Sistering a joist

amateurplumberAugust 6, 2014

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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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.

    Bookmark   August 7, 2014 at 12:59AM
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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?

    Bookmark   August 7, 2014 at 1:50AM
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This post was edited by amateurplumber on Thu, Aug 7, 14 at 1:54

    Bookmark   August 7, 2014 at 1:53AM
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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.

    Bookmark   August 7, 2014 at 2:06AM
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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

    Bookmark   August 7, 2014 at 2:39AM
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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.

Here is a link that might be useful:

    Bookmark   August 7, 2014 at 8:47AM
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Such great info--I'm not sistering any joists, but I'm really glad to have read this.

    Bookmark   August 7, 2014 at 1:39PM
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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

    Bookmark   August 7, 2014 at 5:18PM
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