My dining room floors are too bouncy for my liking, so I went to the cellar and did some measuring. The joists are 1 3/4 by 9, spaced 12 inches on center. The span is 16 feet. Does this seem sufficient? Is it within the building code (I live in Massachusetts)?

I assume your floor joists are really 2x10. You need to know what grade they are and what the floor above is covered with. For example, Southern Yellow Pine in a number 2 can span 18-0 with 40 psf live load and 10 psf dead load, the minimum for a non sleeping room, residential. If you have a poured 1.5in concrete floor in the living room, then the span goes down to 16-11. The span really doesn't say it won't bounce as that is a function of stiffness. The code I think says it has to be built to L/360 which means you take the span (L) and divide it by 360 and you get what deflection is permitted. Which is 16x12in/360=.533inch. You can stiffen the floor to L/480 improved performance or L/600 High performance by putting a beam below and perpendicular to the joists. You will have to determine what the beam must span so you may want to go to your lumber yard or a truss plant to get a computer to figure it. Another way is to double every other joist which may or may not be possible. I don't know what that stiffness will become, but the bounce will be reduced.

Thanks for the info. I re-measured and the joists are 1 3/4 x 9. The wood is SPF #2. Does this seem to be compliant with code?

SPF stands for spruce/pine or fir and a #2 can span 17-3 so it certainly meets L360. You could try nailing a 2x4 at the center, perpendicular to the joists to stiffen them. That and or a drywall ceiling helps to dampen vibration. If the room doesn't have much weight in it ie furniture etc. that may cause the floor to drum. Try the 2x4 if you don't already have a drywall ceiling on the bottoms of those floor joists.

Thank you. So you're saying that this looks to be code compliant, even with the joist sizes being 1 3/4 by 9 (not 2 x 10)?

1 3/4 x 9 is approximately the finished size of a nominal 2x10. It actually should be 1 1/2 x 9 1/4.

Itz must be recognized that the building code is a minimum standard. It does not exist to solve all problems, but only to try to avoid disaster. Floor joists which are structurally adequate in the sense that they will not collapse are the minimum standard. Your joists meet that standard.

If superior performance is desired, then the designer should decide om either a different lumber species, a different grade of lumber, a different joist spacing, or deeper joists, or use engineered composite joists. Most houses are built from builder's plans, and not designed by an architect specifically for an owner. So the designer makes design decisions to benefit the builder, to provide the minimum possible solution that neets the code.

Joists which are near the limit of their span capabilities, such as yours, will often exhibit characteristics such as you describe, but still be code-compliant. If the bounce is truly troublesome, the best solution is to install a supplementary beam below the joists. Often, the use of the basement below prohibits this. The 2x4 nailed to the bottom of the joists will not cost much, but please don't expect miracles. There is no structural reason why it will do anything at all to cure the bounce.

For joist spans such as yours, there should be 2 rows of bridging. Bridging is either cross-bridging, which may be wood X pieces, usually 1x2, or metal X's, or solid pieces of 2x10 conecting each joist to the ones next to it. See if there are two rows of bridging, and what type of bridging you have.

If there is but one row and it's metal cross-bridging, consider adding two more rows of solid bridging equally spaced, and installing solid bridging right next to the cross-bridging. This may or may not be easily done because of pipes or wiring that may be prsent.

Otherwise, more weight on the floor could reduce the bounce. Weight has a dampening effect on vibration, and the bounce you feel is nothing more than a low-frequency vibration. Heavy carpet above, and a drywall ceiling below could help. More furniture, especially toward the middle of the floor can help.

Thanks...I really apprecaite the info. Got another question that may or may not be related. My home is a 3-story center entrance colonial. The laundry room is on the main (1st floor). A few months ago we purchased a new washer that has an extremely fast spin cycle. Even though I know it is level to the floor, when the spin cycle commences, I can feel the vibration in the room on the 2nd floor (above the laundry area). My question is: can this vibration cause any structural damage? Can it loosen any nails in the framing? Could it be a factor in the bounciness of the floors?

It's more the other way around...the fact that you feel the vibrations from the washing machine is a function of the felxibility of your floor joists.

Is it possible that the problem is not your floor joists, but your subfloor? I've inspected houses with that same problem, replacing the thin plywood subfloor with a thicker, more stable wafer board cured the problem. Of course, this is assuming that is the type of floor you have, and that you're bothered enough to remove the existing flooring and replace the subfloor.

The logical DIY approach would be to start simple and cheap - does adding nails or screws through the subfloor into the joists improve things (assuming the finish flooring is carpet, of course)? Inadequate fastening between the subfloor and floor joists could allow more flexing than is necessary. If that doesn't work, or doesn't work well enough, try adding the 2x4 support or additional cross bracing between/under joists, etc cetera.

While you can play around with all these suggestions, you'll probably save money in the long run by having a structural engineer look at the problem and tell you specifically what the best solution is. I did inspect a house in which the builder had forgotten one of the footings in the center of the house (2 stories over a walk-out basement). The building inspector didn't catch it and it took 5 years for the house to sag enough there for anyone to notice a problem. Took the engineer 2 minutes to figure out what happened and to recommend a fix for the problem that didn't require too much damage to the adjacent living space.

So, good luck. I hope your solution is simple and cheap!

"Bailey" makes a good point. In modern construction (say newer than about 20 years), the subfloor is typically nailed AND GLUED to the joists, so that they act together to resist loads, more so than if the subfloor had only been nailed. Notice that I've said "typically" and not "universally"...it's usually but not always done.

If your subfloor is 3/4 inch thick, that is usually adequate, and again, that thickness is typical. Occasionally, in premium custom construction, an underlayment is added over the subfloor, which would add at least another 1/4 inch to the thickness. This is rare, however, in speculative construction.

It's probably too late to glue the subfloor to the joists, but screws driven between the nails (if that's how it was installed) might help. But again, don't expect miracles, because the joists are near the limit of their span capability. They meet the 1/360 deflection limit of the code, but premium construction often aims for 1/480 as a deflection limit, and that would typically be done by deeper joists, closer spacing, a stronger grade of lumber, or engineered composite joists.

Regarding the washer wobble: No engineering justification here but it might work. The next time you see one of those synthetic "alligators" (tire retreads that have been driven off tractor trailers) in the road take it home and use a hacksaw to cut up four 2-3" pads for the feet of the washer. It will definitly subdue some of the vibration meanwhile protecting the floor from any rust discoloration through condesation on the metal legs. I also saw another use for these retreads against vibration on "ask this old house". Tom Silva use a 6" strip of it to hang the motor of a garage door opener to reduse the vibration transferred through the house. By the way, look for a retread that doesn't have the twisted wire running through it. It's a bear to cut through.

The washer vibration on wood floors is a common problem with new, high spin speed front load washers. Is that what you have? Some appliance stores are highly recommending that these washers are only installed on concrete floors, due to vibration complaints from customers.

Sorry, no solutions, just an observation.

Floor bounce is defined as deflection. Quite simply, the deeper the structural member, the less the deflection will be. I've found that you can increase the depth of the 2x10's by glueing and screwing a continuous 2x4 (on the flats) along the full length of the bottom of the joists. This will make the floor joists about 1 1/2" deeper than they were previously. Before you install the 2x4's, lightly jack them up from the bottom. I think you will find a great improvement to your bounce.

I like your idea, but I could use a bit more explanation on how to do it...can you go into greater detail?

all you are doing is making the floor joists deeper by adding the 2x4 on the bottoms. Any carpenter can do this. As a registered structural engineer, I constantly work on all kinds of similar issues. My method will make your floor joists somewhat like engineered lumber joists, except the webs will be 2x10's with a bottom 2x4. It does work. Use panel adhesive and wood (or drywall) screws about 3" long on 4 inch centers. Good luck.

Let me get this straight: I should cut some 2X4's to whatever length the bottom of the joists are, and glue/screw them flat side to the joists. Can you elaborate on the part about what needs to be jacked up? And is this method more effective than sistering the joists?

This method is more effective than sistering the joists because you will be increasing the depth of the member - you only increase the width of the member when you sister them. Further the structural deflection equation uses the depth of the member to the third power which means that a little increase in the depth will dramatically reduce the deflection. Jacking should be performed at the mid span of the member to take some of the load off of the joist while you are attaching the 2x4's to the bottom.

Thanks again for your input. Should I do every other joist? Do I absolutely need to do the jacking if the joists are not sagging? If so, I should apply the 2X4 at the mid-span of the joist, then jack it up at that spot just enough to relieve some weight while I'm gluing/screwing the rest of the span, correct?

"KMAN"

In your Nov 7 post, you state, "...and glue/screw them flat side to the joist." This ran a yellow caution flag up my pole and so, just to be clear, I add the following.

It is probably obvious, but the axis of the 2x4 described by STRUCT ENGINEER is turned 90 degrees from the 2x10. In other words, the 4" side of the additional 2x4 is to be centered on the 1-1/2" bottom edge of the 2x10, not along the 10 inch dimension of the joist. Yes, I realize this is extremely elementary and probably laughable, but I have seen seemingly simple instructions interpreted amazingly different on the project. That is why we architects draw pictures, rather than trying to describe construction in words.

Thanks for clarifying. So the idea is to have the bottom of the joist resemble an I-beam, correct?