What in the World is L/360?

Have you heard the term L/360 before? Looks kind of geeky don’t you think? In the residential construction industry L/360 is the maximum allowable Live-Load deflection. Live-Load is considered everything in your home that can move. Yes, the upright piano that has sitting in your home, in the same spot, for twenty-years is considered Live-Load.
Another term that you will see, or hear of, is L/240. This control is for the Total-Load deflection, which by the way, is the Live-Load plus the Dead-Load. For sake the conversation; Dead-Load is all the building material in your home.

Well now…that is just about as clear as mud. Maybe an example is in order. Let’s look at a typical floor joist, and let’s assume the joist has a span of 15-feet.

Tip: The L in the equations above represent the span, and it has to be in inches.

Step 1: 15-ft x 12-inches per foot = 180-inches.
Step 2: Allowable Live-Load deflection = 180-inches/360 = 0.5-inches (1/2-inch).
Step 3: Allowable Total-Load deflection = 180-inches/240 = 0.75-inches (3/4-inch).

So given the above condition and a loading pattern that would push the controls to their limit these values would be the vertical displacement we could expect to see in our floor system.
So now…that was not so bad, was it?

I bet you are thinking right now, where is my MBS Blog writer!? He wouldn’t write this kind of….

If you are thinking this, then you are correct! Let’s say that I too have access to MBS’ Blog. So before I get caught with my hand in the cookie jar, I want to know is there anyone out there that can tell me what the Dead-Load deflection would be in our example. Email us your answer.