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yellowkid 03-20-2013 10:36 AM

floor joist span
I'm starting to get in to the details of an addition I'm planning and have a question on the floor joists and maximum span. its 2-story and 18X18' -- upper story is a bedroom with closets and downstairs will be bath/laundry/mudroom with partition walls. crawlspace foundation. when i look at the tables for max span it looks like I can do 18' with 2X12 doug fir joists and no piers/bearing walls etc. And possibly 2X10 for the bedroom? But i look to you guys who have experience with this -- is it a good idea? want it to be solid! thanks as always this place is so helpful, and so much quicker than my local inspector! jp

cleveman 03-20-2013 09:33 PM

You can't span over 16' with a 2x10 16" oc.

Sounds like the 18' is doable with 2x12's. If you want a stiffer floor, use solid blocking 6' in (2 sets). Or space 12" oc.

Now that I think about it, look into 2x10's with 12" spacing and see if you can span 18' with them.

ddawg16 03-20-2013 09:38 PM

You should be good with 12"OC using the blocking as cleveman suggested....

And if I may suggest....use 1 1/8" T&G for the subfloor....It's what I did.....does a great job of making things stiffer...

GBrackins 03-20-2013 10:44 PM

2x12 at 12" O.C. for the lower level would give you about a 1/2" of deflection in the joist based upon 40 psf live load, the second floor with 30 psf live load using 2x12 at 16" again would give you about a 1/2" deflection. This is based upon douglas-fir #2 joists. To use 2x10 on the first or second floors would require a spacing of 8' o.c. and would have more deflection than the 2x12.

an option would be to use I-joists or floor trusses.

Good luck!

Nailbags 03-20-2013 11:18 PM

Go with engeineered joists save money and can span alot more. Pluse no warping no twisting cracking and it makes the floor quiter too.

yellowkid 03-21-2013 09:00 AM

guys great thanks for the thinking -- had not thought of either the I-joists, tighter spacing or blocking as options and never occurred to me that the T&G subfloor would make a difference in stiffness. stuff that you don't get to no matter how many books you read. really appreciated. I'm sure I'll be back with more as I get in to the working drawings.

shazapple 03-21-2013 09:37 AM

Why not have a built up beam or lvl run down the middle and use smaller joists? A 3ply built up beam would only require one post. Ask the guy you are getting your trusses from if he can specify an lvl that will span the whole distance. 18' 2x12 are a pile of money, but you'd only need a 2x6 or 2x8 if you used a beam. Engineered josits would probalby be less expensive than the 2x12, but more than a floor with a beam. I'd definitely use a beam for the 1st floor, no reason to span a crawlspace with such expensive lumber.

jagans 03-21-2013 11:08 AM

I completely agree with No. 7 WHy in the world would you spend all that money to eliminate a center post or even 2 in a crawl space????? 2 x 8's would give you more useable height in the crawl space, for the cost of one or two supports that could be as simple as stacked flue block filled with concrete???? You also do not need more than 3/4 inch T&G sub flooring.

18 foot anything in demensional lumber is considered premium with cost comensurate to "Premium" Think about the trees needed to produce such lumber.

yellowkid 03-22-2013 08:30 AM

jagans thx thats the kind of straight talk that i come here for -- i guess the point is just because you can doesnt mean you should and this is the kind of knowledge that only comes with experience. agree no great need for an "open" crawlspace! but i am looking for an open 1st floor plan which would get me back to premium 2X10 or 2X12 for the second floor joists or I-joists which would be worth the expense. given the lighter load requirements for a bedroom 2X10 should work tho? all of this will obviously give me back some room as well. thx again.

GBrackins 03-22-2013 03:39 PM


Originally Posted by yellowkid (Post 1142817)
given the lighter load requirements for a bedroom 2X10 should work tho? all of this will obviously give me back some room as well. thx again.

see my post #4 above for doug-fir #2 floor joist spans. Bedrooms are to be designed for 30 pounds per square foot live load minimum. I typically recommend designing all floors for at least 40 psf.

Good luck!

lylsutherland 04-03-2013 07:10 PM

Check out the IBS2000 at They have a premanufactured bridging product that will give you a stiff floor

AndyGump 04-03-2013 10:32 PM

Get someone to design the structural for you, you will save time and money and headaches.


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