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Fire Separation Distance

Building Code Issues

By Steven R Winkel, FAIA, CASp, The PREVIEW Group, Inc., Architects providing regulatory solutions &  Kerwin Lee, AIA, CASp, I CC – Certified Accessibility Inspector and Plan Examiner, ICC – Certified Building Plan Examiner

Steve says:

There is often confusion on the part of architects/designers and sometimes even plan reviewers about the impact that “fire separation distance” has on wall and opening protection requirements and other design considerations for buildings. The key to understanding the use of this term is in its definition. All definitions are now located in Chapter 2 of the 2013 CBC (California Building Code). The definition of this term reads:

FIRE SEPARATION DISTANCE. The distance measured from the building face to one of the following:

1. The closest interior lot line;
2. To the centerline of a street, an alley or public way; or
3. To an imaginary line between two buildings on the property.

The distance shall be measured at right angles from the face of the wall.

The key to understanding this term is the last sentence. This distance is to be measured perpendicular to the face of the building. This can sometimes be a factor in wall and opening protection requirements when the adjacent property line is angled in relation to the wall. The distance should always be taken 90 degrees
from the face of the building wall, which could mean using arc distances for a curving wall. The distance is never to be measured at 90 degrees from the property line (or imaginary line when assessing requirements between buildings on the same property). Note also that you should take advantage of Item 2 in the definition and use the widths of public streets when they abut a property, since the fire separation distance is to be measured to the centerline of the street, not to the back of the sidewalk where the property line occurs. This makes sense when you see buildings in real life with open glazing on the street, or even windows in exit stairs that face a public way. They are making use of the allowance for fire separation distance
to be measured beyond the property line.

The places where fire separation distances come into play include the following critical design considerations:

■ Exterior wall fire-resistance ratings per CBC Table 602
■ Overhang projection distances per CBC Table 705.2
■ Percentages of wall openings in exterior walls per CBC Table 705.8
■ Fire-resistance rating for openings above the roof line of adjacent buildings per CBC Section 705.8.6
■ Parapet requirements per CBC Section 705.11
■ Exterior Wildfire Exposure, Chapter 7A
■ Location of exterior exit stairs per CBC Section 1026.5
■ Use of combustible materials at exterior walls per CBC Section 1406

Kerwin says:

The basis for fire separation distance is to protect your neighbor from you and visa versa. Item three of the definition is very important when dealing with buildings on a common piece of property, like a school campus or any campus of buildings. The imaginary line, which is treated as a property line, can be placed anywhere between the buildings. The placement of an imaginary line can be used to take advantage of
existing exterior construction. In the drawing below, this imaginary line (assumed property line – old term) was located/placed between two buildings to take advantage of where openings can be located and to avoid opening protection.


The game plan changes when you get into Wildland Interface, under Chapter 7A of the CBC. In California with our suburban sprawl and our love of nature and the outdoors, exterior wildland exposure is a special issue, especially in drought year. Distance is still a consideration, but under this section of the code it is called defensible space. Although there is very little one can do in defense of a large forest fire, additional measures are required to protect the exterior of our buildings from a wildland urban interface fire. This is supposed to address places like the Oakland Hills from small vegetation fires. This can also be the case in a forest area when the ground level vegetation and fuel (dead wood) is maintained to a minimum/manageable level so that a ground fire cannot jump into the trees.

Chapter 7A addresses a variety of exterior construction requirements for those building within the Fire Marshals designated wildland areas. This includes ignition resistant construction, additional protection of exterior walls and openings, including roof and eaves. Any added projections, such as decks and separate out structures are also addressed.

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