A Notch Above Boring
(Notching and) boring rules for dwelling sprinkler systems
NFPA Journal, May/June 2010
My first opportunity to observe the installation of a sprinkler system in accordance with NFPA 13D, Sprinkler Systems for One- and Two-Family Dwellings and Manufactured Homes, was in the late 1970s. In that particular installation, holes had been bored in successive wood joists through which the sprinkler piping would pass. Because of the location of ductwork in the joist channels, however, the holes were bored near the bottoms of the joists, many in mid-span to accommodate sprinkler locations in the center of rooms. Someone obviously knew that this practice was prohibited by the local residential building code and had prescribed some remedial action. Shield plates had been attached to the undersides of the joists below each hole, since the low holes made the piping vulnerable to nail penetrations when gypsum board was attached to the ceiling.
While nail penetrations of piping may be a legitimate secondary concern when holes are drilled in the lower part of wood joists, the major concern is structural integrity. Building codes contain specific rules to ensure that the strength of structural members is not compromised by boring holes or notching the undersides of beams or joists.
When a structural member is used in a typical horizontal position to support loads while it is supported at both ends, there are two main ways in which the member is called upon to resist the applied forces. The ends of the structural member must resist shear forces caused by supporting the total load and handle the moments applied along its length, which it does by resisting compression along its top surface and resisting tension along its bottom surface. A simple way to visualize the effect of these forces is to bend a flat rubber eraser. If simulating a beam or joist bent downward under the weight of applied floor loads, the eraser’s top surface will tend to crinkle under the compression loads, while the bottom will eventually split open under the tension. The forces are highest at the center of the span. The middle third of the beam depth generally remains neutral, even at the center of the span, and does not contribute to the strength of the member. This is why holes in the mid-span of a joist should be located in the middle third of the joist depth to avoid compromising strength.
NFPA 13D does not contain rules for boring holes and cutting notches in wood joists, although annex diagrams showing proper insulation practices depict both practices. Under residential building codes, however, specific rules apply. Notches in solid joists cannot exceed one-sixth the depth of the member, cannot be longer than one-third of the depth of the member, and are not permitted in the center third of the span. Notches at the ends of members may not exceed one-fourth the depth of the member, while notches in the tension side, or underside, of members 4 inches (100 millimeters) or more in nominal thickness are only permitted at the ends. For boring or cutting holes, the hole diameter may not exceed one-third the depth of the member, and such holes may not be closer than 2 inches (50 millimeters) to the top or bottom of the member, nor to any other hole or notch. Similar rules apply to lightweight metal construction.
Proper attention to structural rules can ensure that residential fire sprinkler systems are not a problem in residential construction. Avoiding problems will be a key to their widespread acceptance.
Russ Fleming, P.E., is the executive vice-president of the National Fire Sprinkler Assocaition and a member of the NFPA Technical Correlating Committee on Automatic Sprinklers.