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Posted by Thomas Harrington on Wednesday, February 6, 2013

Windows, Sill Height, Safety, and Safety Glazing

Struck by that eerie sensation one gets standing at a floor-to-ceiling window in a tall building, a friend recently asked me about the relationship between the height of a window sill above the floor, the necessity of safety glazing, and fall protection. The answer requires a discussion of the “building code’s” way of balancing our desire for dramatic window lines with its concerns about the safety of people, especially infants.

Building Code's Historical Development

Here’s a look at the historical development of the code’s requirements regarding widow design. As recently as the 1950's, glazing systems - even in large patio doors - were made of plate glass. As acknowledged by author Stephen A. Van Note, CBO and Senior Technical Staff of the International Code Council (“ICC”), in the recent ICC publication "Building Code Basics: Residential," there is a need "To prevent serious injury from shards of breaking glass..." In the 1960's, tempered glass replaced plate glass in a variety of locations where people were likely to injure themselves by accidentally striking it. Tempered glass is much stronger than plate glass, and if someone does crash into it, tempered glass breaks into small relatively dull pebble sized pieces instead of sword-like shards of destruction. .

Through 2007, while the International Conference of Building Officials' Uniform Building Code (“UBC”) was used as the model code for the California Building Code(“CBC”), it was determined that if each of the following four conditions existed in an "...individual fixed or operable panel..." the "light" or window pane had to be made of safety glazing material.

First, if the "Exposed area of an individual pane [is] greater than 9 square feet..." This is because the size of the individual piece of glass was proportionate to its strength, and because, as explained by Stephen A. Van Note, "Large panels of glass lack the visual cues or physical barriers to prevent people from accidentally walking into them."

Second, if the exposed bottom edge of the pane is closer than 18 inches above the floor. This addresses a number of issues. The presence of the barrier at 18 inches or higher provides a visual cue of the window’s location. It reduces the area of glass that can be broken by an individual's lower extremities. It also provides for some infant fall protection, which will be discussed later in this article.

Third, if the top exposed edge of the pane is higher than 36 inches above the floor of the occupied story

Fourth, there must be a walking surface(s) within 36 inches of the window.

As explained in previous articles, in 2008 California stopped using the UBC as its model code and switched to the ICC's International Residential Code (“IRC”) and International Building Code (“IBC”) for its model codes. These model codes kept the same four conditions requiring safety glazing, with one notable exception as applies here. If a bar of a specified size and strength is installed at a height of 34 to 38 inches above the floor, and it is not in contact with the glass, then the glass assembly does not need to be safety glazing. This bar operates as a guardrail providing both a visible cue and a barrier to accidental breakage. It is important to stress here that the bar is only required if the design of the window does not include safety glazing.

Protecting Infants

I previously referenced the 18-inch sill as providing some degree of fall protection for infants. The CBC now goes even farther in addressing this. I call it the 4/24/72 Rule, and simply put it requires that windows with sills 72 inches or more above the exterior grade and that allow a 4-inch sphere to pass through the window opening, must have a sill height of 24 inches or more above the floor. The use of the standard of a “4-inch sphere” indicates a specific concern for the safety of infants. It is the same standard applied to stair rails and guardrails to prevent infant injuries. It does allow for exceptions, but only when approved devices are used to limit the opening or provide an artificial sill height. All of this is done because, as explained by Stephen A. Van Note, "The 24-inch sill height is typically above a small child's center of gravity, reducing the likelihood of the child's toppling over the sill."

Protecting Adults

So what about fall protection for adults? The short answer is that there are none incorporated in the code, and for that matter even the fall protection for infants is actually limited. In multiple-story buildings, large operable windows are perfectly acceptable in occupied stories high above the ground as long as the sill height is at least 24 inches (or approved devices are used) and the window, where required, uses safety glazing. (Safety glazing is the use of a high strength glass in an assembly that has passed the required standards for the location(s) where it will be used.) While safety glazing provides more resistance to an adult falling through the glass, it provides no protection when an operable window is open. And while the 24 inch sill may be above an infant's center of gravity, it is nowhere near the 34-to 42-inch height required elsewhere in the code to account for an adult's center of gravity. Worse yet for infants, people often put furniture in front of windows, allowing access by an infant to get above the 24-inch sill.

In short, regardless of their construction, windows - like a well-designed staircase - offer utility and drama but will always present a certain degree of risk.

As stated by Mr. Van Note "In protecting the health, safety, and welfare of occupants, the International Residential Code (IRC) sets minimum requirements for a safe means of exiting the building, protection from falls...and from hazards associated with breaking glass..." Indeed, there can be no substitute for personal vigilance and care.


Van Note, Stephen A., Building Code Basics: Residential. Based on the 2009 International Residential Code. International Code Council, 123-141.

1994 Uniform Building Code. Volume 1. Administrative, Fire- and Life-Safety, and Field Inspection Provisions. International Conference of Building Officials, Chapter 24

1997 Uniform Building Code. Volume 1. Administrative, Fire- and Life-Safety, and Field Inspection Provisions. International Conference of Building Officials, Chapter 24

2010 California Residential Code, California Code of Regulations, Title 24, Part 2.5. California Building Standards Commission, Chapters 3 and 6

2010 California Building Code, California Code of Regulations, Title 24, part 2, Volumes 1 and 2. California Building Standards Commission, Chapters 14 and 24

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