Advanced Guidelines for Deck Steps Spacing and Sizing – Continued
This past week, we started looking at an outdoor or exterior set of steps that led to a wood deck. The deck really looks good, but when we say that, we’re referring to the visual aesthetic only. From a code compliance and construction perspective, the deck has several defects that go back to the original construction. It should be noted that this particular deck is not here in Washington DC and the same rules don’t apply. Nonetheless, it’s a perfect point of comparison to explain some of the rules that do apply here to all of us in Washington DC.
When talking about the building code and, particularly here, it’s a good time just to mention so that everybody is aware when reading this article that the building code varies significantly, not just from state to state, but from even small municipalities to another. Even within neighborhoods in Washington DC, as well, the type of construction, based on the type of building and the particular use of the building may very significantly and each different type of application may have a different set of rules.
The way we explain or look at this type of deck construction though, is based on a typical residential or commercial use and although the rules may vary from one application to the next, these are some guidelines that we often recommend.
Here, at these stair stringers, as shown in the picture below, the spacing is set way more than 36 inches apart. The spacing of the stringers is important because the stair treads depend on the stringers for structural support. When the stringers are set too far apart, they will allow the treads to sag when heavy people walk on them. People weigh different weights, that’s normal and typical. Small kids might only weigh 50 pounds and the overall distance or spacing between the stair stringers, in this case, might easily support a little human of only 50 lb. Large men or women, though, might weigh much more.
A typical professional offensive lineman generally weighs over 300 lbs. When somebody of that size and weight steps in the middle of a stair tread seated on top of a overly spread out set of stair stringers, they might come crashing through the steps. That’s dangerous, and the way that the building code looks at it, you can’t just simply say you will never allow large people to walk on your stairs. In commercial locations such as restaurants and retail spaces, you can’t prohibit people to use the stairs because they’re too large.
It’s just not legal to discriminate on that basis. So therefore, your space must accommodate people that weigh more than small children. Also, even in private spaces, you have a right to maybe not allow large people to use your private property, but there are times when emergencies happen and if a building is built unsafely, it will not be able to accommodate someone like an emergency responder who happens to be larger than a small child. Therefore, the building code has requirements that apply uniformly to large classes of types of construction.
Also, the rise to run ratio is all wrong. Here, in this example, shown in the photo below, the ratio is off and the risers are too tall and the treads are too short. A few different rules of thumb are typically used for determining good practice for the rise to run ratio for stair treads and risers. Basically though, and in the simplest terms, the treads need to be deeper or wider than the rise or height of the stairs. Here, just at a glance, you can see that the treads are not deep enough and the rise is too tall.
The rise and run ratio, of a deck stair’s treads and risers, might sound like it’s all simply about ergonomics and comfort which does matter in good construction, but it’s also about safety. For example, the International Residential Code (IRC) sections R311.7.5 and R311.7.5.1 specify dimensional requirements for stairs to prevent accidents, like tripping and stumbling, which helps to ensure safe vertical movement.
This has a lot to do with simple ergonomics of walking, but the building code puts parameters in place that make it specific and not just abstract. Typical dimensional lumber used in deck stairs usually includes 2×12 pressure-treated lumber for stringers, 2×10 or 2×12 for treads, and 1×8 boards for risers. These standard lumber sizes are often chosen to accommodate the IRC’s specific dimensional requirements.
The codes often require that stair risers cannot exceed 7.75 inches in height, and the treads must be at least 10 inches deep. It’s important to note that for the riser the dimension given is a maximum and for the tread, it’s the opposite, a minimum value
These requirements touch on some of the fundamental differences in human locomotion between walking on flat ground and climbing stairs. When walking forward on level ground, a typcal human stride is about 2-2.5 feet. In contrast, climbing stairs normally uses a more deliberate, shorter step with additional leg lift and decreased forward momentum.
We will discuss some additional details about handrail design and upcoming and future art articles as well. The IRC requires handrails to be positioned 34-38 inches above the stair nosing, which corresponds to the grip height of an typical adult. This height accommodates most people while providing stable support. For children, who are shorter, this standard placement might be less comfortable, which is an example of the complexity of creating universally accessible building design guidelines.
Structural considerations also come into play in this stair layout and configuration Stringers must be spaced no more than 36 inches apart, as we mentioned above. It’s related here as well. This maximum spacing helps to provide sufficient structural support while maintaining the stair’s overall rigidity and bearing capacity.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities. You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com/contact-us/ and you can email us there as well at https://dupontdeckspatiosdc.com
