According to an article written in Applied Ergonomics titled “Stairways risers and treads: acceptable and preferred dimensions”, it was discovered that the ideal riser was in fact 7.2 inches and a tread depth of 11 inches (or 12 inches). There are many rules out there to guide you in creating your treads and risers and they all seem to conflict with each other just a bit. Back in the late 1600’s Nicolas-Francois Blondel also known as “The Great Blondel” and French architect determined the ideal stair would follow the equation of 2xR + T = step length. It was determined that average step length, according to Arizona State University Extension, was 30 inches for a man and 26.4 inches for a woman. Sounds like we need his and her stairs! In 1675 this equation, created by The Great Blondel, was approved by the Royal Academy of Architecture (I am not a member because it is has been defunct for over 200 years). The Academy was a leader of influencing architectural theory and education back in the day. They determined 2R + T = 2 Paris feet was the appropriate proportion. Here we go again some strange measurement that was not covered in my previous blog “Do You Measure Up”. A “Paris Foot” going back to the Carolingian system of measurement is based on the “royal foot” coming in at approximately (in today’s measurements) 1.066 feet. Therefore according to this formula, the end product should be a touch over 25 ½ inches! So approximately 315 years later this equation still holds up. I wonder how much money and time they spent on the Applied Ergonomic study.
A stairway is a common place for injury for young and old alike. So if these are the ideal ratios, why does the IRC NJ Edition only require a maximum riser height and a minimum tread depth? According to Section R311.7.5 Stairs treads and risers, a riser can be a maximum of 8 ¼ inches and a tread depth shall not be less than 9 inches. These dimensions ironically fall pretty close to the ratio, however, the code does not maintain any requirement as such. So a riser could 8 inches and a tread 12” and bam we are not even close! I believe many states have adopted ratios and lower maximum riser dimensions; however, this is not the case for NJ. What about that 8 1/4 inch number? That is quite odd. Well, I believe the builders once again dictated the code back in the day so they could get away with just 12 risers in a house with 8 foot ceilings. However the houses I work on today rarely have 8 foot ceilings, they have grown to 9 and 10 feet. Maybe it’s time for New Jersey to add a little more safety into their residential code. Sounds like the convenience of “7-Eleven” is not so convenient!