Suspended Ceilings

Based on 1998 California Building Code Standard 25-2, which applies to systems with an installed weight (including fixtures) of less than 4 pounds per square foot.

Note: The terms 'drop', 'T-bar', and 'suspended' are used interchangeably with respect to ceilings.

The Grid and Vertical Support

The main runners shall be supported at 4 feet on center with 12 gauge hanger wires hung vertically from the supporting structure with fasteners sufficient to support a 100-pound load. These hanger wires may not exceed 1 in 6 out of plumb. Three wraps are required at each wire connection. Hanger wires or other approved supports are required at the ends of each main runner and cross runner along the perimeter of the room. Hanger wires shall not be deflected by any duct, pipe, etc. Splices and intersections of runners shall have a positive connection with a minimum load capacity of 60 pounds. Perimeter wall angles are considered aesthetic closures with no structural value unless they are part of an approved structural system.

Horizontal Bracing and the Compression Strut

Horizontal bracing shall be installed not to exceed 12 feet on center in both directions and at not more than 6 feet from perimeter walls. It shall consist of 4-12 gauge wires secured to the main runner within 2 inches of a cross runner, and splayed at 90 degrees from each other at an angle not exceeding 45 degrees from the plane of the ceiling. At the point of connection of the horizontal bracing wires on the main runner, compression struts shall be attached to the main runner, extended vertically and be attached to the supporting structure above.

Lighting Fixtures

All fixtures shall be positively attached to the grid. Hanger wires shall be attached to the main runners within 3 inches of each corner of the fixture. Lighting fixtures weighing less than 56 pounds shall also have two hanger wires attached at the diagonal corners of the fixture. Air handling diffusers and pendant lighting must have dedicated hangers.

Above Ceiling Wiring

The wiring above a drop ceiling cannot be secured to the ceiling hanger wires. For exceptions refer to the 1996 edition of the National Electrical Code, Section 300-11(a), (1) and (2).


If you need additional information, please call the Building Division at (650) 991-8061.


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