In place of a 2x10 rim joist for an exterior deck, Can two stacked (not sandwiched) reach the same or greater load potential?

Thanks for your help.

In place of a 2x10 rim joist for an exterior deck, Can two stacked (not sandwiched) reach the same or greater load potential?

Thanks for your help.

Short answer is No! Randy will be along to fill you in on the engineering reason or you can look at the building code for the answer.

Michael,

You will have to look at bending capacity and deflection separately:

- A single 4"x4" wooden beam will deflect 1/4 as much as two 2"x4" beams stacked on top of each other for the same load.
- A single 4"x4" wooden beam will carry twice the load before it breaks then two 2"x4" beams stacked on top of each other.

This is based on ignoring any sliding friction between the two 2"x4" beams and assuming the ends of the beams are pinned and free to rotate. Also the width of the beams are the same and the height of the two stacked beams are each 1/2 the height of the single beam.

The information above just compares the engineering properties of the two configurations. The answer to your question is two beams may work if the deflection and bending capacity are within the allowable limits. I would have to know the exact loading and geometry.

Randy this is great I think one more thing should be clarified. If you were to take 2 - 2x4s and glue and screw them together you still would not get the strength of 1 - 2x10 in fact you would not even get the strength of a 2x8 in my opinion.

One 2x4 has a max span of 3-6 for 20 (psf)

One 2x8 has a max span of 6-10 “” “”

Since he asked about replacing a 2x10 we are talking about 8-5 for 20 (psf)

So the load would not matter since we are talking apples to apples.

Correct me if I am wrong.

Kevin

Maybe this will help clarify this issue (or not). The deflection and load capacity of a single beam or one beam composed of two sections or four sections (left picture) is theoretically the same as long as the overall width and height stays the same (apples to apples). This is only true if the glue holding the individual sections is stronger than the wood. Basically this is a Glu-lam beam. However, Harry the home owner would not be able to produce a factory quality Glu-Lam beam so in reality the single beam would be stronger. Your statement about 2- 2x4 not being as strong as one 2x8 or one 2x10 is technically correct because the height is different for each one (not apples to apples).

Referring to the code tables is a little misleading. Double stacked beams (not glued together) each carry half the load. So the allowable span for a double stacked beam would be based on a table developed for 10 psf not 20 psf for the single beam. Does this help of just muddy up the water?

Accept that I said glued and screwed and I did not need the clarification however I thought that Michael did, the 10 psf is very interesting though. Thanks again for making sure this is answered correctly.

More data: This is a 12 foot square deck with support in the middle of the rim joist, so the span is no greater than 6 feet. The floor joists are 2x8. The rim joists were intended to be 2x10, but a single rim joist was installed that was a 2x8. Rather than removing the joist all together, just adding a second 2x8 should be more than adequate.

I can imagine this ever twisting.

As long as the gap is flashed this should be no problem.

Michael,

Not sure where this deck is but if your in the 60 psf snow load area of Michigan then the deck floor joists and rim joist are under sized. Design loading combination for the deck is likely DL + 0.75(LL+SL) which equates to the following joist load based on 16" spacing:

DL= 10 * 1.33 = 13.3 lbs/foot

LL= 40 * 1.33 = 53.2 lbs/foot

SL= 60 * 1.33 = 80 lbs/foot

Total Design Loading = 13.3 + 0.75(53.2 + 80) = 113.2 lbs/foot (floor joist)

This loading requires a 2x10 joist depending upon the grade of wood.

The outer rim joist carries 1/2 of each joist load which equates to 6 x 113.2 = 679.2 lbs every 16 inches. This equates to 511 lbs/foot which requires a double 2x10 rim joist to stay within the code limits.

As bad as this sounds the most likely failure mode of most decks I have investigated are the connections. I suspect if your deck was checked in detail the connections would fail first.

Good point Randy as I was basing the info on 20 psi just for example.