All too often on the jobsite, I’ve heard the phrase, “Well, I was always told the rule of thumb is….”. You can fill in the blank as to what code requirement someone has distilled into an easy to understand and 9 times out of 10, incorrect assumption. So, what is a “rule of thumb”, and why should you be wary of following any simple “rule” as far as codes go? First let’s start with the concept, and then follow with some extremely common and incorrect rules of thumb”.
So what does the phrase “rule of thumb” mean?
Gary Martin, Author and Founder of www.phrases.org.uk provides the following meaning and history of the phrase:
Rule of Thumb: A means of estimation made according to a rough and ready practical rule, not based on science or exact measurement.
The phrase itself has been in circulation since the 1600s. The earliest known use of it in print appears in a sermon given by the English puritan James Durham and printed in Heaven Upon Earth, 1685, “many profest Christians are like to foolish builders, who build by guess, and by rule of thumb, (as we use to speak) and not by Square and Rule.”
The origin of the phrase remains unknown. It is likely that it refers to one of the numerous ways that thumbs have been used to estimate things – judging the alignment or distance of an object by holding the thumb in one’s eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down.
I like Martin’s definition, specifically the phrase, “not based on science or exact measurement”. As the building and fire safety codes are based entirely on exact measurements, testing procedures and other such criteria, making any code related decision based on a hunch, or estimation would be rather unwise. While it’s understandable that anyone in the construction (or design) fields may want to simplify the rationale for doing something to direct their teams, workers, or supervisors quickly, this often results in a very watered-down version of a specific code requirement. Given enough time, the “rule of thumb” passes through so many people that it doesn’t even resemble the original statement. If you have never played the “telephone” game in grade school, I encourage you to Google it.
Rule of Thumb No. 1, “Electrical boxes that are in different stud cavities do not need putty pads”.
This rule deals with the requirements for metallic or non-metallic (plastic) electrical boxes that are installed in a fire-resistive wall assembly. The mere mention of a “stud cavity” exists nowhere in the commonly used building codes or standards that exist in the United States today. The International Building Code (IBC) and NFPA 101, the Life Safety Code (LSC), both require protection of any penetrations in a rated assembly to prevent its failure. Electrical boxes are specifically required by NFPA 70, the National Electrical Code to be installed in accordance with their listing. So here is what that listing says (taken from UL’s Guide QCIT.GuideInfo, Metallic Outlet Boxes, on www.ul.com):
Listed single- and double-gang metallic outlet and switch boxes with metallic or nonmetallic cover plates may be used in bearing and nonbearing wood stud and steel stud walls with ratings not exceeding 2 hr. These walls have gypsum wallboard facings similar to those shown in Design Nos. U301, U411 and U425, as covered under Fire Resistance Ratings – ANSI/UL 263 (BXUV). The boxes are intended to be fastened to the studs with the openings in the wallboard facing cut so that the clearance between the boxes and the wallboard does not exceed 1/8 in. The boxes are intended to be installed so that the surface area of individual boxes does not exceed 16 sq in, and the aggregate surface area of the boxes does not exceed 100 sq in per 100 sq ft of wall surface.
Boxes located on opposite sides of walls or partitions are intended to be separated by a minimum horizontal distance of 24 in. This minimum separation distance between the boxes may be reduced when Wall Opening Protective Materials (QCSN) are installed according to the requirements of their Classification.
The boxes are not intended to be installed on opposite sides of walls or partitions of staggered stud construction unless Wall Opening Protective Materials (QCSN) are installed with the boxes in accordance with Classification requirements for the protective materials.
You’ll notice very quickly that the words “stud cavity” do not appear in UL’s listing for Metallic Outlet Boxes, and to my knowledge do not appear in any other code, standard or listing. The 24” horizontal separation for boxes on opposite sides would definitely correlate to a wall built with studs at 24” centers, but it’s only a coincidence. In this case, the “rule of thumb” is 100% incorrect.
Rule of Thumb No. 2, “No storage is permitted anywhere within 18” of the ceiling in a building protected by sprinklers”.
This rule is simply a misunderstanding of the code requirements surrounding possible obstruction to the flow of water from a sprinkler head when activated. In this particular case, keeping all storage at least 18” away from the ceiling would definitely help in avoiding sprinkler obstructions, but when the same rule of thumb is used in reverse (to cite violations of the code), it would be very wrong.
NFPA 13, the Standard for the Installation of Sprinkler Systems, 2010 edition, states the following for standard upright and suspended (pendant) sprinkler heads:
NFPA 13, §18.104.22.168 Obstructions to Sprinkler Discharge Pattern Development
22.214.171.124.1.1 Continuous or noncontinuous obstructions less than or equal to 18 in. (457 mm) below the sprinkler deflector that prevent the pattern from fully developing shall comply with 126.96.36.199.
The following figures and charts from NFPA 13 show the required distances from any obstruction to the sprinkler head.
You’ll notice that obstructions less than 24” in depth are permitted when up against the wall of a room protected by a sprinkler (as depicted). This means that storage is permitted within 18” of the ceiling, but only in accordance with this section. Is it safer to just keep everything 18” below the ceiling? Yeah, it probably is, but spreading that requirement as a “rule” that uninformed people use as a “requirement”, only leads to confusion and conflict later (especially when used as a justification by inspectors).
What to do when everything you were told is wrong:
So I may have burst your bubble on two rules of thumb that are really not correct; what other rules of thumb are out there that could be wrong? I’d love to know. With some research, patience and willingness to challenge your past experience, we could eliminate those estimations and guesses; replacing them with the (sometimes simple) truth.
If you have some examples to share, or would love me to weigh in on, please write them in a comment. The more we discuss about what is actually correct or required the better off we all are.
Sources and Copyrights:
Excerpts from UL Guide QCIT.GuideInfo are reprinted from the Online Certifications Directory with permission from UL, © 2017 UL LLC
NFPA 13 and NFPA 101 are copyrighted by the National Fire Protection Association, and reproduced here for reference and educational purposes only