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Joist 34' o.c bouncy

Posted by bluebirdhouse (My Page) on
Sat, Mar 20, 10 at 6:57

Hi there I read alot of the excellent post on bouncy floors. Although did not find any to match my problem... I bought a cottage built in the 20s which I am converting to a fourseason home.

The floor joist are all 2x8 rough 34" o.c with spans if 12'

I was thinking of adding a 2x8 (1.5x7.5) joist in the middle which would equal approx 16" o.c and shimming th .5" missing with s piece of hard wood floor my question was with reading the posts using 1/8 metal strapping seems pretty efficient any guidance would be greatly appreciated


Follow-Up Postings:

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RE: Joist 34' o.c bouncy

What is the sub-floor?

With 34 OC joists you need a really thick sub-floor, like 2x lumber, to avoid bounce between the joists.

My span tables and books are at work, but 2 inch rough will be stronger then 1.5 inch surfaced that most tables are designed for.
The other problem is wood grade.
Without a grading mark you are pretty much stuck with the lowest strength grade if you can identify the wood species (not always easy on old rough lumber).

If you decide to add new joists between the old it would be better to place the packing at the ends of the new wood were the transfer the load to the foundation.

If the top of the foundation is not well above grade (a couple feet at least) the packing pieces should be pressure treated.


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RE: Joist 34' o.c bouncy

The existing floor is so understructured that metal straps would not help enough. I'm surprised there isn't a permanent sag to the joists.

Adding 2x8's should bring the deflection to about the code limit which is not terribly stiff but tolerable to some people. It would help to use Hem-Fir or a higher grade of lumber to get a higher Mod. of Elasticity than No. 2 Spruce-Pine-Fir.


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RE: Joist 34' o.c bouncy

Two inch net wood on 32 inch centers will behave about like 1.5 inch wood on 24 inch centers.

The joists are 33% stronger from the added width.

Depending on lumber grade and type they may be 'legal.'

That does not make them any more solid as the engineered joists folks found out when they created large span i-joists and such. Using 20 foot i-joists at 1/360 makes a floor acceptable from an engineering point, but not from a 'comfort' point. The deflection is simply to great.

1/360 deflection is not a safety issue limit but is for ceilings installed on the joists below to avoid cracking.


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