What does L/360 deflection mean?

The notation: L over a number

L is the span. The number underneath is the fraction of that span the member is allowed to bend under the table's design load. L/360 therefore means "no more than one three-hundred-and-sixtieth of the span", and because L is expressed in inches, so is the answer.

This is a serviceability criterion, not a strength one. A joist at its deflection limit is nowhere near breaking. The limit exists because a floor that sags visibly, cracks its ceiling finish, or bounces when someone walks across it is unacceptable long before it is unsafe. That is why the limits differ by member: what counts as too much movement in a finished floor is not the same as what counts as too much movement in a rafter.

One important detail: the limit applies to deflection under the table's specified load, not to whatever the floor is carrying on a given afternoon. It is a design check performed against an assumed load case, and it says nothing about how your particular floor behaves under your particular furniture.

Working the arithmetic

Convert the span to inches, then divide by the denominator. A span of 15 feet is 180 in; 180 divided by 360 is 0.5, so an L/360 floor joist spanning 15 feet may deflect half an inch under the table's design load. The same member under an L/180 limit would be allowed a full inch.

Worked out across a few spans, the pattern is easy to hold in your head: the allowance grows with the span, and doubling the denominator halves it.

Two things follow from that arithmetic. Long spans get a larger absolute allowance, which is why very long floors can feel livelier even when they satisfy the same criterion as a short one. And a stricter limit costs span: to satisfy a smaller allowable deflection, the same member has to span less, or get deeper, or be spaced closer.

Why floors, ceilings and rafters use different limits

The four datasets published on this site carry three different deflection limits, and each one matches what the member is expected to do.

Because those limits travel with the table rather than with the lumber, you cannot carry a span from one context to another. A 2x10 sized as a ceiling joist has been checked against L/240 and a 20 psf attic load; the same 2x10 in a living-room floor faces L/360 and 40 psf, and the published span is materially shorter.

• Floor joists - L/360. The strictest of the three, because finished floors carry furniture, cabinetry and people who notice movement, and because the ceiling below is usually a rigid finish that cracks.
• Ceiling joists - L/240 in the limited-attic-storage table published here. The load is lighter and the consequences of movement are mostly cosmetic.
• Rafters - L/180 in the 30 psf ground snow table published here, for a roof where the ceiling is not attached to the rafters. Roof framing is allowed to move more because there is no rigid finish fixed directly to it.

The same joist, four tables

Here is a single Douglas Fir-Larch No. 2 2x10 at 16 in on center, read out of each of the four tables this site holds. The lumber is identical in every row; only the load case and the deflection limit change.

The ceiling joist number is nearly five feet longer than the floor joist number for the same stick. That gap is the combined effect of a lighter live load and a looser deflection limit - the table does not separate the two, and neither should you when quoting a span.

It also explains a common misreading. Somebody sees that a 2x10 rafter is published at more than eighteen feet, concludes that a 2x10 spans eighteen feet, and applies that to a floor. The floor cell for the same lumber is nowhere near it.

Strength or stiffness - which set the number?

A published span cell is the result of more than one check. The table's author works out how far the member can go before it exceeds its allowable bending stress, how far before it exceeds shear, and how far before it exceeds the deflection limit, then publishes the smallest of those answers. The cell does not record which check governed.

That matters when you are tempted to make a substitution. Moving up a grade raises the design stresses but often does far less for stiffness, so on a span controlled by deflection a better grade may buy almost nothing. You can see this directly in the tables here: at 16 in on center in the 40 psf floor table, Spruce-Pine-Fir publishes an identical span for No. 1 and No. 2 in every size. Depth, by contrast, helps both checks, which is why going deeper is usually the effective move.

If you need to know which check is controlling a particular member - because you are substituting material, or because a client is complaining about bounce - that is an engineering question, not a table-reading one.

Stricter limits, and the bounce complaint

Meeting a code deflection limit is a minimum, not a target. Designers frequently specify something stricter than L/360 for long-span floors, tile installations, or clients who are sensitive to movement, and floor systems are sometimes assessed with vibration criteria that a simple deflection ratio does not capture at all. This site publishes only the tables it holds, and none of them is a stricter-than-code table - if you need one, that is a design decision for a professional.

In practice, the useful takeaway is this: a floor that satisfies L/360 is code-compliant and is not going to fail, but "compliant" and "feels solid underfoot" are different bars. Deciding which one you are building to is worth doing before the joists are ordered, not after the floor is down.

To see what a given deflection limit does to real spans, the table pages on this site state the limit in the header of every table, and the span lookup tool repeats it with every result along with the source table and edition it came from.

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