Floor System Performance Experience Request
Hello everyone,
My wife and I are currently building our forever home, and we have found a great builder that has really worked hard with us to meet our expectations. My question is about the performance of the floors we have designed.
First, and foremost, the floors pass minimum code. By a lot. What the builder and structural engineer have been working on is finding a cost effective way to construct a floor that meets my requirements. My wife and I are coming from a home that has a concrete subfloor and we just love the sturdiness of the floor. There’s no bounce, creakiness, or any other movement when walking, jumping, etc. We’d love to get close to that performance with our new home.
Overall, the TJ-Pro rating is 50 which is above average I believe.
The largest clear span is 28′. The floor will be framed 16″ O.C. and will be using 1 1/8″ Advantech subfloor glued and screwed. The I-joists are 560 series 20″ depth, and some blocking will be utilized.
The builder and structural engineer are confident that the floor will meet our expectations including my request that they take frequency of the floor system into account, but I am concerned since I have no practical experience on floors designed, and have only book knowledge.
It would be helpful if anyone could share their experiences on the performance of their floors, the TJ-Pro rating system, and if I should talk with my builder about increasing joist depth to 22″ or 24″, or any other options to have a joist floor system mimic a concrete subfloor.
Replies
The code standard for deflection is L/360, meaning that in the center of the span, the floor can sag 1/360 of the joist length under the design load. That's what you'll find in most tract houses, and any carpenter will be aware of the flex as they walk over the floor. A typical upgrade takes the deflection standard to L/480. It sounds like your design is L/560. You'll never get the feel of concrete out of wood, but you're far above code.
If you like a concrete subfloor, why not put one in?
A gypcrete underlayment on top of the advantech will add mass to the floor and make it feel more like concrete. I think the designed stiffness of you floor system is already at the point of diminishing returns.
Thank you all for the responses so far.
I think what I was looking for was feedback from builders or homeowners that have walked on 28'+ spans that have had bounce, some bounce, and no discernable bounce, and if they knew what depth the joists were. Or put another way, what are some of the characteristics (joist depth, span, and joist spacing) of floors individuals have walked on that have no bounce?
I believe the final standard deflection is L/740 for the floor system. For a 28' span the total deflection is .45" which is okay. My initial request was to get a maximum total deflection of less than or equal to 7/32". I got that number from guidance published by BC with deflection recommendations from the Marble Institute of America.
https://www.bc.com/wp-content/uploads/2019/06/GE-6-Tile-Floors.pdf#:~:text=The%20Tile%20Council%20of%20America,than%2014'%2D0%E2%80%9D.
The main reason I asked my builder to use that deflection requirement was to future proof the house for adding a radiant floor heating system which requires a 1.5" - 2" concrete topper. However, after pricing out the cost of the radiant heating system, we decided it was too costly for our budget.
Based on conversations with our structural engineer, adding a concrete topper would cause a lot of issues because the span is so long. It would dampen the frequency, but the additional dead load over that span would require a much deeper joist in addition to a thicker OSB web among other design issues.
Greg E, my builder and structural engineer stated something similar. Increasing the floor performance becomes more cost prohibitive after a certain point. I could pay for it, but the cost would likely not justify the additional performance.
My builder is extremely flexible. Once framing is up, and the subfloor is constructed, my builder and I will walk/jump on the floor and see how it performs. There are some additional options to stiffen the floor up if the feedback is too bouncy for my wife and I, but it'll cost more after the fact to beef up the floor.
I would definitely spec 5/8 drywall on the ceiling below. This will help with both rigidity and reduce the bounce in the floor.
Sound transmission........maybe.
How so on rigidity?
Sorry, I misspoke. Adding mass to the floor system will increase it's perceived rigidity by reducing the bounce of the floor. 5/8 drywall is a great way to achieve this. It's anecdotal, but I've had several clients say their floors felt stiffer after finishing the basement.
how much more expensive to go 12 on center rather than 16? added blocking between the joists may also be a cost effective way to increase the stiffness
you may want to research other radiant floor system that don't use concrete - tile over cement underlay for thermal mass might work and not be so heavy
good luck
I would like to go to 12" O.C., but that would cause issue with ductwork that is planned. My builder and I are still working on cost effective solutions. It may make sense to go 12" O.C. for most of the floor and 16" O.C. between joists that will have ductwork.