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I am replacing part of a floor because of water damage. I have removed three layers of flooring. The joists were unaffected by the rot (as was the bottom floor, but I removed it anyway).

I only intend to put in one or two layers of 3/4" plywood for the new subfloor. I don't want to build up ~2.75" inches of flooring and try to match the thickness of the original floor. Instead, my plan is to sister the joists in this area in order to lift them to the appropriate height. So the entire weight of the floor will rest on top of the new sisters.

The sisters will cover only ~6 ft of the original joists' ~9 ft span. The original joists are undersized by today's standards (they are 2x5s [4.5" actual—never seen that before!] spaced 16" o.c.), but there is no noticeable sag, and replacing the entire floor is out of the question at this point. So I'm not looking for the sistering to create a stronger composite system, but simply to build up the floor.

Is it enough to glue the joists and nail them? (If so, is a couple of 8d common nails every 8 inches good?) Or is there occasion to use screws or bolts also?

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AFAIK, you don't even need the glue, though I'd probably use it myself.

8d might be a hair short. 10d driven at a slight angle (to avoid stickout with whatever size they shaved 2" lumber down to in the latest revision, 1-1/2" is so late 20th century...) might be better, especially if the old joists are from a thicker time in the history of 2" lumber.

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    I always hesitate to use nails, even heavy nails, to take shear loads... – keshlam Sep 10 '14 at 2:18
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    Nails are actually better than screws at shear. A solid, smooth chunk of steel .vs. one with delightful stress concentrations in the form of threads. Likewise, screws are generally harder - which also means they are more brittle. – Ecnerwal Sep 10 '14 at 3:58
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    @Ecnerwal - The difference between wood screws and properly sized structural wood screws. No stress risers in the shear zone. Always something to think about. – Fiasco Labs Sep 10 '14 at 14:04
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    @keshlam What kind of screws? At what spacing? That is saying "screws" is problematic in that 1: Most screws in the world are not suitable for structurally attaching 2 pieces of lumber; 2: Screws suitable for attaching lumber come in many varieties; 3: Unlike nails, guidance for proper attachment is not baked into the building codes nor is the knowledge common in the construction industry [most people don't own a copy of NDS which is where the fastener calculation formulas required to design a screwed wood connection reside]; 4: This means it's really easy to get screws wrong. – ben rudgers Sep 10 '14 at 14:09
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    @Ecnerwal the nails should be driven perpendicular to the face, not at angle. Deliberately driving nails at an angle will reduce their rated capacity to that of toenailing from an engineering standpoint. – ben rudgers Sep 10 '14 at 14:32
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Unless the new joist is undersized, there is no need to sister the new joist to the existing. If the new joist is undersized, sistering to another undersized joist may not provide enough additional section modulus [additional beam width has much less impact than additional beam depth]. If the existing joists are damaged, then it is even less likely they will add sufficient section modulus to make a new undersize joist adequate.

In any event, if the existing and new joists are inadequate, an engineered design is really required both in terms of sizing members and bonding them into a composite system. General construction practice however is to use 10d nails when face nailing a pair of 2x members. So long as the wood does not split due to nailing, more nails than the minimum number is acceptable. Construction adhesive is a nice to have, but does not compensate for inadequate nailing under prescriptive design - engineering analysis is required to consider it as contributing.

If on the other hand, the existing joists are adequate, or would be adequate but for damage, the alternative to installing taller joists is to match the existing joist depth for any new joists and to build add additional elevation to the finished floor in the new finished floor system via, thicker subfloor and/or thicker floor finish system.

The advantages of making up the difference in floor system are:

  • Easier to undo later.

  • Provides for more flexible selection of floor finishes for the current project.

  • Keeps the top of structure consistent with existing construction.

  • Separates concerns - i.e. the structural system is not determined from the floor finish system.

  • Thanks for the answer. I have edited my question a bit to clarify that I'm sistering just as a convenient way to add elevation, and I'm not looking for the composite system to make up for the inadequacy (by today's standards) of the old joists. – Vebjorn Ljosa Sep 10 '14 at 14:23
  • Then, as I said, or at least a I think I said, there's no need to sister. Just install new joists and attach them to the existing support structure. Also, as I think I said, I'd strongly consider building up the floor system - e.g. on sleepers versus doing it with new joists because it comes apart more easily for future renovations or if there is an elevation bust late in the construction process. – ben rudgers Sep 10 '14 at 14:28

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