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Discussion Starter · #1 ·
Hello,
I purchased an old house, 2 story.
I want to strengthen the second story floor.
Currently the joists are 2x8, spanning 13lf.On 16 inch center. The joists are not rotten want a live load/ dead load closer to 40/20.
Should I sister the joists with 2x8 full length or should I add 2x8 in between the 16inch centers. Thank you kindly for your response.
 

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Discussion Starter · #3 ·
Are you working from below or above, Any chance you could sister with 2x10s?
Thank you for responding,
I am working from below. I can’t imagine I could use 2x10 with out taking out the whole top/second story floor. From slab to ceiling joists it is only 7’8” don’t want to lose any height. Already need to recess lights.
 

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retired framer
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yes the floor is bouncy, after researching the lumber size, type, and span, I discovered I am at the max span and want a solid floor I am comfortable on.
Does it have bridging or blocking mid span?
My experience is with 2x10s, and we do bridging or blocking no more than 7 ft from a bearing wall, I am wondering if you should have 2 rows. Even if you sister them.
Bounce is usually caused by the bottom of the joists moving sideways mid span under load,
 

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Discussion Starter · #7 ·
No there is no blocking or bridging.And ok I can do that. I will sister them full span with 2x8 and then add 2 rows of blocking. Thank you again for responding.
 

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retired framer
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No there is no blocking or bridging.And ok I can do that. I will sister them full span with 2x8 and then add 2 rows of blocking. Thank you again for responding.
I would do a test first, screw 2 2x4s across the the bottom like strapping, and see how that effects the bounce.
Maybe you just need blocking and not the sisters.
 

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Discussion Starter · #9 ·
Ok I will do that!
If I do need to sister, how much of the joist (minimum) should be placed on the sill plate. Current joists have electrical running through joists, located at sill plate. Have about 3 inches on each side for sister joists to sit on. I don’t want to notch each board for the Electrical wires.
 

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If I may add to what Neatw has already said.
When you use blocking, you effectively tie one rafter to the two on both sides of it. That greatly reduces the bounce if not eliminate it altogether.
Block with the same size lumber as the joists.
 

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retired framer
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Ok I will do that!
If I do need to sister, how much of the joist (minimum) should be placed on the sill plate. Current joists have electrical running through joists, located at sill plate. Have about 3 inches on each side for sister joists to sit on. I don’t want to notch each board for the Electrical wires.
The old joists are carrying the weight now so you don't really need any but it always looks better if you have 1 1/2" on the wall. If you cut a wedge off the top of one end you could go the all the way in one end and then back it out to center.
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