By Jason Shumaker, P.E.
When you bought your last vehicle did you want a soft, smooth ride or did you prefer something that was stiffer and more responsive?
What about a couch or recliner—do you want firm cushions or do you prefer to be enveloped when you sit down?
Are you a firm mattress person or soft mattress person? A firm mattress with a pillowtop? A mattress where you can dial in your preferred level of support because your spouse doesn’t want the same thing as you?!?!
Everyone Has Performance Expectations
Home buyers assume the floor of their new home will meet code requirements and be structurally sound, but they also have an expectation of performance even if they never thought or talked about it before. These expectations are variable and subjective, often dictated by past experience and socioeconomic considerations. A homeowner moving from a slab-on-grade structure is likely to have a higher floor performance expectation than someone who had lived in the third floor of an apartment complex. Similarly, a buyer moving to an upscale community may have a different perception and expectation than a first-time homeowner whose college dorm was acceptable just a handful of years earlier.
Builders and specifiers have historically relied on span ratios—L/360 code minimum or even more stringent ratios like L/480—to determine what size joist they need. While this approach selects a joist that works structurally and satisfies code prescribed deflection requirements, these criteria alone do not always indicate how a floor will feel. TJI® joists have become more popular in recent decades as home sizes and floor spans have increased. More recent shifts in home designs have continued the trend of longer spans and have added open concept layouts that also impact floor performance. The TJ-Pro® Rating System can be used to select a desired level of floor performance by accounting for this evolution in home styles as well as the floor components that contribute to how a floor feels.
After the Home is Built is the Wrong Time to Address Floor Performance
It is critical to identify homeowner expectations prior to construction because improving floor performance after the home is built is often an expensive and impractical idea. While our Technical Bulletin discussing floor performance mentions options that can be used to help improve how a floor feels after it is installed, a simpler and more cost effective approach is initially designing the floor for the desired performance. Factors like joist stiffness and depth, joist spacing and span, bearing conditions (wall or beam), sheathing thickness, whether a gypsum ceiling is attached to the joists, location of perpendicular partition walls and weight of the floor finish material all have an impact on how a floor feels.
The TJ-Pro® Rating System is included in Forte® and Javelin® design software and can be used to target a desired floor performance while accounting for the factors mentioned above. Ideally builders, specifiers and homeowners could walk floors that are already built to gain an understanding of relative floor performance and determine what level of performance is expectedàthe goal being able to state ‘I want a TJ-Pro Rating® of at least X’. In some cases this won’t be possible; fortunately the analysis behind the TJ-Pro® Rating System (based on data obtained from over 800 TJI® floor systems) can be used as a guide. As shown in the table, selecting a floor with a higher TJ-Pro® Rating increases the probability of satisfied customers.
One of the most common questions regarding TJ-Pro® Rating is some variation of ‘What floor rating is ‘normal’ for my house?’. The answer varies region by region, builder by builder, year by year and even room by room. On average though, entry level homes are typically toward the lower third of the scale while luxury custom homes will often target the higher end of the scale, especially in high traffic areas of the home.
Common Span Increases can Dramatically Affect Floor Performance
Consider a scenario where a series of joists are 16 feet long and adjacent to an area where the foundation shifts outward 4 feet, creating a span of 20 feet. This situation is not uncommon along the rear of a house, where a great room bumps out 4 feet relative to the kitchen or the kitchen bumps out 4 feet to accommodate an eating nook. In either scenario, while code deflection criteria would simply indicate that the longer joists could deflect a little more, the floor performance ramifications are much greater. In fact, assuming the same bearing conditions, sheathing thickness and joist spacing, the 25% increase in span would require the longer joists to be doubled to achieve a comparable floor performance to the 16’ span!
Put another way, that 25% increase in span required a 100% increase in joist stiffness to achieve a similar “feeling” floor.
Fortunately the TJ-Pro® Rating System can compare several alternatives to achieve a desired floor performance and capture the feel of the floor; not just deflection. As a result, more economical solutions can often be designed rather than doubling the joists in a floor system.
Certain Areas Need More Consideration
How a floor feels is subjective. As a result, the TJ-Pro® Rating System is the best option available to anticipate floor performance and select a floor system. However, even highly rated floors can be impacted by factors not directly associated with the floor structure and affect a homeowner’s perception of how a floor feels.
Kitchen islands have become a standard feature in nearly any size home and are commonly as narrow as 2 feet wide and 6 to 8 feet long. Frequently these narrow islands run parallel to the floor framing and often straddle a single joist. As a result, a joist near the island that is stepped on may have minimal deflection but can still result in visible movement at the island countertop (rattling dishes and ripples in drinks). This visual observation can create the perception of a low performing floor system.
In addition to visible cues that can influence perception of floor performance, floor finish can also play a role. For example, great room floors with carpet and pad offer both sound absorption and cushioning for each foot fall throughout the room. A hardwood floor over this same floor allows the physical impact and audible noise of each step to be noticed resulting in a perception of reduced performance.
Area rugs and window treatments that help absorb sound and even furniture selection can make a hardwood floor seem similar to a carpeted floor. Blocking located below the kitchen island (and extending a couple joist bays beyond the island) can reduce island movement. Discussing focus areas like these ahead of time with your local Weyerhaeuser Territory Manager and homeowner can help identify options and reduce the risk of customer dissatisfaction.
TJ-Pro® Rating: more to come
As Weyerhaeuser continues to raise awareness of floor performance and the benefits of using the TJ-Pro® Rating System, I invite you to share your floor performance thoughts and experiences. Whether its successes using the system, thoughts on how we can help explain or demonstrate why floor performance matters to a homeowner, experiences with areas similar to the kitchen and great room mentioned earlier or any other floor performance experience I’d welcome hearing from you. Our goal is to assure builders and specifiers understand the TJ-Pro® Rating System so they can differentiate themselves to prospective buyers and reduce the likelihood of a dissatisfied homeowner. Please feel free to e-mail me your experiences, thoughts or questions at email@example.com.
JASON SHUMAKER, P.E.
Jason Shumaker, P.E. is a Product Support Engineer at Weyerhaeuser. For over 15 years, he has been providing engineering support for Trus Joist and Weyerhaeuser products to homeowners, builders, dealers, architects, engineers, and code officials.