# Double Floor Joists Under Non-bearing Walls?



## fj80

Are double floor joists required, and/or common practice, under non-bearing walls that run parallel to the joists?


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## fatboy

Practice........


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## Pcinspector1

I see the Doubled FJ's under *bearing walls* with and with out a space between to allow plumbing sometimes. 

Common: When Doubled FJ's are under a non-load bearing wall to eliminate the floor flex that creates a drywall crack above a door header on the wall above, typically between a bedroom and master bath. 

Were only dealing with minimum codes, right?


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## fatboy

I should have said "good/best" practice..........


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## fj80

Pcinspector1 said:


> I see the Doubled FJ's under *bearing walls* with and with out a space between to allow plumbing sometimes.
> 
> Common: When Doubled FJ's are under a non-load bearing wall to eliminate the floor flex that creates a drywall crack above a door header on the wall above, typically between a bedroom and master bath.
> 
> Were only dealing with minimum codes, right?



Primarily concerned with minimum codes, but want to know if it's good common practice as well.
I've worked at several residential firms where we never showed this in our drawings, but am working at a new residential firm where they say it's common practice. Just want to educate myself on which is best practice. 
Maybe at my past firms the contractors were using double joists under the parallel walls with me noticing it...


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## Pcinspector1

Keep in mind, that the double FJ's do not always carry the load depending on your span. I have seen a triple 2x used on occasion due to the cost difference of LVL's, etc. etc.


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## north star

*& >*

Also, ..."good practice" may not make good financial sense
in some markets.

IMO, ...if cost is not a concern, then Yes, Double FJ's make
good structural sense.

fj80, it may be that your "new" Residential firm cares
more about quality & integrity of their product than some
other firms.......Have you asked why this is considered
"best practice" at your new digs ?


*< &*


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## fj80

north star said:


> *& >*
> 
> Also, ..."good practice" may not make good financial sense
> in some markets.
> 
> IMO, ...if cost is not a concern, then Yes, Double FJ's make
> good structural sense.
> 
> fj80, it may be that your "new" Residential firm cares
> more about quality & integrity of their product than some
> other firms.......Have you asked why this is considered
> "best practice" at your new digs ?
> 
> 
> *< &*


north star, I asked my boss why and he said he's just always done it this way because once an engineer told him it was a good idea for walls that are parallel to the joists so they're not concentrating their load on one joist or even just the plywood subfloor if the wall falls between joists. It makes sense to me, I had just never heard of the idea until now and wondered how many architects and builders do it this way.


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## fatboy

I'm sure the plumbers don't like it.


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## ICE

Double floor joists come in handy when nailing bottom plates.


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## FLSTF01

It is my opinion that 3/4 plywood or Advantech does not flex enough to create a drywall crack in a non-load bearing wall, running parallel to the floor joists.  I think it's a hold-over from the days of 1/2 inch CDX, then 5/8 cut to fit up to the walls.

Edited to add that solid blocking every few feet would likely do as much if not more than a doubled joist would to prevent any drywall cracks that may or may not happen.


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## tmurray

There is a requirement in our code that non-loadbearing walls are supported on a floor joist (singular) or on wood blocking between joists.


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## Pcinspector1

The construction company I worked for worked really hard to eliminate call-backs. We had a problem with the drywall on the master bedroom door wall cracking above the header The floor would flex on the room side but was solid on the outside wall side due to that wall being over an exterior foundation. We added an additional FJ and another king stud which made the issue go away. Had nothing to do with floor sheeting flexing. 

Another area we had drywall cracking was in an outside wall corner, we installed a horizontal block midway up from the FF in the wall cavity, another crack issue resolved. Flex bead drywall corners solved another issue.

If I give you anymore trade secrets I'll reconsider being a BO and go back to making real $$.


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## Paul Sweet

Double floor joists under partitions has always been considered best practice.

A typical residential floor load is 40 PSF live plus 10 PSF dead.  This is 67 PLF on joists spaced 16" OC.

A typical wood stud partition weighs 6 to 8 PSF, or 48 to 64 PLF for an 8 foot high partition.  Putting a partition directly over a single joist increases the load on the joist by 71% to 95% (it's even worse if the joists were sized for a 30 PSF bedroom load).  You can get away with this on short spans, but you're likely to have problems if the span is near the maximum.


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## ICE

I have always asked for two joist under walls and I don't recall anyone complaining about it.


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