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Sistering a joist

9 years ago

Hello, I have to sister a small length (4 feet) of a joist, and I've got a few questions. Nails are preferred over screws, right? 16d nails are most commonly used, correct? What should my nailing pattern be? Should I angle the nails? Should I nail from both sides? I'm using adhesive as well. Thanks!

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Comments (8)

kudzu9
9 years ago
I'd use screws over nails any day in a situation like this. Screws have more holding power and will draw the pieces tightly together, which will enhance the effectiveness of the adhesive. Use quality screws meant for load bearing (not sheetrock screws), apply adhesive to both mating surfaces, draw the two pieces of wood together with clamps (like parallel clamps), and drill pilot holes through the first piece of wood you will be penetrating which are just big enough to allow the screw to pass through with a little friction. Use of an impact driver would be preferred, but you can use a regular driver if that is all you have.
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amateurplumber
Original Author
9 years ago
Thanks for the advice! I have heard from a couple people now that screws would be better, but everywhere else it says that nails would be better because of shear strength. Just drill a pilot hole through the first joist, not the second?
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Okay. If you're just trying to level out the floor versus strengthen it, all you would need is something like 2x6's, but you'd want them to span wall to wall, so that the subfloor would be fully supported. It will be fully supported so long as ONE of the two-- the original joist, or the sistered joist, hit the bottom of the subfloor when laid down flat (the operative word there being FLAT). As for order of materials, If you're just doing a normal ceramic floor, you want a layer of 3/4" exterior grade plywood, then either 1/4" cement board or 1/8" Ditra, and then your tile. NO tar paper, unless you're going to do a mudset. Figure on 3/8" for the thickness of tile and thinset, and if you need any height past what these layers would give you, then my recommendation would be to do it in plywood for anything over 1/4", and for under, switch up with your underlayment by using 1/2" cement board, or something along those lines.
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Trimming the sister shorter does not make much difference since sag is not a bearing area issue. The problem with any type of jacking is that the old joist may have a set that will not go away very quickly. If you want to try and straighten the old joist before sistering watch what is happening very carefully as you start to lift the middle. Some joist will give enough, but many will not and the end(s) can actually be lifted off their bearing location. Raising very gradually (1/8 inch per week type numbers) can give the old wood some time to move back into a straight line. Glue is not normally needed, and in most cases is assigned a zero strength since application is so variable. All you need is a slightly thicker glue line to render it useless, and correct glue line thickness is very thin (thousandths of an inch over the entire glue area). Clamping is required to even get close to a correct glue line. 16d nails are more than adequate. While bolts and lag screws may appear better if you use fewer of them tan you would nails you can easily exceed the compression limits of the wood. There are nailing rules for new work that can serve as a starting point, but older lumber may have been ungraded or just over spanned, making a more frequent nailing schedule needed.
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I don't think that steak example is parallel at all. And given that it is *not* that hard to do, why not tell them what they need to know, so they -can- do it themselves? Or, maybe it is parallel. It's not -hard- to figure out how to eat a steak for the first time, but a person who had only eaten lasagna or cooked grilled-cheese sandwiches might benefit from a little extra info. Would you seriously suggest that a person hire someone else to feed him a steak, or would you just say, "Plastic would work, but it'll be less frustrating if you use metal. And it'd be easier with a steak knife instead of a regular table knife." Would you seriously say, "You obviously don't know enough to cook a steak; go to a restaurant"? Or would you say, "Use X flame, X minutes; you can tell it's by when it feels like this. Don't poke it with a fork to turn it, and don't use tongs; use a spatula to flip it so you don't squeeze the juices out." People who have cooked one simple thing are sensible to ask for expert advice when they're ready to expand. But wanting (or even needing) advice before doing so doesn't mean the person isn't capable of doing it successfully. And there are supportive ways to tell someone that you suggest they get someone w/ more expertise to help them. There was a beautiful example of that elsewhere on this forum. But I think that the level of detail given by people here and on the other thread on this topic, and the disagreements on this thread, are proof that this was a very sensible question to ask, and it in no way indicates that our OP is unable to hammer a nail or drive a screw. Or that the OP is unable to successfully support this notched beam. It's not that hard; but there are smaller pieces of info that will make the OP much more effective at this. If a person is just ridiculed for ever wanting to learn how to do things like this, how will they ever expand their abilities? Isn't that the purpose of forums like this, to transfer experienced knowledge to the brains and lives of those who are inexperienced but willing to get more info?
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"... in the UK, the preferred method is with through bolts or (threaded rod), large square washers, and large nuts." The problem that occurs with bolts is you need a decent number of smaller bolts to not exceed the allowed loading of the wood (compressive loading) at the fasteners, and to make sure the loads are transferred as evenly as possible. I would guess that they are not using 2x lumber, but something thicker (and possibly even hardwood). 16d nails are worth around 300-600 pounds of shear load per nail. The actual value allowed depends on wood species and grade. Lots of 16d nails has the advantage of spreading the loads out among the fasteners to avoid exceeding the compressive strength of the wood. even in flitch beams, fastener size and hole clearance through the metal is critical. There have been tests using powder actuated drivers to run the nails through the wood and metal in flitch beams. It appears to work well since the nail punches a tight hole through the metal flitch.
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amateurplumber
Original Author
9 years ago
Deleted.

This post was edited by amateurplumber on Thu, Aug 7, 14 at 1:54
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grubby_AZ Tucson Z9
9 years ago
Yep. Do exactly what Kudzu said.

The screw hole thing is a general concept. The screw should catch well in the second board but slip in the first board (the one with the screw's head digging into it). Thus the screw pulls the slip board down onto the caught board (highly technical terms) by the head. This does not exclude having a pilot hole for the threads to catch in.
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kudzu9
9 years ago
It's true that nails have good shear strength, but they don't resist torsion or withdrawal forces along their length, including seasonal expansion and contraction of the wood, and any tendency of the joists to twist. Sheetrock screws are somewhat brittle, which means they don't have good shear strength, which is why I recommend you use general purpose construction screws. You can get these with square drive heads or star drive heads, which will allow long screws to be driven easier without slippage than, say, a Philips head.

Unless the joists are really hard, you should be able to drive through the second joist without a pilot hole. The point of putting a hole in the first joist is that, once the screw bites in to the second joist, it will pull the first joist nice and tight as the screw threads will not be able to bite into the sides of the first joist. If you drill too small a hole in the first joist, or the same size pilot hole in the two joists, the threads will bite into both joists and will keep the first joist from being drawn as tightly to the second as possible. Does that make sense?

This post was edited by kudzu9 on Thu, Aug 7, 14 at 14:08
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sombreuil_mongrel
9 years ago
2 7/8" headlock bolts are just for this purpose.
spaced 16" on an up/down zigzag pattern. I would predrill the holes with a 3/16" bit to prevent splitting the old wood.
Casey

Here is a link that might be useful: Fastenmaster.com
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talley_sue_nyc
9 years ago
Such great info--I'm not sistering any joists, but I'm really glad to have read this.
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amateurplumber
Original Author
9 years ago
Thanks for all the info! I'll go get some Spax #10 3" construction screws if that sounds good. I should get the ones that are NOT fully threaded up the shank, right?

I'd really much prefer to use the screws anyway, since i can maneuver easier with an impact driver than a hammer. I wish I could find shear strength values somewhere.

IF i can use some coated 3" deck screws I already have that would be freaking amazing. Then i wouldnt have to buy any more screws...

Thanks again all!

This post was edited by amateurplumber on Thu, Aug 7, 14 at 17:37
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