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I have a rotted joist in my floor that I can access through my basement, but is above several pipes. I could squeeze a 1x10 through the gaps but can't get a 2x10 to fit. So wondering if I got two 1x10 to fit, then affixed them together, would that have the same strength that a single 2x10 would have?

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In theory, yes, if you're able to bond them together adequately. This would typically mean a glue bond across the entire surface, or closely-spaced screw fasteners. You might even consider a triple.

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In theory, yes. But do consider the type of wood. Look online to compare the strength of various species of wood before you go this route. The reason for caution here is that there are strong woods that you would typically find good quality 2x framing members cut from. On the other hand the typical wide boards (1x material) that you see are cut from much softer and less strong types of wood.

Another thing to consider is the portion of the log that 2x beams and 1x boards are cut from. Study the wood's grain pattern on the end of the beams/boards before committing.

If your length is not longer than the lengths that you can get in plywood you may want to consider composing a beam out of lengths of plywood that you sandwich together in place. With enough layers you may even be able to stagger butt joints in the lengths of the plywood strips to achieve longer lengths. Glue bonding with fasteners to draw the strips together would be essential for this type of sandwiching.

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It’s probably stronger if same grade...(whether they are fastened together or not,) because: 1) same cross-sectional area, 2) same area from same distance from neutral axis, and 3) more surface area seen during grading.

1) Strength of beam is based on several factors, one of which is cross-sectional area. If they match, they are the same strength.

2) Extreme Fiber in Bending: Because the two 1x10’s have the same surface area as the single 2x10 from the neutral axis, it’s the same strength.

3) Lumber is graded on what is seen and not seen by the grader. More surface area of small pieces of lumber can be seen in small pieces of lumber than larger pieces of lumber, so the grader is more certain of the quality of the wood and therefore has a higher grade strength. That is to say, the grader can’t see the inside of a large piece, so there’s a safety factor in the grading (and thus structural design) that reduces its strength.

Strength has nothing to do with how to fasten the two pieces of lumber together.

I’d fasten them together with minimal number of fasteners, because they’ll tend to “buckle” under extreme loading before the wider lumber.

However, if you’re fastening sheathing to the top, you’ll need to be more careful to fasten directly into the skinnier 1x10’s.

  • The buckling is the issue to me. It's why 2 sheets of paper stuck together are much stiffer than 2 loose sheets. You need to arrest the boards' ability to slide past each other. That'll take more than a few nails. – Harper Dec 15 '17 at 15:59
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    @Harper The question is, “Do they have the same strength?” Yes, the slenderness ratio changes with a skinnier joist, but as long as they remain plumb, they have the same strength. – Lee Sam Dec 15 '17 at 16:05
  • But if they are loaded comparably, will the poorly bonded 1x's buckle first? Call me peculiar, but my definition of strength is how well they will perform in the field in service under normal loads. So in my book, if they buckle sooner with less load, then strength is not equivalent at all. – Harper Dec 15 '17 at 17:18
  • @Harper Actually, just a few nails will keep the boards vertical and just as strong as a 2x. A good example is a roof truss. It has a huge slenderness ratio, but to keep it from buckling, all is needed is a brace or two attached to the bottom chord with a couple of nails so it stays plumb which allows it to carry a huge load. – Lee Sam Dec 16 '17 at 16:25
  • ...because roofs are designed for that application. Where I'm disagreeing is the claim that a few nails is as strong as gluelam. If you're saying it's not as strong, but strong enough for this application, then sure I'd believe that, I'm not a structural engineer.... – Harper Dec 16 '17 at 17:23
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Yes, but it will be much weaker. Here. Get some sheets of paper. Kids' construction paper would be best, but any will do. Take 2 sheets at a time and do these things:

  • one pair, do nothing
  • one pair, draw dots on a 2” (50mm) grid and staple the paper everywhere there are dots.
  • one pair, mix up a 1/4 teaspoon of Elmer's or any wood glue thinned with water, and brush it lightly across one sheet and lay the other on it. Press them together and let them dry.

Now see which one is the stiffest. The unaltered paper will flop like a noodle. The stapled paper will be stiffer, but the staples will "work", enlarging their holes and reducing stiffness. The glue-laminate will in hands down.

A great glue to use is West System epoxy with bonding filler added to it - not too much, a lot of the epoxy will soak into the wood and it won't take the filler with it. The wood will be pretty well-finished just by virtue of being the finer one-by stock; if not, a thin pass through a planer would not be uncalled for.

It might also pay to "triple" it.

  • I'm not sure I like your analogy. In it, the adhesive is a much more substantial component of the system than in the case of the lumber. The fact is that screw fasteners on say a 4" vertical and 6" horizontal grid would eliminate the likelihood of catastrophic buckling in either member, resulting in a system roughly equivalent to a solid board. The varying grain plays a beneficial role, too. – isherwood Dec 15 '17 at 16:59
  • @isherwood Sure, if you can make sure the screws remain effectively clamping and transferring force by compressive adhesion, and not by placing the screws in shear, instead of by shearing the screws, which in softwood will work their holes quickly. Put it another way, I've seen quite a few glue-lam structural beams, not too many screw-lam. Also, the epoxy will penetrate a fair bit into softwood, making the epoxy layer thicker than you'd expect. You could make the epoxy layer a formal layer by laying and saturating several layers of fiberglass matt between the boards. – Harper Dec 15 '17 at 18:25
  • Of course the fiberglass layers would complicate the heck out of clamping, because any screw you'd use would snag and bunch up the fiberglass, creating air gaps and messing up the gluing quite badly. It would have to be all clamped. But strong as daylights! – Harper Dec 15 '17 at 18:34

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