Go Back  DoItYourself.com Community Forums > Interior Improvement Center > Framing, Flooring and Sub-Flooring
Reload this Page >

Do joists 24" OC req. cross braces prior to subfloor install?

Do joists 24" OC req. cross braces prior to subfloor install?


  #1  
Old 03-29-05, 11:03 AM
enetprof
Visiting Guest
Posts: n/a
Do joists 24" OC req. cross braces prior to subfloor install?

Hi Folks,

I just moved into a newly constructed home which has a "bonus room" above the garage. The garage rafters and joists were all designed for this room. It has electricity and HVAC but no floor, walls, etc. since we planned to do that ourselves to shave some cost off of the new house. There is drywall on the garage ceiling, but that is about it for the room. Well, now the time has come for me to finish off that room and I am concerned about the 24" OC joists, and the subflooring flexing across that span.

Can I just put down subflooring on joists spaced 24" apart, or should I put in some cross braces first? By cross braces I mean some 2x4 or 2x6 boards between the joists (in the joist cavities), and flush with the top edge of the joists to help support the subflooring.

Thanks for any assistance!
 
  #2  
Old 03-29-05, 01:25 PM
J
Member
Join Date: Jan 2005
Location: United States
Posts: 455
Received 0 Votes on 0 Posts
1. 24" spacing is a little much for 3/4" plywood - it will make for a very flexible floor.

2. Bridging requirements depend on the size of the joists.

3. Joist size and spacing depend on the span distance.

You say that the joists were designed for the room. How long are the joists and what size are they. If there's no subfloor there now, then it's questionable if the construction was done properly because the plywood subfloor should extend all the way out to the perimeter for shear purposes.

If the joists are properly sized to support the load then you should consider using thicker subloor material such as 5/4 plywood - but it's heavy and would not be easy to carry up and install.
 
  #3  
Old 03-29-05, 04:55 PM
enetprof
Visiting Guest
Posts: n/a
Hi Joe,

The joists are 2x12's and span the width of the rafter (25ft). At about 7 ft from each end of the joist is a 5ft knee wall connecting the joist to the rafter. So, 7 ft of both sides will essentially be unfinshed "attic", and the finished room will only be about 11 ft wide, with 5ft walls at the knee wall, and angling up to just shy of 8 ft at the ceiling. The room is about 20ft long (~10 joists across the length).

The builders did leave the perimeter exposed at the ends for connecting the subfloor onto, so I believe that was done correctly.

Thanks!
 
  #4  
Old 03-29-05, 09:33 PM
J
Member
Join Date: Jan 2005
Location: United States
Posts: 455
Received 0 Votes on 0 Posts
2x12's @ 24"o.c. are not sufficient to support a floor load spanning 25 feet.

You need to take a set of plans to the building department, but from what you've said so far, it isn't going to fly. I have a living room floor that has 2x12's @ 12" o.c. spanning 19'-4" and that's just about minimum for structural purposes.

If the "knee" walls somehow make the Joists/Rafters into a sort of truss (unlikely but possible) it still isn't sufficient to support a floor loading condition. 25' is an extreme span for 2x12's - definitely not code for floors.

Also, it sounds like the average ceiling height in this room would be abou 6-1/2 feet which would also not be acceptable for most building codes. If you were sold this house with a representation that this space could be legally finished as a habitable room then you might have legal grounds to go after the builder.
 
  #5  
Old 03-30-05, 01:38 AM
enetprof
Visiting Guest
Posts: n/a
Hi Joe,

Yes, the knee walls are in fact formally trusses built into the rafter and joists. I likely confused the terms here. So the 25ft span does have support from the rafters/trusses 7ft from both ends of that span, and the center 11ft section is the longest unsupported span of the joist.

However, even with the additional support of the integral trusses, you feel that the 25ft span is suspect for normal floor loads, correct?

The builder is one of the largest in this area, and is known to be very reputable. They have been building houses for 4 generations, so I would be very surprised if they did not design to code. But I will review this with their architect to allow him to clarify the loading and code compliance.

Thanks for your feedback!
 
  #6  
Old 03-30-05, 06:56 AM
J
Member
Join Date: Jan 2005
Location: United States
Posts: 455
Received 0 Votes on 0 Posts
If the trusses are properly designed and constructed it might work. Note that 2x12's should have solid 2x12 blocking at 8' maximums. As I said before, 24" is a pretty wide spacing for floor sheathing. In fact, it's a bit much for drywall ceilings.

I'm sure the Architect can give you some pointers on what you can or can't do.

The average ceiling height is still a question in terms of code.
 
  #7  
Old 03-30-05, 08:51 AM
G
Member
Join Date: Mar 2005
Location: Chesapeake, VA
Posts: 340
Received 0 Votes on 0 Posts
I believe the IRC allows 3/4" T&G Plywood for 24" OC spans, and 1/2" (Type-X) drywall if it's not supporting insulation, or a slightly better thickness (5/8" IIRC) if it is supporting insulation. How it would be in practice is another issue entirely, but considering this is a bonus room over a garage and not a main living space, I don't see that it would be much of an issue. You will need to get a copy of the engineering data on the room-in-truss trusses to find out the design load of the truss; if it's only designed for a 20/10 load, then you're very restricted in use for the room- light storage only, and it probably can't be considered habitable. Ideally, it will be rated 40/10 or better, which allows pretty much unrestricted use for a house.

3/4" Sturdi-Floor is prominantly stamped that it's rated for 24" OC use.

To be a habitable room by IRC, it has to have a minimum of (IIRC) 60 square feet of floor space. And at least half that ceiling height has to be at least 84" tall, and no square footage counting towards that 60' can be less than 5' tall. So, if at least 30 square feet of the room has a ceiling height of 7' or more, and the room meets the other HVAC, glass area and eletrical requirements, it can be considered "habitable". (At least that's how I read it... If I'm interpreting the IRC wrong, someone please let me know, because I'm designing the new addition to my house along these lines!)
 
  #8  
Old 03-30-05, 09:48 AM
J
Member
Join Date: Jan 2005
Location: United States
Posts: 455
Received 0 Votes on 0 Posts
You might get in trouble with the 60 sq.ft. / 50% rule.

For as long as I can remember this meant that a habitable room had to be at least 60 sq. ft. and that at least 50% of the ACTUAL area had to have at least a 84" clear height.

So if the room is only 60 sq.ft. then only 30 sq.ft. would need a 84" clear height. But if the room is 140 sq. ft.then 70 sq.ft. would need a 84" clear height.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: