Replies

1. 
   PedroTheMule | Mar 31, 2009 06:51am | #1

   Hi cirino,

   More 2x8's will probably still have to much deflection.

   I bought a house that had 2x8's spanning 16' 12" OC.....It was like a trampoline.

   I cut 3/4 plywood the same depth as the 2x8's, used yellow glue dispensed on a roller head which evenly coated the plywood and stood it up and topped the edge with poly subfloor glue, then slipped it in and up against the 2x8 and subflooring. Used drywall screws to hold it snugly in place until the glue dried....yes I left the screws in after setup. The polyglue kept the plywood from squeaking. As I did each one, I used a bottle jack to raise each 2x8 1/2" at the center....this created an artificial crown.....did one floor joist per weekend for months.....only 20" of clearance in the crawlspace....for a diy project that was about as fast as I could stand.

   I applied the plywood to both sides of each 2x8 and staggered the joints. Probably unnecessary but I gorilla glued the end joints together.

   It turned out to be the sturdiest floor in the house....much sturdier than the 2x10's 14' span 16" OC.

   Depth is always better than width in this case but my fix of an existing floor with limited clearance was well worth the effort and expense.

   I had another floor that needed extreme rigidity.....I jacked 2x8's under existing 2x10's....had a high basement ceiling to work with....I then added the same sistered plywood as described above.....it felt like is had less bounce than a concrete slab on grade....just my perspective.

   Pedro the Mule - love a sturdy floor below my hoofs

1. 
   JTC1 | Mar 31, 2009 04:17pm | #5

   Plywood sisters...... have done same to stiffen floor joists for tile installs.

   Someone once told me that plywood on edge has 2x the stiffness of dimensional lumber.

   Don't know if that is 100% true, but the plywood sisters you described have worked for me. Might be the jacking in a crown is what does it - I just know it stiffens up a floor.

   Jim Never underestimate the value of a sharp pencil or good light.

1. 
   wane | Mar 31, 2009 06:45pm | #6

   all, this is new construction, why would you sister something that doesn't yet exist ...

   Edited 3/31/2009 11:46 am ET by wane

1. 
   Piffin | Mar 31, 2009 07:01pm | #7

   I wouldn't worry about losing 2" in new cost, but 2x10 is still minimal for a 16' span. I would go with LVLs, but that adds a lot of dollars so it is up to the bill payer whether it is worth the money. Adding strapping at bottom can make for a diaphragm sandwich to take a lot of the bounce out.But this is all off the books since it exceeds the normal charts and tables. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

2. 
   JTC1 | Mar 31, 2009 08:50pm | #8

   ...new construction.....

   How'd I miss that in the OP?

   I would not sister in new, but the plywood sisters seem to work well in existing.

   LVL, I Joists, maybe even floor trusses for new. Kind of depends on what is going on that 2nd floor.

   Jim Never underestimate the value of a sharp pencil or good light.

2. 
   Piffin | Mar 31, 2009 02:03pm | #2

   You just lost your way is all

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=118394.9

    

    

   Welcome to the
   Taunton University of Knowledge FHB Campus at Breaktime.
    where ...
   Excellence is its own reward!

3. 
   jimAKAblue | Mar 31, 2009 03:12pm | #3

   I would use some form of engineered joists. You might find that 7 1/4" microlams would work for you as joist. You might have to double some, or all  of them to create the floor system of your dreams..

1. 
   wane | Mar 31, 2009 03:19pm | #4

   agreed, engineered floor joists, or as you said, you need to run a beam down the middle to break the span.  If you do this, make sure you set the joists higher than the beam so when they dry out they don't shrink below the beam and you get a ridge telegraphing thru the subfloor .. my pref would be engineered, otherwise it's real hard to predict where the floor levels will wind up ..

4. 
   Davo304 | Apr 01, 2009 07:43am | #9

   Not for nothing...but according to my book on span charts and calculations, unless you are going to use SELECT grade Hem-Fir , you cannot  safely span 16FT using  2X10s on 16 inch centers. None of the other species listed, including Spruce-Pine-Fir, Doug Fir,or Northern Species group could meet that span.  And, Hem-Fir maxed out at 16Ft-9....which just barely makes it and in my book is really no good either. Also, my area lumber yard doesn't carry SELECT grade. No1/No2 grade is best I can get.  That's typical just about everywhere.

   Now, if your 2X10s are going to be placed 12 inch OC, then using No1/No2 grade will yield you the following results...

                                                      SPF = 17-3  

                                                     Doug Fir =17-6

                                                      Hem-Fir = 18-0

                                                      Northn Species = 14-9

    

   Looks like engineered lumber and/or beam to break up midspan is the definate way to go.

    

   Davo

    

1. 
   JohnFinn | Apr 01, 2009 03:23pm | #10

   Depends on the floor loading. For second floor bedroom loads using 30LL/12DL 2x10's will safely span 16'-0" but will deflect over 1/2" (No.2 SPF).

1. 
   cirino | Apr 01, 2009 04:43pm | #12

   Hey JohnFinn, Deflection is a problem unless your building a trampoline.

   Looks like more wood is the only real option here.

   Thanks for your help!

   cirino

2. 
   Davo304 | Apr 02, 2009 09:40am | #14

   The 30LL (30psf for "Live Loading") and 12DL (12psf for "Dead Loading") is rated for "Sleeping rooms and Attics".

   I never use this loading. I use a minimum of 40LL and 10DL which falls under the category of "All rooms except sleeping rooms and attics."

   The span calcs I gave was based on that loading configuration.

   Although a 1/2 inch deflection in 16ft may be permissable, I'd rather opt for a stiffer floor.

    

   Davo

1. 
   JohnFinn | Apr 02, 2009 07:16pm | #16

   The span calcs I gave was based on that loading configuration.Although a 1/2 inch deflection in 16ft may be permissable, I'd rather opt for a stiffer floor. I figured as much. I guess my point was it may be possible to use 2x10's in this instance, dependent upon what the floor loading is. I too would always opt for the stiffer floor with the least amount of deflection given the choice, and would probably frame this with
   9.5"/230's @ 16" o.c.

2. 
   cirino | Apr 01, 2009 04:39pm | #11

   Hey Davo304, Thanks again this info is the most helpful yet!

   cirino

1. 
   Davo304 | Apr 02, 2009 09:22am | #13

   Here is a pretty good website for checking span loads. Log onto:

   http://www.cwc.ca

   When this homepage pops up, look for Span-Calc and click on it. This is an online program that will tell you the permissible spans for floor joists, rafters, headers, etc. You just plug in your  lumber dimensions and Span-Calc will compute you the answers.

   This website. cwc stands for Canadian Wood Council. I bought their book titled U.S. Span Book for Major Lumber Species. I paid $6.95 for it back in 2001. It has helped me a lot.

   Davo

5. 
   sullivanjh | Apr 02, 2009 05:47pm | #15

   we just finished a 21 by 21 addition using TJI engineered joists and I can tell you that when they are covered with 3/4 in Advantech you will not get any deflection. The TJI's let us clear span the entire 21' and other than being somewhat awkward to maneuver, they are super.

1. 
   Piffin | Apr 02, 2009 07:45pm | #17

   are you saying you can clear span 16' with a 7-1/4" wood joist? 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

1. 
   BamaTom | Apr 02, 2009 08:16pm | #18

   Could someone please clarify what is being referred to when "Span" is mentioned?  The original poster says his addition is 16' x 16', presumably out to out of studs.  Is the span, therefore, 16', or is it the 15'-5" clear space inside to inside of studs?  This has always confused me.  Help please?

1. 
   Piffin | Apr 02, 2009 08:37pm | #19

   The clear space, but lots of times is rounded up to the full length for a safety cushion.In this case, the suggestions that will meet minimal code with allowable deflection can work, but still be unsatisfactory because of vibrations and bounce. generally it takes more depth to reduce vibration. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!