Floor structure span too great

2 x 10 floor joists spaced 16" o.c., spanned 17’-9". My understanding is the maximum span for this joist is 16’-5" or 15’-5" depending on what standard you follow. There was a double 2 x 4 girder framed on the flat, supported by a couple of columns at the approximate center to help support the beams, although it did not run the full length of the basement. Settlement noted in floor where “girder” was missing.

My belief is the span is too long where it is not supported at any other point, and the 2 x 4 on the flat are basically useless. Obvious crack prevalent, and cracks in the drywall where the 2 x 4 were covered.

I am recommending further evaluation by a licensed engineer but like to hear your thoughts.


It depends on what the live load is and the lumber used.

Max. live load 30lbs/ft2, #1 board is 18’-5" or #2 17’-2"

Max. live load 60lbs/ft2, #1board is 13’-11" or #2 13"-00"


40 lb. live load is what is typically used. And those maximum spans I posted were based on that load.

Report what you see…the floor sags or has settled, etc.


Anytime you see extra supports a red flag should go up.

The problem could be poor design or the design was good but was over loaded or a combination of poor design and over loaded. In any case I would just refer to a contractor or an engineer depending upon the severity of the problem.

Maximum span calculator


Nice link, Joe. Thanks.

I definitely referred it. Looks like the supports were an after thought.

Randy, I believe I am correct in saying the 2 x 4 on the flat should not be considered structurally supportive, even if doubled up. Is that reasonably accurate?

It is “supportative”, but it is an improper and inadequate support.


A 2x4 has load carrying capacity in both directions. As long as you do not exceed the deflection limits, shear limits and moment capacity set by the applicable standards then the design is O.K. regardless of orientation.

Now to clarify the capacity of an upright 2x4 versus a 2x4 laying on its side you will need to pick a common denominator such as deflection or moment capacity. For example a 2x4 oriented upright will hold a concentrated load 5 times greater than a 2x4 laying on its side for the same amount of deflection, i.e. in general terminology its 5 times stiffer. Using moment capacity as your limiting factor an upright 2x4 can hold 2 times the concentrated load before the board breaks than a 2x4 laying on its side. The shear capacity is the same in either orientation because shear strength is based on cross sectional area which is the same.

That makes sense about it having some level of bearing capacity in either direction. I am just not used to ever seeing a 2 x 4 on the flat used for any type of load, as it would usually make better sense to turn it on edge, because of it’s increased deflection limit and moment capacity.

PS: Thanks for the clarification and buzz words. :slight_smile:

The other question to ask is if the post for the 2x4 are on a footing or not. If they just sit on the slab they would also be incorrect. Sounds like the floor needs further evaluation and repairs needs at the discrecian of a PE.

I just plugged in the info into Joes recommended calculator for construction grade Douglas fir and the span came back at 15’ 8" at 16"OC. In an existing floor I would measure the clear span and the deflection at the center and use the L/360. So 17’ 9" is 204+9=213/360=0.5916666666 X 32=19/32 inches deflection. If less then 9/16" OK, if more them recommend a structural engineer evaluate the structure.
Unless you are a structural engineer you can not go past a referral to a structural engineer.

The beam is laid wrong.
The lam should be vertical not horizontally laid…
It should be loaded on the thickest proportion.