[hewaschuk]

The first floor of my 30 year old two story house consists of 5/8 inch plywood on 2 by 8 joists every 12 inches. When I walk around the living room or dining room the glasses etc in the china hutch or bar rattle because the floor is so bouncy.
I tried correcting the situation by gluing and screwing a sister joist (consisting of 3/4 inch by 7 1/4 inch plywood) to the joists. The situation improved only marginally. I am building a rec room in the basement below the living room. The basement ceiling height is only 7 foot 6 inches and I do not want to lose any more height, so I don't want to put in sister joists the are deeper than 7 1/4 inches.
Is there a way of reducing the bounce.

[nmwirez]

The first floor of my 30 year old two story house consists of 5/8 inch plywood on 2 by 8 joists every 12 inches. When I walk around the living room or dining room the glasses etc in the china hutch or bar rattle because the floor is so bouncy.
I tried correcting the situation by gluing and screwing a sister joist (consisting of 3/4 inch by 7 1/4 inch plywood) to the joists. The situation improved only marginally. I am building a rec room in the basement below the living room. The basement ceiling height is only 7 foot 6 inches and I do not want to lose any more height, so I don't want to put in sister joists the are deeper than 7 1/4 inches.
Is there a way of reducing the bounce.

First up, what is the span for the 2 x8 existing joists? The scab-ons or "sister joists" you applied are undersized, that is why they still deflect. Using 2x8 'I' joists scabbed to the existing joisting will stiffen up the floor considerably, but if the span is in excess of 14 ft, the original framing should have been at least a 2 x 10 df 2btr, even with 12" o.c.'s. Get a reading from the joisting supplier for deflection on the span and 12" centers to eliminate the deflection. Nm

[hewaschuk]

First up, what is the span for the 2 x8 existing joists? The scab-ons or "sister joists" you applied are undersized, that is why they still deflect. Using 2x8 'I' joists scabbed to the existing joisting will stiffen up the floor considerably, but if the span is in excess of 14 ft, the original framing should have been at least a 2 x 10 df 2btr, even with 12" o.c.'s. Get a reading from the joisting supplier for deflection on the span and 12" centers to eliminate the deflection. nm

The span is 13 feet. Since I wrote my original request, I have scabbed on a third sister joist consisting of 2 inch by 8 inch spruce and glued and bolted it to the sister joists. I did this to 3 joists at the end of the room where the cabinets are that rattle so much. The floor above is almost as bouncy as it was. Do you think the weight of the dry wall that I will be installing in the rec room will reduce the bounciness any?

[nmwirez]

The span is 13 feet. Since I wrote my original request, I have scabbed on a third sister joist consisting of 2 inch by 8 inch spruce and glued and bolted it to the sister joists. I did this to 3 joists at the end of the room where the cabinets are that rattle so much. The floor above is almost as bouncy as it was. Do you think the weight of the dry wall that I will be installing in the rec room will reduce the bounciness any?

No, and it may crack the joint seams. You may try it on the ceiling with 5/8 rock, staggered, but I have been there and done that. The span deflection is caused by lack of fiber stress density in the existing wood due to cross-size (and wood type) of the lumber. Stiffness is needed to reduce deflection. Standard deflection allows up to 1/4" by design to avoid anomalies such as cracking problems. The structure will hold together with the current size, it is just that the flexibility will cause adverse effects.

The engineered 'I' Joisting for 2x8 is the stiffest method compared to sawn lumber. The joisting material may also be something other than a dense fiber wood such as Douglas fir, say compared to hem fir, and the same span would make a big difference of about a half inch in physical deflection. If your flooring is moving around like jello, scabbing and gluing may help some but may not cure the problem with inferior wood densities. Nm

[nmwirez]

BTW, the original subfloor should have been laid with 3/4 inch ply T&G to reduce oil canning deflection even with a 12" oc joisting. JMO and experience with high end projects. Nm

[hewaschuk]

How much does cross-bracing reduce the bounciness, does it really make a big difference? Would 2 or 3 rows of cross-bracing help even more?

[nmwirez]

How much does cross-bracing reduce the bounciness, does it really make a big difference? Would 2 or 3 rows of cross-bracing help even more?

Yes, if you use freeze blocking. (Not the old two stick method.) The freeze blocking if real tight may reduce the bounce. Just depends on what is there now. What type of tree in the forest did they use for the 2x8's? Nm:)

[nmwirez]

How much does cross-bracing reduce the bounciness, does it really make a big difference? Would 2 or 3 rows of cross-bracing help even more?

In referring to deflection of sawn lumber, the modulus of elasticity is the reference point used for deflection integrity. Old growth quotes are used but new growth bends more. A strength comparison will give an idea... Doug Fir=1.97(million psi) White Spruce=1.45. The static bending is considerable difference. Nm

[glavine]

Not To Lose As Much Head Room. You Have To Go With A Steal Angel Plate Of Some Kind, Cause At 13ft You'd Be Using A 2x10 , You'd Also Need To Get Some Floor Jacks To Support This Beam , They Are Adjustable To Take The Load Off The Floor, Hope This Helps , Nichols Trim Carpentry