# Floor Deflection for Comfort and Sections of Tile

| Posted in General Questions on

I am working with the engineer to finalize my new house plans and am trying to sort out how far above code we need to be to get a tight floor and to properly support tile in bathrooms and the mudroom.  Please note that all the floor joists in this house will be 11 7/8″ TJIs and right now the maximum joist span is around 17′.

I understand that L/360 will meet code but may not feel solid and may not be ideal for tile sections.  I also understand that within a set joist depth that I can reduce deflection by reducing the spacing and upgrading the grade of the TJIs.  I also understand that we can design to L/480 or above.

Given this information, what is the best way to quantify a “stiff” floor?  for example, will L/480 result in a stiff feeling floor?  Is there a guideline for staying a certain % below max spans listed on TJI tables that will give a stiff floor?  Would my tiled sections need to be even stiffer?

Thanks!

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### Replies

1. Expert Member
| | #1

What we humans terms as good floor is actually the natural frequency of the floor. Natural frequency is a combination of span and deflection so deflection alone does not guarantee a good floor.

Generally bellow 20' span, anything near or a bit above code min is a good floor, once you get past 20' is where you have to be careful with natural frequency. This is a good read about the topic:

https://seaoo.org/downloads/Presentations_CONF/seaoo_2012_conference___woeste___design_of_wood_floors_to_mitigate_floor_vibration_problems.pdf

Tiles only care about deflection. Some large format tiles might need L/480 or higher, I would check the installation requirements of the tile of your choice.

1. | | #3

Akos, Thanks! I assume that you comment on sub-20' spans applies to engineered lumber as I am in a house with 2x10 southern yellow fine floor joists spanning 15' or so on 16" centers and they are bouncy.

1. Expert Member
| | #4

When you get up near the extremes in the span tables for any side of lumber, you start to notice more "bounce" in the floor. Sometimes solid blocking will help a little, but usually you either have to sister the joists, add a 2x4 on the lower edge as a lower "flange", or put a header under the center of the span to essentially cut the span in half.

Bill

2. Expert Member
| | #5

Sawn lumber floors tend to have less vibration than an engineered wood floor with the same calculated deflection, because the variation in wood density makes it less likely to find a common harmonic frequency than with the more-uniform density of engineered lumber.

I typically specify that at and above a 16' span, deflection should be L/480, even when L/360 meets code. I also follow the lead of professional engineers I work with and limit maximum deflection to about 0.6", regardless of the span.

3. Expert Member
| | #7

Code tables have a lot of legacy values that don't necessary meet even code min deflection values. I've recently run into this while sizing a beam, the code min beam was way undersized for deflection based on detailed calculation.

The 2x10 at 15' is in this category, at best it is borderline.

1. Expert Member
| | #8

I was going to argue with you, as I often find it faster to size my own beams/joists/rafters/headers but then check code tables, and I usually find that they are right in line. But I just checked your example, using #2 SPF (MOE=1,400,000 psi), 40 psf LL + 10 psf DL and L/360 deflection, and found that the joist should be 9.8" tall, not the standard 9.25". The MOE would have to be 1,700,000 psi for a 2x10 to work mathematically, which might be possible with southern pine or fir but more than I've heard of for SPF.

1. Expert Member
| | #9

Don't forget that SYP underwent a design value revision in 2013. It's no only about as good as SPF or white pine. No. 2 visually graded is still 1.4E6 psi.

I'm surrounded by it here in the south east. The growth rings on newer stuff could be up to a half inch apart in some lumber I've come across. I use to prefer it, but now it's just heavy, dense, and harder to work with IMO than SPF.

2. Expert Member
| | #10

Response to Kyle, #9: NELMA revised the values for SPF as well; smaller dimension members went from 1.4 to 1.1 E6 psi. I believe the idea is that larger-dimension members come from higher-grade trees, which matches what I've seen--sometimes I'll buy a 2x10 or 2x12 and rip the sides off for beautiful, straight-grained, quartersawn spruce. I don't believe the code council adopted the NELMA changes but I could be wrong.

3. Expert Member
| | #11

I'm guilty of buying oversized and ripping out the middle for a good quarter sawn board as well!

2. Expert Member
| | #2

BrunoF,

Depending on how far you want to get into it, John Bridge deals in depth with these questions.
https://www.johnbridge.com/

3. Expert Member
| | #6

I specify L/480 deflection in kitchens and bathrooms, and L/720 when the tile is large-format, with one dimension 24" or more, after reading on the John Bridge forum and checking with manufacturers.

1. | | #12

The Tile Council of North America (TCNA), also specifies minimum L/720 for any stone flooring of wood framing.

2. Expert Member
| | #13

Michael,

I suspect the trend for oversized tiles is going to come back and haunt us.

1. Expert Member
| | #14

Malcolm, I suspect the same, though I also love the look of larger tiles.

Dan, you're right--that had escaped my memory!

2. Expert Member
| | #15

Tiles are only going to get bigger. I'm seeing showers with the full size tiles, it all had to be water jetted to fit.

1. Expert Member
| | #16

Akos,

I like the lack of grout lines and all the attendant problems they can cause, but on floors it's asking a lot of the tiles, which are bound to have stresses in some parts under load. For now I'm favouring a pre-fabricated base, and big tiles on the walls.

3. | | #17

I don't think we will have any tiles greater than 12" in any one direction in this house.

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