# what kind & size floor joists needed to span 15 feet?



## Ron6519 (Mar 28, 2007)

You chose the one the Architect/Engineer tells you is appropriate for your situation.
Ron


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## redss (Jul 9, 2009)

What architect/engineer? I have consulted with some general contractors, and they all have different opinions.


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## Gary in WA (Mar 11, 2009)

You need a local Structural Engineer as Ron said, in order to build it safe. Your building department would require this as your family and future occupants are relying on you to build it correctly. Then when you go to sell, you can leave it there. Your Insurance company will cover it if done legally as well. 
Hint..... the General Contractors will use a Structural Engineer to take the liability (your family's safety) off of them. Be safe, G


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## Scuba_Dave (Jan 16, 2009)

To span 17' I went with I-joists to eliminate any possible bounce
They were spec'd out at one size & I went to the next size
Around a 2x10 will work, a 2x12 would be better
Exact size would depend upon species/grade of wood
Is it exactly 15'?


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## redss (Jul 9, 2009)

Well to be exact, it'll be about 178 inches minus the ledger board on each side so 176 inches.


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## Scuba_Dave (Jan 16, 2009)

I like the I-joists myself, I like building above code
If you go to a lumber company (not big box) many (most?) can size them for you
My lumber company even sent the plans out for an engineer review & stamp - free


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## aeridyne (Jul 10, 2009)

Heh, i'm really starting to love everyone saying "structural engineer" for pretty much any question. I've come to grow rather... UN fond of them. Not to knock anyone, I understand their role and importance, and I'm not arguing that, but everyone seems to be WAY too uptight about everything and a simple question goes unanswered, not the case here, as someone did pretty much answer your question and pretty well. There is a little book i got on amazon, it is a flip binder type thing, it's called code check, wonderful book. If it's a 2nd level and you aren't going to be putting elephants up there, i agree that 2x10 or 12 should be fine, of course there might be other factors here and that is assuming you don't have any oddities to your structure, I'd be assuming something very simple, if simple was the case though and you wanted to be extra sure 2x12s at 12" on center would be double overkill, haha.


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## aeridyne (Jul 10, 2009)

Scuba_Dave said:


> I like the I-joists myself, I like building above code
> If you go to a lumber company (not big box) many (most?) can size them for you
> My lumber company even sent the plans out for an engineer review & stamp - free


Who is your lumber company? People around here all act like they've been sued a million times, they don't even want to give you a quote most of the time without blue prints or a structural engineer's stamp, its maddening.


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## Scuba_Dave (Jan 16, 2009)

Actually 2 different lumber companies offered the same thing
One was Cape Cod Lumber, the other was Hingham Lumber
1st has 2 locations, the 2nd only has one location


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## Maintenance 6 (Feb 26, 2008)

If you are planning on a standard 40 pound per square foot live load, then a 2x10 #2 fir joist will span 14'11" on 16" centers (some charts show longer). To stiffen the floor, go up to 2x12. In either case add bridging. That will help to reduce vibration. It does not increase load bearing capability. FWIW, sleeping rooms can use 30 psf live loads in the calculation. In my opinion, engineered joists or LVLs are way overkill for a 15' span unless you are spreading your centers or planning on an excessive load.


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## redss (Jul 9, 2009)

Maintenance 6- what is "bridging"?
Thanks for the replies, everybody.


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## redss (Jul 9, 2009)

Ok thanks to google I now know what bridging is. I assume I only need one one row of bridging in the middle of the 15' span? I am thinking solid bridging would be sufficient rather than cross bridging?


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## Gary in WA (Mar 11, 2009)

Cross bridging is better but solid is faster and easier. Bottom of page 748-749: http://books.google.com/books?id=zj...=X&oi=book_result&ct=result&resnum=1#PPA92,M1

Or metal: http://www.strongtie.com/products/connectors/nca-tb-ltb.asp

Joists blocked at all ends and intermediate supports. 

Joists >2x12 blocked or bridged @ 8' on centers IRC 502.7.1 Under that size, not required. Be safe, G


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## Aggie67 (Dec 20, 2008)

aeridyne said:


> Who is your lumber company? People around here all act like they've been sued a million times, they don't even want to give you a quote most of the time without blue prints or a structural engineer's stamp, its maddening.


That's true in my area as well. Unfortunately, there has been enough bad advice out there that the level of litigation has forced insurance companies to tell all of their clients "don't offer design services, or you will lose your coverage and you will be on your own. Engineers carry insurance for this, let them bear the risk." That is basically the speech you get when you try to get insurance these days. That's not to say that 100% of the advice out there is bad, just that there has been enough on the bad side to compel the general liability carriers to draw a line in the sand and state what they will and will not cover.


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## Carpenter (Jul 11, 2009)

*build it right*

Use 2x10's on 12" centers with 2 courses of solid blocking, which is superior to cross-bridging..... Use 5/8" T & G screwed and glued for best results. Don't forget to create a solid air barrier around the rimboard using DuPont Tyvek. That's what I'd do.


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## Gary in WA (Mar 11, 2009)

To ress, my reason I stated cross- bridging is better than solid, for what it's worth: 

Cross is stronger - it can be wedged into place making a tight connection between joists. This transmits the load more effectively. If there is a 1/16" gap between the block/joist, times 8 block bays= 1/2" of dead space not transmitting. 

More importantly, as the article quoted brought out:

"Cross bridging is considered by far the best, as it *allows the thrust to act parallel to the axis of the strut, and not across the grain *(diagonally) as must be the case where single pieces of boards are used."

I may be nit-picking with engineering facts but now you know my reasoning. How will you be attaching the rims to the middle of the two story studs? Be safe, G


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## Carpenter (Jul 11, 2009)

*Bridging vs. Blocking*

Touche, Mr. Gbar. Your engineering results are very valid; but from my experience, i`ve had to pull the bridging to one side to make it flush to the bottom many times more than once, as the spacing in between joists is rarely uniform regardless of 16" or 12" centers. Unless the spacing in between joists is dead-nuts every time, bridging will not work as it should; meaning that you will have to 'modify' it to work in an odd spacing. The result will be a break in the system and will be as useless as tits on a bull. Blocking, when nailed in 3 spots (top, middle and bottom) and cut-to-fit at the time of installation will never have this problem, creating a solid member through the joisting that will be uninterrupted. I have used both methods over a thousand times each and would never opt to use bridging over blocking.


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