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Bouncy floor
Hallo you all. I am new to the forum and would like to introduce myself before asking a question about a little problem that i have and that I hope someone might be able to answer. My name is Anthony and I live in CT.

Recently I moved into a new home that I had build and I notice that some of the floors are not as stiff as I would like them. The kitchen floor, one of my bedrooms floor and one of the bathrooms floor are bouncy so that if someone walks on these floors you can feel the vibrations. The subfloors are all made up with 3/4 inch plywood over 2 by 12 douglas fir joists 16 oc. The longest span of the joists is 15 ft. The bedroom is carpeted and the kitchen and bathrooms are tiled.

Not only the vibrations are noticeable, but the grout on the tileds floors cracks and pops out from time to time so that it has become a real pain to continue replacing it. A cople of the tiles have cracked and a couple of them sound like they have come loose when you step on them.

Is it possible that the 3/4 inch plywood is insufficient or can there be a more serious problem? How can this situation be rectified? By the way, I have access to the floor from below only as far as the kitchen goes. The bedroom and bathroom are on the second floor and I would have to rip the first floor ceiling sheetrock if access to the joists is necessary.

Thank you for any advice that you may be able to give me.
Posts: 5 | Registered: 22 October 2005Reply With QuoteReport This Post
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Thank you, Homebild. Hopefully CT Engineer can provide some additional input. As a follow up question, is there any way that I can find out what grade lumber the joists are and/or if there is some other factor that is contributing to the problem? I seem to recall that the lumberyard where the material was purchased mentioned No. 2 DF.

Also, since the lumber yard calculated the lumber requirement from the blueprint (I think they called it "take out" or something similar) shouldn't they be knowledgeable enough to provided the right grade of lumber for the job? Or shouldn't they at least have mentioned if there was a problem given the spans and other factors?

Thanks again for your advice.
Posts: 5 | Registered: 22 October 2005Reply With QuoteReport This Post
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It's not the responsibility of the lumber yard to design the home.

That is the job of the architect or engineer, if any, or contractor if he acts as the designer.

All the lumber yard does is price and supply the materials from the 'take off sheet' and send what has been requested to the jobsite.

Plans need to be approved by the local code office and most states now require that plans be reviewed by certified plans examiners before a permit is even issued.

It's possible that the house was built prior to the such plan review requirements or someone screwed up along the way by not planning for the additional loads that ceramic tile can add.

All lumber is required to bear an manufacturer's grade stamp indicating the species, the grade, the degree to which it has been dried among other things. You will find this imprinted on the side of every joist.

The framing inspectors use this information to assure that the lumber used and in place is that which is called for on the plans.

But mistakes happen, and miscalculations can slip though even when engineers design and examiners review.

Sometimes its just that adding extra loads like tile floors are done as an afterthought or well after the house has been constructed and installed without consideration of the added loads placed on the structure.

Ultimately who is at fault is the one who designed the home and the loads, whether that is the designer, architect, or engineer or the contractor if he took that responsibility on himself, not the lumber suppliers.

Don't know what else to say.

I'm going to ask an architect friend to check out this thread and give his opinion.
Posts: 453 | Registered: 19 July 2005Reply With QuoteReport This Post
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#2 Douglas Fir - Larch is commonly specified for joists, and my span tables (in the New York code), for a 40-psf live load and 10 psf dead load, indicate a maximum allowable span for 2x12's of 17-10.

I am frankly mystified by the bounce in your floors. Of course, SOME deflection is normal. It is nearly impossible to make an absolutely stiff structure.

In my experience, floors in office buildings and shopping malls can be bouncy because they are never subjected to the loads they are designed to support. I have seen office building floors act like a trampoline (I exaggerate for effect here) when the building is empty, but the vibration is gone once the partitions, desks, files, etc. are in place. So it MAY be possible that you have not approached the 10-psf design dead load, even with ceramic tile in place. Do you have a great deal of heavy furniture, or a smaller amount of lighter furniture? And if you'll pardon the question, what is your weight?

Another possible cause may be the absence of cross-bridging between the joists, although there is some doubt that cross-bridging actually accomplishes anything. The old standard for cross-bridging was 8 feet on center, so one row would have sufficed for your condition by the old standard.

A subfloor of 3/4-inch plywood is pretty much standard, so that should not cause the problem. If the subfloor is both glued and nailed or screwed to the joists, it does stiffen the floor slightly. Do you know if this was done?

Is your ceramic tile installed by adhesive method, or is there a mortar bed?

Another question is: are all the joists full depth, or did the plumbers notch or cut some to accomodate the plumbing? Depending on where and how deep the notch is, the structural capacity of a joist can be reduced drastically by notching, drilling or cutting. This could cause quite a bit of deflection.

I'm sorry I cannot find a definitive answer to your problem. By the way, I'm the architect that "homebild" referred to.

Architect (NY) and Home Designer (PA)
Posts: 2870 | Location: Tobyhanna, PA | Registered: 24 October 2005Reply With QuoteReport This Post
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Thank you for your efforts, Homebild and Mr. Hetzel:

I will try to confirm that the joists are # 2 DF, but I am pretty sure that the lumberyard told me that they are. To address some of the issues presented by Homebild, the plans were drafted by an architect and were filed and approved by the Town engineering department prior to construction. The Town inspector was on site at different stages of the building process and all permits were issued without any indication by the inspector that something was not done according to code. Obviously, I think that the inspector would have required anything done wrong to be redone. The blueprint also provided for tiles to be installed in the bathroom and kitchen. The only thing that was changed from the original plans, as far as I can remember, was to substitute marble tiles for ceramic tiles in the bathroom.

As far as the questions posed by Mr. Hetzer, the joists have been installed with solid blocking placed midway within the span. Some minimal drilling has been done to accomodate the electric wire, especially on the joists supporting the first floor, and 2 or 3 of the kitchen joists were drilled to allow the plumbing for the hot water heater to be placed above the basement ceiling. All the holes are very close to the end of the joists, maybe one foot or so. As far as I can remember the joists of the second floor bedroom and bathroom have not been touched at all since all the wiring and the pipes were run through the walls. The walls top and bottom plates have been notched or drilled in a few places to allow room to run the drain pipes and the water pipes within the wall, but I don't know if that would have any effect on the floors.

With regard to the kind of weight placed on the floors themselves, in the kitchen, besides the wall-to-ceiling cabinets (which cover 2 and 1/2 walls), the oven, a double-door refrigerator and a pantry, I have a center island with cooktop an a granite countertop and a light table with four chairs at the other end of the room. I have relatively light furniture in the bedroom and nothing at all in the bathroom except for 2 cabinets supporting the sinks and placed on 2 different walls. There is a shower stall and a jacuzzi on the opposite wall, which has been used and filled with water only a couple of times.

The subfloors were nailed and glued to the joists, but I don't know how good of a job at gluing the contractor did, at least in the bedroom, since I noticed squeeks in a few spots. Also, the tiles themselves have been installed with thin set mortar, not glue.

I do understand that sometimes is very difficult to diagnose a problem, especially when there could be a number of causes, without visiting the site and based only on someone's description and I really appreciate you taking the time to try to give me at least an idea of what could be wrong. Is there anything that can be done to pinpoint the exact cause, or is there anything simple that I can try to see if the problem is eliminated or at least reduced? For example, from the description of the office building issues described by Mr. Hetzel, I seem to gather that if I were to increase the weight of the items in the room the bouncing problem should be reduced if caused by the fact that the load limit has not been reached. By the way, I weigh 184 Lbs. and my wife weighs 140 Lbs.

Thanks again.
Posts: 5 | Registered: 22 October 2005Reply With QuoteReport This Post
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Joists are designed to support two kinds of loads. They are called Live Load and Dead Load.
Live Load is the weight of people and furnture and anything else one may place on the floor. Dead load is the weight of the structure itself, including finishes such as tile, and things like piping, wiring, ductowrk, lighting and plumbing fixtures, etc.

If the dead load were greater than the 10 pounds per square foot that the joists are designed to carry, the result would be permanent excessive deflection (sag) in the joist but not necessarily bounce. Thinset tile should not cause the dead load to exceed the 10psf design load. All joists will deflect somewhat. Normal maximum deflection is 1/360 of the joist span. For your 15 foot span, that would be 1/2 inch. That is to be expected.

The mystery is because your joists, if they are indeed #2 Douglas Fir - Larch, should not be stressed anywhere near their limit. Your own weight certainly is quite normal. I weigh considerably more, and I don't cause bounce in peoples'

You might be able to check the deflection by measuring the ceiling height at the walls, and in the center of the room. If the room is approximately 15 feet wide, the ceiling should be no lower than 1/2 inch in the center than the height at the walls. That measurement would be interesting to know.

I'll keep thinking and will do a little research when I get a moment.

Architect (NY) and Home Designer (PA)
Posts: 2870 | Location: Tobyhanna, PA | Registered: 24 October 2005Reply With QuoteReport This Post
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I have calculated the maximum deflection to be expected for your 2x12 joists spanning 15 feet, for 50 psf total load. It comes out as 0.218 inches, or less than a quarter of an inch, IF the floor is FULLY LOADED at 50 psf. Your actual deflection should only be a fraction of that. This is plenty stiff enough for tile floors.

I found a discussion of deflection under tile floors, in which the writer referred to a recommendation of TWO layers of 5/8 inch plywood as a subfloor under the tile. He calculated the allowable deflection in the subfloor for 3/4 inch plywood as 0.02 inches, or about a millimeter, based on the 1/360 criterion. This would be an almost unattainable stiffness in the subfloor.

So at this point, I'd suspect the plywood subfloor as the cause of the grout cracking, but that still doesn't fully explain the bounce.

I have a reference book coming, which may arrive today or tomorrow, and it may shed some light on the subject. Stay tuned.

PS: the book didn't explain anything.

Architect (NY) and Home Designer (PA)
Posts: 2870 | Location: Tobyhanna, PA | Registered: 24 October 2005Reply With QuoteReport This Post
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WWP stands for Western Wood Products; S GRN means that the lumber was surfaced green, rather than surfaced dry. Surfacing is the process that reduces a 2x12 to 1 3/4 x 11 1/4, by planing the lumber. The other choice would be "surfaced dry"(S-DRY). Lumber is often kiln-dried to 19% maximum moisture content. "S-GRN" indicates that the moisture content exceeded 19% when the lumber was manufactured, and the actual finished dimensions of the lumber would be fractionally larger, so that shrinkage will reduce the size to the standard size. Simpson is the mill that produced the lumber.

All of that is good news. You are correct in surmising that the joists are at least #2 Douglas Fir, and some are #1 or better. Probably #2 Douglas Fir was specified, and you got that or better than that. So it appears that there is nothing structuraly incorrect.

As far as I can tell, your architect, your builder, and the lumber dealer all did nothing out of the ordinary, and certainly nothing wrong.

However, the Western Wood Products Association "Western Woods Use Book" discusses the effect of moisture content, and states that there is a "natural gain in strength and stiffness that occurs as lumber dries". So it could be that your joists, being fairly new and in a season of unusually wet weather, may not have achieved their full strength and stiffness. If you measure the depth of your joists, if dry (19% or less moisture content) they will measure 11 1/4 inches; if still fully unseasoned, they will measure 11 1/2 inches. If they are somewhere in between, then the drying process is continuing, and the problem may lessen when they are fully dry.

Architect (NY) and Home Designer (PA)
Posts: 2870 | Location: Tobyhanna, PA | Registered: 24 October 2005Reply With QuoteReport This Post
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