Beware the ‘rule of thumb’

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.

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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 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

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, § Obstructions to Sprinkler Discharge Pattern Development 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

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

Codes must be followed to be effective; Immediate lessons from the Grenfell Tower tragedy.

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A huge amount of information has flooded the internet and media outlets regarding the possible causes and now future ramifications of this tragedy. As the exterior cladding of the building is being initially blamed for the fire’s rapid propagation, the presence of such a material on dozens of other government owned housing blocks has led to large-scale evacuations of residents, putting thousands of people out of their homes.

This fire will no doubt lead to civic and criminal investigations in the UK, but what can those of us in the United States learn from such a disaster? The worst possible outcome for those of us watching from across the Atlantic would be complacency, “Oh, that wouldn’t happen here.” Really? Are you sure?

What went wrong?

Initial investigations by local authorities and news organizations has focused on a “flammable” exterior cladding installed during a recent renovation project to the Grenfell tower and others like it. The product installed (allegedly confirmed by the manufacturer) was a Metal Composite Material (MCM) as defined by the International Building Code (IBC), a model code that has been adopted in some form across all fifty states in the US. The MCM installed on Grenfell Tower is a product called Reynobond PE, manufactured by Alcoa Architectural Products, located in Eastman, Georgia. Reynobond PE consists of layers of Aluminum sheets over a polyethylene core (foamed plastic). The panel as a whole meets IBC flame spread requirements (Class A per ASTM E84), however the polyethylene core on its own does not. The product is manufactured in accordance with US standards and is permitted for installation on buildings as high as 75 feet tall per the IBC, with VERY specific exceptions. The primary exception is that such a product cannot be installed on a building higher than 40 feet above the ground unless that building is equipped with an automatic sprinkler system, and then never installed above 75 feet above the ground. Alcoa also produces a product called Reynobond FR, which has a mineral board core that meets the ASTM E84 flame spread requirements on its own.

Another likely cause of the fast growing fire was the way in which exterior columns were clad in MCM panels. Quickly looking at the plans of the Grenfell Tower, listening to reports and interviews of tenants, and reviewing images of the devastation itself, one can quickly see several problems that possibly led to the fire’s growth and more importantly made it extremely difficult to for residents to escape the building before conditions became untenable.

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Grenfell Plan

  • There appears to be a relatively large gap between the cladding material and the structural columns of the building itself along the exterior. This kind of cladding style is called a “curtain-wall”, as it is attached to the face of the building, and does not terminate at each floor, but creates its own cavity on the building’s perimeter. looking at the pictures following the fire, the space between the structural column and the cladding is quite large. Per the IBC, this kind of curtain wall assembly must be
    Grenfell Colored Elevation.jpg
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    firestopped at each floor level using a Perimeter-Fire-Containment System such as those tested and listed in the UL Certifications Directory (systems such as CW-D-1001).

  • Grenfell Tower had only a single exit stairway. For a 24 story residential high-rise, which would have an occupant load of no less than 22 people per floor (based on the floor plan of Grenfell Tower and the 2003 IBC), a minimum of 2 stairways would be required without exception.
  • The building did not appear to have an automatic sprinkler system or manual fire alarm system interconnected with automatic detection devices.

All of these factors likely contributed to the fire’s rapid growth and the inability for residents to evacuate fast enough.

Codes have to be followed to be effective:

You may say then, “Well, those issues can’t happen here because our codes don’t allow it”. This is where the truth really matters. It can happen here. Having the rules to follow doesn’t mean that everyone follows them. Having laws that limit the speed on every highway in America does not keep thousands of people from breaking them every day. I could not begin to list all of the code violations I have witnessed over my career that were either simple mistakes, intentional omissions, or a lack of understanding about what the code really requires. The third reason is actually the scariest one. Honest mistakes happen, and I’m sure intentional code violations exist as well, but in my experience, the most common reason codes are not followed, is because designers, owners and contractors don’t understand them. Ignorance is not bliss, its dangerous.

Although I have not seen a building constructed with too few stairways, I have seen plenty of stairs that were not protected from the rest of the building, had blocked exit doors at the bottom, were too narrow, had locked doors going into them, or some other issue that essentially eliminated them as a possible exit. Having the stair doesn’t mean anything if you can’t use it.

Having a fire alarm and fire sprinkler system is absolutely worthless if the systems are not installed correctly and then routinely inspected and maintained to ensure they work. Having a sprinkler system means nothing when it fails to work because someone unintentionally blocked a sprinkler head or closed a valve.

The exterior columns at Grenfell Tower, if it does turn out to be the issue it appears to be, is due to a lack of understanding on how a fire acts, and why the building codes are written to limit the spread of a fire. This exact issue could happen in the US if a contractor substitutes a less-expensive product (like the Reynobond PE instead of using the FR version), having no other intention than saving the owner money, but the design team is either not part of that decision, or fails to understand its ramifications. Whether the PE or FR product was used, a building official in the US could easily miss the requirements to firestop the perimeter of each floor at such a system. Such omissions could result in a similar fire without anyone even knowing the issue exists.

What can you do?

Educate yourself. Surround yourself with educated people if you can’t understand the requirements yourself. The costs are too great to downplay or ignore anything having to do with fire safety and building codes in any type of construction.

Ignorance is not funny and not acceptable. Education and knowledge are our greatest asset in preventing tragedies such as the Grenfell Tower fire from happening again.


Notice: The commentary above regarding possible causes and circumstances surrounding the fire at Grenfell Tower are personal speculations and assumptions. Educate yourself on the facts. Listen to the evidence presented to you and research the actual laws and codes that were applicable. That’s my entire point.