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Old 10-09-2018, 07:07 PM   #1
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Sister 2x6 @ 24 OC Joist


I have a 1926 cottage that was built in a different era. The floor joists are 2x6 (Fir) on 24" centers spanning between 6.5' to 7.5' . It is 1x4" T&G fir on top of the joists.


I am looking to have a new floor laid over the top of the old one but the material we like is a vinyl plank flooring with stone substrate (stone product composite (SPC), Goodfellow Dubai Collection) . The "Planks" are 5.5" wide and 48" log and can withstand a maximum deflection of 3/64".


I believe my current floor is around an L/389 design, so on 48" would deflect close to 8/64 or much more than the 3/64 spec.


Wondering if I sistered a 2x10" to the 2x6 I would get at least L/1208 which should meet the 3/64" requirement. Its just crawl space under the floor so it doesn't have to be beautiful.



The 2x6's rests on a beam either end so I would have to notch the 2x10 to get it to rest on top of the beam. Wondering if notching is a bad idea (creates a stress to split the lower 3-4" off the joist) or if I just cut 2x10 to butt up to the beam and just glue and nail it to the original 2x6. I'm reading that all the stress of importance is in the middle of the span thus it doesn't matter much what you do at the ends.



Any suggestions appreciated.
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Old 10-09-2018, 07:48 PM   #2
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Re: Sister 2x6 @ 24 OC Joist


You don't want to notch them, You can solid block between the joists on top of the beam and but the new joist up to the beam and hang it from the blocking.
I would change to 16" on center with the new joist so every third one would be sistered and need a double hanger.
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Old 10-10-2018, 12:27 AM   #3
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Re: Sister 2x6 @ 24 OC Joist


For joists that are SYP or Douglas Fir, in good condition, 6 inches tall, 2 inches wide, 24 inches on center, and 7.5 feet long between supports, the deflection calculated is 0.166 inches.

This translates to a deflection of L / 542.
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Old 10-10-2018, 11:59 AM   #4
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Re: Sister 2x6 @ 24 OC Joist


Quote:
Originally Posted by Nealtw View Post
You don't want to notch them, You can solid block between the joists on top of the beam and but the new joist up to the beam and hang it from the blocking.
I would change to 16" on center with the new joist so every third one would be sistered and need a double hanger.


Good thought. I was primarily thinking about joist deflection but your suggestion will improve subfloor deflection between joists as an added benefit.

Itís a dirt crawl space so Iíll need to get motivated to do this job....



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Old 10-10-2018, 12:23 PM   #5
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Re: Sister 2x6 @ 24 OC Joist


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Originally Posted by curiousB View Post
Good thought. I was primarily thinking about joist deflection but your suggestion will improve subfloor deflection between joists as an added benefit.

It’s a dirt crawl space so I’ll need to get motivated to do this job....



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The dirt needs to be sealed under a heavy sheet poly
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Old 10-10-2018, 12:27 PM   #6
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Re: Sister 2x6 @ 24 OC Joist


96" long 2" x 6" 's are not that expensive. Just buy more of them and slip one in between each joist run and make it a 12" O.C. floor joists system thereabouts.
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Old 10-10-2018, 12:40 PM   #7
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Re: Sister 2x6 @ 24 OC Joist


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Originally Posted by Gregsoldtruck79 View Post
96" long 2" x 6" 's are not that expensive. Just buy more of them and slip one in between each joist run and make it a 12" O.C. floor joists system thereabouts.
Up here a floor is always 2x10 or bigger, never less.
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Old 10-10-2018, 05:17 PM   #8
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Re: Sister 2x6 @ 24 OC Joist


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Originally Posted by Nealtw View Post
Up here a floor is always 2x10 or bigger, never less.


OP >> I have a 1926 cottage that was built in a different era. The floor joists are 2x6 (Fir) on 24" centers spanning between 6.5' to 7.5' . It is 1x4" T&G fir on top of the joists.




Well yeah, if the OP is building a new room or house then sure, use 2" x 10"s. + joists. But he is trying to make a 1926 model floor work without tearing down the room I would think.

My book states a 2" x 6" x 10' 9"' long joists using #2 Southern pine spaced on 12" O.C will withstand 60 pounds per sq. ft. of live load and 20 pounds per sq. ft. of dead load. The OP's span is even less than 8 ft. on the joists, even making the load capability even higher. JMO
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Old 10-10-2018, 05:34 PM   #9
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Re: Sister 2x6 @ 24 OC Joist


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Originally Posted by Gregsoldtruck79 View Post
OP >> I have a 1926 cottage that was built in a different era. The floor joists are 2x6 (Fir) on 24" centers spanning between 6.5' to 7.5' . It is 1x4" T&G fir on top of the joists.




Well yeah, if the OP is building a new room or house then sure, use 2" x 10"s. + joists. But he is trying to make a 1926 model floor work without tearing down the room I would think.

My book states a 2" x 6" x 10' 9"' long joists using #2 Southern pine spaced on 12" O.C will withstand 60 pounds per sq. ft. of live load and 20 pounds per sq. ft. of dead load. The OP's span is even less than 8 ft. on the joists, even making the load capability even higher. JMO
He also said.


Wondering if I sistered a 2x10" to the 2x6 I would get at least L/1208 which should meet the 3/64" requirement. Its just crawl space under the floor so it doesn't have to be beautiful.
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Old 10-10-2018, 06:02 PM   #10
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Re: Sister 2x6 @ 24 OC Joist


Oh I see. The OP must be going to park his F250 pick up truck perpendicular on that 7' 5 " span floor. My apologies belated now, to the OP for my ignorance. .
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Old 10-10-2018, 07:11 PM   #11
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Re: Sister 2x6 @ 24 OC Joist


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Oh I see. The OP must be going to park his F250 pick up truck perpendicular on that 7' 5 " span floor. My apologies belated now, to the OP for my ignorance. .
It will be good for tile for the next owner.
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Old 10-10-2018, 07:49 PM   #12
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Re: Sister 2x6 @ 24 OC Joist


Besides the joists, is 1x4 decking ok? My house is also 1x4 only, not underlayment and a finish floor. Is it smooth? Check with the manual or manufacturer. Modern engineered materials need modern substrates. Check the defection for marble flooring and that may be your goal.
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Old 10-10-2018, 09:33 PM   #13
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Re: Sister 2x6 @ 24 OC Joist


To me, its the 24" on center that is the issue and probably cause for deflection. If you sister using 2x10's notched, I would add a hanger to prevent split out at the notch. It probably would also be a good idea to change it as others have stated to either 16" oc or 12" oc. You may also have to have some form of underlayment put down to smooth out the plank subfloor so details don't telegraph through the new flooring to the top.
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Old 10-11-2018, 07:57 AM   #14
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Re: Sister 2x6 @ 24 OC Joist


OP here.


Thanks for the many suggestions and ideas. No I am not parking my F250 on the floor.



..."Floor must be level to 4.7mm per 3m radius (3/16" per 10 foot radius). Maximum deflection should be no more than 1.1mm (3/64")."...



The spec for the flooring (mentioned in OP) I am considering is no more than 3/64" deflection and the sections are 48" long. I compute that as 48/.0468 = 1024 or L/1024 which does seem arbitrarily stiff especially in light of L/360 IRC spec. Maybe I am not doing this calculation correctly?


If it is L/1024 then my existing 2x6 @ 24" center is L/389 or not even close to the spec. I found that [email protected]" center is L/1208 which meant sistering the joists. NealTW had a good idea to just go 16OC and sister every thiird joist.



Taking that a step further I am wonder if I put a joist halfway between the 2x6 existing could I get better than L/1024. If the joist is 2x6 making them all uniform, then 2x6 @12" OC is L/779 which is not enough.



So now I want to calculate if the new midspan joist were a 2x8". The calculator I am using assumes all the joists are the same. Wondering how I compute deflection with a mixed bag of of joist sizes. I'm thinking I can get away with 2x8" mid span.



I know 2x10 will work because [email protected]"OC is L/1208 so ignoring the 2x6 completely [email protected]"OC meets spec.


Not sure how to compute deflection for [email protected]"OC with [email protected]"OC staggered 12" apart.


Could it be as simple as saying [email protected]"OC is L/779 and [email protected]"OC is L/1468 so the staggered arrangement would be midway between the two or L/1123?

Last edited by curiousB; 10-11-2018 at 08:06 AM.
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