Roof and Ceiling Terminology Question

[rmallaire]

I am reviewing allowable spans in Roof-Ceiling Construction from https://www2.iccsafe.org/states/Virginia/Residential/PDFs/Chapter%208_Roof-Ceiling%20Construction.pdf

I was wondering if anyone knows what L/Δ means

As an example-
"Ground snow load = 30PSF, ceiling not attached to rafters, L/Δ=180"

Thank you,

Randy

[MountainDon]

Good question Randy.

Deflection.   Δ is the deflection factor.  "delta"

To use  L/Δ=180 as the example....
L = length between supports for a rafter, joist or beam, in inches. Also called the span.

Ceiling joists 16 feet overall length.
Multiply by 12 to convert to inches = 192

That becomes 192 / 180 = 1.0667 inches

1.0667 inches is the maximum amount the ceiling should deflect (bend, sag) at the midpoint when loaded with the maximum design load.

Different "Delta" are used for different parts of a structure.
A higher delta means less deflection is allowed.

Examples of code-prescribed deflection limits and live load values are:

    * Living room floors L/360 & 40 psf (for tile and stone deflection should likely be less)
    * Bedrooms and habitable attic floors L/360 & 30 psf
    * Attic floors with limited storage L/240 & 10 psf.
    * Rafters  L/240

[Squirl]

Deflection.  The amount the wood is allowed bend under load.  Higher the number the less it is allowed to bend.

180 is standard for roofing without a ceiling material.

240 is standard for ceiling joists.

360 is standard for flooring.

Most tile and rigid flooring materials require 480, so they don't crack and the mortar break.

This is noticeable as "bounce" or springiness.