   April 19, 2009 at 9:54AM
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