Hi, We're totally gutting our master bath & installing a whirlpool tub (where there was only a vanity before). The tub specs indicate the floor must be strong enough to support 50 lb./SF. My question is whether we need to add additional support under the floor. We have 2x6 Douglas Fir #2 joists on 6' spans. The tub will be on the outside wall, so the joist will be resting on the concrete stem wall on one end and piers 6' away on the interior. The subfloor is 3/4" plywood plus there is about a 3/8" underlayment on top of that. Another concern is that we plan to put a heavy porcelain tile over the entire floor, tub surround and separate shower surround. Is this too much weight for my little joists or am I o.k.? If not, what recommendations do you have to beef them up? (The entire room will be about 7' x 14'.) Thanks for your help.
You again? I though we had you taken care of! If everything is as you describe, your little joists should be loaded to about 40% of the maximum permitted load, and that was figured using 50 PSF live load PLUS 10 PSF dead load, over the entire length of the joist, not just where the tub will be located, assuming the joists are on 16-inch centers.
Architect (NY) and Home Designer (PA)
Posts: 2494 | Location: Tobyhanna, PA | Registered: 24 October 2005
To echo what Richard said, what you have is fine, assuming 16" on center, your maximum span is 9'-1", even if your joists are 24" on center your maximum span is 7'-6", which you are fine there too. This all assumes 50 psf live and 10 psf dead loads.
By the way Richard, thought you got rid of me huh?
General Contractor/Home Builder
Posts: 288 | Location: Minnesota | Registered: 15 January 2007
Thanks, guys. That's good news, since I wasn't looking forward to working in the crawl space! No, Richard, you didn't get rid of me. My questions last time were about the room addition I built. I'm proud to say it's up (and still standing)! The exterior is pretty much finished, and we passed our framing inspection yesterday. Now I'm moving on to the bathroom project. Thanks again for your help.
Although the capacity of the joists is more than adequate for the loads, in terms of pure dead weight, the real question is whether you have any deflection in the floor as you walk over it. The reason this is important is that tile, marble, granite, porcelain, etc., have no ability to flex under load, but the wood sheathing over the joists, can absorb plenty of flex, and return to "normal" after the loads are removed. With the growth in size of tiles used in the wet areas of the house (kitchen, bathroom, laundry, etc.), now averaging 16" x 16", the floor beneath this absolutely ridgid tile could flex downward under load, breaking the bond between the underlayment and the tile, eventually leading to loose, or worse, cracked tiles.
What the Tile Council of America recommends for floor deflection under large-bodied tiles is a number calculated as L/480, the deflection value (stiffness value) which will allow the placement of such tiles on the floor without fear of cracking or popping loose.
This message has been edited. Last edited by: W. J. Parker,
Posts: 105 | Location: West Haven, Conn. | Registered: 15 November 2005
Show me a bathroom that gets 50 PSF live load. The allowable deflection over a 6-foot span would be 0.15 inches under full design live load at 1/480 of the span. At full live load, the deflection of the referenced joists would be 0.25 inches, approximately, if in fact the full live load were ever actually achieved.
Architect (NY) and Home Designer (PA)
Posts: 2494 | Location: Tobyhanna, PA | Registered: 24 October 2005
I am not an engineer, any more, and haven't been in that field since 1978. I am a remodeling contractor, specializing in complete kitchen and bathroom make-overs. As such, it has been my responsibility to delve into all the minutia that make each of my jobs a success. I also have been able to learn from past failures, either of my own, or those of collegues with whom I network.
While I will never argue that L/360, or even L/250, the commonly accepted design spec. for floors, is "unsafe", I will state, for the record, that whenever I go into a client's home where the subject of tile floors comes up, I always give the floor the "bounce test"--I literally jump up and land on the balls of my feet on their floor, after first advising the client of what I am about to do, and why, with the client standing nearby and watching, in order to determine the potential for tile failure. If the client can feel my 185 lbs. deflect his floor, while standing 4 or 5 feet away, then he or she gets a real sense of the kind of flex the floor is able to absorb from walking across it, without catastrophic failure.
The point I am trying to make is that there is a minimum design spec. for a "safe" floor, one that will support all ordinary loads imposed, without permanent deflection or collapse, and there is also an "optimal" design spec., one that will not result in cracked or loosened tiles, which will inevitably result in a call-back, and/or a warranty claim. The Tile Council of America has published the L/480 specification, not because it is a safety limit for acceptable deflection, but because it is a standard which will all but guarantee the acceptable performance of the installed product, whether it be tile, porcelain, or stone.
On a floor with "bounce", the only place for the flex to take place is at the grout joints, since the tile is too rigid to accommodate any flex. If the joists, sub-floor, and underlayment beneath the tile deflect under load, the result is a tension shearing of the bonding agent beneath the tile, and subsequent loosening of the tile to the underlayment. Further loading, over time, results in the concavity of the underlying floor system, with the rigid tile being supported at only the edges remaining in contact with the floor assembly, leaving a hollow, unsupported section beneath the tile, which, upon further loading, will crack the tile. The fractional equivalent of a deflection of .15 is approximately 5/32 inches, which probably would not rupture the tile-to-substrate bond. The fractional equivalent of a deflection of .25 is 1/4 inch, which deflection would most certainly result in failure of the tile.
If the floor in a contemplated remodel is bouncy, it either needs to be stiffened, by the addition of intermediate cross supports, such as a new carrying beam, which results in a reduced span for the under-performing joists, or by sistering on new joists, with lots of nails or lag screws, and with construction adhesive at the joist-to-joist interface, so that the new sandwiched assembly behaves as one. Barring that, the client needs to be aware that the only other acceptable tile installation is one involving small tiles, such as 2" x 2", or mosaics on a mat, since the flex will be absorbed through the polymer-modified grout joints.
By the way: a six-foot jetted tub, accommodating two bathers, has a water capacity in excess of 90 gallons, @ 8.4 #/gallon, excluding the combined weight of the bathers. At 756 lbs.of water, plus the weight of two 150 lb. light-weight bathers, the weight on that part of the floor is in excess of 1056 lbs, not including the estimated 100 lb. weight of the tub itself. Add to it the vibration pulsing of the water propulsion system. 6 feet x 2.75 feet, the approximate area encompassed by a six-foot jetted tub, is 16.5 square feet. Dividing the approximate accumulated weight of a filled tub, with two bathers, totaling a conservative 1,200 lbs. by 16.5 sq. ft. gives a load of 72.7272...lb./sq.ft. That's the load that needs to be factored into the flooring deflection calculations.
This message has been edited. Last edited by: W. J. Parker,
Posts: 105 | Location: West Haven, Conn. | Registered: 15 November 2005
I am remodeling a recently bought home. One of the upstairs rooms is a game room and I plan on putting a pool table in it. The original joists were 2x6 16" OC - I glued and nailed another joist on to existing joist - so now it is 4x6 14" OC. If I would add another joist in between, that would make the joist system 2x6 8" OC.
I want to know which one would be better. How can I find out since there are no calculators I can find online that will allow me to get the appropriate spans as a function of sistered 4x6 joists or at a spacing of 8" OC?
The rest of information: Southern Pine grade #2; dead and live load of 10 and 40, respectively; deflection of L/360.
Maybe you could help me with an exact answer or tell me where should I look to calculate myself.
If everything is as you describe, your little joists should be loaded to about 40% of the maximum permitted load, and that was figured using 50 PSF live load PLUS 10 PSF dead load, over the entire length of the joist, not just where the tub will be located, assuming the joists are on 16-inch centers.
