Taking the bounce out of a floor?

[Mike Cutler]

I'm not trying to justify my idea just trying to figure out what others are thinking. I don't need a rock steady floor just much better than I have I can't afford to tear out and replace but would like more strength. When we originally looked at the house and I went into the shop I don't remember this much bounce I was thinking this morning if removing the PO machines has allowed the joists etc to spring back and if putting weight back on would help? Just throwing out ideas.

Jerome

Old houses,barns, and garages, have a distinct charm, but they can present some interesting challenges requiring some non-conventional thinking.
Living in New England, where there are lots of old houses, I've seen some interesting repairs, and refurbishments, to existing structures that would just be to ungodly expensive to rip out and start again, which could also degrade the value of the house in the end.
One big problem is dimensional lumber. It just doesn't work when the house was made with "true" lumber. You end up ripping every board you buy to size, which gets not only tedious, but expensive.

The PO's machines may have absorbed some of the vibration. Hard to tell. I think if you re-inforce the floor the way you're describing, it will help a lot. Adding load to the floor with machine weight may help too.

You're on the right track.

[Jim Becker]

If I install between the existing 2X6's it would be 12" centers I am confused as to why you are saying it would be 24"? QAs for the 1X6 floor right now each one is toe-nailed to the existing joist and they are run at 45* to the joist I plan on using flooring screws into the new joists.

If the tops of both the existing 2x6 and new 2x8 are level with each other, you will, in fact, have 12" OC supporting the floor. But if the tops of the new 2x8 are proud of the existing 2x6, then you're still at 24" OC. What's missing is how you are positioning the new lumber.

[Al Launier]

From what I've read above the focus is on re-joisting and that is correct, but in my opinion only partially. My concern would be supporting these ceiling/floor joists not only at mid-span as you have noted, but as importantly at the outer walls. Not knowing if the existing wall studs are 2x4s or 2x6 will have a direct bearing on the support & rigidity of the structure. Considering the potential "living" use of the upper floor of your garage, or for heavy storage, the walls should be constructed of 2x6s set on a good 8" foundation wall on a good footing(8"x16").

If the sill plate on the foundation is 6" and the existing wall is 2x4, then I would consider building another wall inside the existing wall with the new wall studs staggered between the existing studs at 16" with a crosswise stud sitting on the sill plate where the 16" matches an existing 24" stud and then supporting the new 10" joists on top of the new double top plated wall & the mid-span walls. The tops of the new joists would match the tops of the existing 6" joists. This would reduce the inside wall-to-wall dimensions of the garage a little, but would provide the support needed & you would not be concerned about this in the future.

[Bruce Wrenn]

When we encounter a "bouncy floor," we added a hanging beam (usually a pair of 2 X6's nailed together,) and hung it off bottom of existing joist with 2 X 12 joist hangers. The beam runs perpendicular to span of joists, and is located at center of span. You have to make sure EVERY joist makes contact with the beam, or it doesn't work. This means either jacking the beam up, or loading the floor above so as to drop it down onto the beam. The beam is supported by a joist hanger at each joist.

[Alex Zeller]

How much headroom do you have? If you are not looking to remove the existing sub floor and replacing it with larger joists could you build a second sub floor on top of it going perpendicular to the original floor? It may not be the ideal way to go but should solve the problem.

[Steve Rozmiarek]

One more thought, balloon framing supports the joists via a ledger board that is inset into the studs. The ledger is usually a 1x4 or similar, and will support no load in between the studs. The joists are nailed to one side of the studs and sit on that ledger. Using a 8" additional joist on a 6" setup will requires notching the ends to fit the 6". I'll also require using the other side of the stud, so the 8" joist will only be 1 1/2" away from the existing 6". You won't be able to move the new joist away from the stud to get a different spacing as you would be able to on a platform framed building.

[Lee Schierer]

It is probable that your 2 X 6 floor joists are spaced too far apart and are exceeding their allowable span limit. You can identify what type and grade of lumber that was used and enter the span and spacing in this calculator and determine the maximum allowable span. Span calculator. For a 12 foot clear span you will likely need at least a 2 x 10 and 16" centers.

[Jim Dwight]

When the floor joists are too small and too far apart the consequence is normally too much deflection, not a floor that caves in. The span tables assume that the maximum deflection that is acceptable is 1 inch over 360 inches and that is at maximum load. For the first floor, normally that is 40 lbs per square foot. Sometimes they use 40 lbs/ft squared live load and 10 lbs per square foot dead load. A shop with heavy tool may need to be designed to hold more than 40lbs/ft2 or the tools may need to be placed close to a support pier. Deflection will be greatest mid span.

I think my room over the 2 car garage is just 2x10s on 16 inch centers spaning 20 feet. It has been there for decades. But I may hire somebody to come in and double up the 2x10s. With good grade lumber, I should be about at code for the 30 lbs/ft2 that area needs to be designed to support. The floor is somewhat bouncy. We don't use it much and it doesn't have any where near 30lbs/ft2 in it. So I'm not worried about safety but if several ~200 lb people are up there, it is a little bouncy.

2x8s are good for 13+ ft at a 40lb live, 10 lb dead load. But 2x6s are not. But if your 2x6s are really 2x6 instead of 1.5 inches by 5.25 inches that would help. If you calculate the moment of inertia you can get a feel for the difference. I get 18.09in4 for the 2x6 and 36in4 for a really 2 inch by 6 inch joist. So deflection would be half as much. But a 2x8 is 47.63. So if you mix these, the deflection of the areas supported by a 2x8 will be less than the areas supported by he old full 2x6s.

[Jerome Sidley]

You are correct there is a 1X4 ledger I was just going to nail the new ledger the appropriate gap over this existing and into the studs. I see no advantage in notching the new ledger if anything I would drive a wedge block between the two.

[Steve Rozmiarek]

When the floor joists are too small and too far apart the consequence is normally too much deflection, not a floor that caves in. The span tables assume that the maximum deflection that is acceptable is 1 inch over 360 inches and that is at maximum load. For the first floor, normally that is 40 lbs per square foot. Sometimes they use 40 lbs/ft squared live load and 10 lbs per square foot dead load. A shop with heavy tool may need to be designed to hold more than 40lbs/ft2 or the tools may need to be placed close to a support pier. Deflection will be greatest mid span.

I think my room over the 2 car garage is just 2x10s on 16 inch centers spaning 20 feet. It has been there for decades. But I may hire somebody to come in and double up the 2x10s. With good grade lumber, I should be about at code for the 30 lbs/ft2 that area needs to be designed to support. The floor is somewhat bouncy. We don't use it much and it doesn't have any where near 30lbs/ft2 in it. So I'm not worried about safety but if several ~200 lb people are up there, it is a little bouncy.

2x8s are good for 13+ ft at a 40lb live, 10 lb dead load. But 2x6s are not. But if your 2x6s are really 2x6 instead of 1.5 inches by 5.25 inches that would help. If you calculate the moment of inertia you can get a feel for the difference. I get 18.09in4 for the 2x6 and 36in4 for a really 2 inch by 6 inch joist. So deflection would be half as much. But a 2x8 is 47.63. So if you mix these, the deflection of the areas supported by a 2x8 will be less than the areas supported by he old full 2x6s.

Hi Jim, you sound like an engineer, so can I ask a question? From my rudimentary physics education, I remember moment of inertia pertaining to rotational forces. How do they apply to floor loads? I've heard this before but never had the chance to ask.

[Bill Dufour]

I wonder if blocking between the joists would spread the load out and make it less bouncy. I do not think it adds to the load limits.

[Lee Schierer]

I wonder if blocking between the joists would spread the load out and make it less bouncy. I do not think it adds to the load limits.

You are correct. A 12' span with 24" centers is just too much for even a full size 2 x 6..

[Al Launier]

Hi Jim, you sound like an engineer, so can I ask a question? From my rudimentary physics education, I remember moment of inertia pertaining to rotational forces. How do they apply to floor loads? I've heard this before but never had the chance to ask.

Jeremy, what Steve is referring to when he mentions MOI is the ability of a structural member, i.e. its cross-section, to resist bending. Keep in mind that it is the depth of a structural member that provides its rigidity/stiffness rather than its width. Adding thickness does add to the rigidity, but that's not as effective as adding depth.

[Jim Becker]

You are correct. A 12' span with 24" centers is just too much for even a full size 2 x 6..

Agree. I suspect that the original structure wasn't actually intended to have an upstairs that was "in-use"; for storage, maybe, but it sounds more like it was mostly to support a ceiling for the area below.

[Jim Andrew]

Before we had building codes, 2x6 on 24" centers was common. I took down an old house on my property that had 16' 2x6 @ 24" oc, but it was built in the 1860's.