Sistering joists

[Cooler]

If you use screws, use the specific screws designed for joists.  It is important that the screws have sufficient shear strength.

If you are going to do a lot of joists, you need to put in plenty of nails to unify the two joists as one unit.  I would recommend a palm nailer.  It will make the job simple and fast and it will be far less tiring than using a hammer.  Lots faster and cheaper than using screws. 

They are recommending 10d or 16d nails.  Two rows 16" on center.  When I did it I used 8" on center in a zig-zag pattern.  If you use screws, Stimson makes the screws for structural use.  Do not use drywall or standard construction screws.


https://homeguides.sfgate.com/sister-flo...42337.html

[blackhat]

I’m wondering about a lack of stiffness in 2x8 10s. There should be little give in that span. Spongy from a lack of sheeting I can buy. Are these on 24” centres?

[Roly]

I’m wondering about a lack of stiffness in 2x8 10s. There should be little give in that span. Spongy from a lack of sheeting I can buy. Are these on 24” centres?

If it is the sheeting, placing a 2x8 between the existing joists instead of sistering may be best.   It made a big difference in a deck I worked on that was at 24" centers.  Roly

[Cooler]

My house has 3" x 8' floor joists and no cross bracing.  I don't know if that is to code or not.  No floor flexing at all.

I think 2" x 8" joists in our area require cross bracing.   Here is an article on that subject.  It is faster, cheaper and easier to add cross bracing than it is to sister up the joists.  But if you add cross bracing and you still need the sistering, you will have to remove the cross bracing first.

https://floortechie.com/floor-joist-cross-bracing/

If you live in an older home that’s over 50 years old, the floor probably bounces and squeaks a lot when you walk on it. This would be because the joists underneath probably lack any additional, lateral support.

Such support comes in the form of wooden blocking, or the more popular cross-bracing. Therefore, renovating the floor framing by installing cross braces will serve to keep the joists rigid, thus minimizing floor bouncing and squeaking.

For a new home, you’ll want to install cross braces during the construction of the floor frame, to avoid the aforementioned floor problem in futures. Basically, cross-bracing your floor joists makes your wood frame floor system stiffer, consequently preventing twisting, deflection, squeaking, sagging, and bouncing.

[carwashguy]

You can use steel strapping 14 guage if I recall.  You start on one side at top end, angled down to the middle of the span, buy strapping with screw holes and screw it on to the joist every couple inches, wrap the band around to the other side and back up to the top on the opposite end.

If you can jack the joist in the middle a 1/8" before adding the strapping it works even better.

Adding 2x6 will not help much with the bounce, deflection is mostly a matter of the width of the joist, not the thickness.  

As as alternative to blocking, you can run 2x4s perpendicular to the joists, 6 ft from each end, screwed in to the bottom of each joist.

My deck with 2x8 joists has a little bounce to it.  I was told a similar fix to WoodworkerTom's steel strap solution.  I was told to glue and screw a 2x4 laid flat against the bottom of the 2x8, parallel not perpendicular.  i was told it would create a I-joist.  The 2x on the bottom inhibits deflection.  i can't tell you if it works or not because I've never had time to do it.  Just throwing the idea out because it is easy to do and cheap.  maybe someone else will let us know if it works or not and save us some $$

[Cooler]

200 Simpson Strong-Tie joists cross braces for under $70.00.

https://www.amazon.com/Simpson-LTB20-200...4463&psc=1

[messmaker]

A Lally  column works great for support and if any jacking is needed for floor leveling, support.

A lally column and a cross brace under the joist for the jack to push on would make things rock solid, but it would intrude into your basement.

[Cooler]

A lally column and a cross brace under the joist for the jack to push on would make things rock solid, but it would intrude into your basement.

Plus most basements have 7 foot ceiling heights so that the space does not get counted as living area and thus affecting your property taxes. 

If you drop a 2" x 8" or a 4" x 8"  cross brace you will be at just 6' 4" height.  Most, but not all people can walk under that without hitting their heads. It is something to consider.

[MstrCarpenter]

If you're going to sister the joists, use 2x8 fir. You can leave one end short so you just have to jack it up if necessary. If you can make them full length it's better. Full length means you'll be rolling them. It's easier to plant the top against the existing joist and beat the bottom over. It's even easier if you rip a 3/4" wide x 1/2" tall triangle off the top, outside and the same at the ends only at the bottom inside.  Glue is good too, but in my experience the glue has already begun to skim over before it's compressed, so it's basically just a spacer at that point. Columns of 3, 12d sinkers spaced a foot or so apart will do basically the same thing. (Really sink them then cinch the points.) When you're done, screw the floor to both joists.

Some comments were made about cross braces or bridging. That's not too easy if the floor sheathing is already in place. A row of solid blocking does the same job better. Just snap a line on the bottoms, mark square line on both sides, cut (square) blocks to fit snugly between (measuring at the top only), and nail in place alternating left/right side of line. To really take advantage of solid blocking you should glue and nail 1 x 4 to the bottoms of the blocking and joists. When you're done, screw the floor to everything.