Joist span?

Eggman · Post by **Eggman** » Tue Jan 04, 2005 7:46 am

Happy NY everyone!!

I am now working on my control room. My live room worked out great with isolation that far exceeded my expectations.

I have a quick question regarding floor load and joist spans. I would appreciate some input. My room is in the basement of a 1916 house. The control room will be under the living room. The joists are 2x8 and are 24" oc. The span is 10'6". Like in my tracking room, I plan to add 3 layers of sheetrock between joists (1/2 " 5/8" 5/8") then RC and 2 more layers of 5/8".

QUESTION...Can I hang 5 layers of sheetrock on 2x8 joists spaced at 24" spanning 10'6" and still be within allowable load limits. My floor above is 3/4 T&G subfloor with 3/4 hardwood on top. Right now, it feels very solid with zero deflection.

QUESTION..If I need to beef up the floor, could I just sister a 2x6 to each joist?

As always, thanks for the help.

Doug

z60611 · Post by **z60611** » Tue Jan 04, 2005 8:55 am

Eggman:

I don't know about you, but when I asked my Building Inspector how much weight I could add to my 10' span 2x8 basement joists on 16" centers with a single layer of T&G above them he said two layers of 1/2" drywall max.

I don't know about sistering the joists, but room-in-a-room is predictable.
But for 5 layers on the ceiling, you might want to have a look at Paul Woodlock's room. Not just the ceiling construction, but also his floating floor and HVAC and doors.

Eggman · Post by **Eggman** » Tue Jan 04, 2005 11:00 am

Thanks for your thoughts.

just to clarify, three of the five sheetrock layers will be up against the subfloor, between the joists, held on by cleats. Two will be hung from the joists on the RC.

Can anyone calculate the load and determine if my joist system can handle it?

Thanks

Doug

Jas · Post by **Jas** » Tue Jan 04, 2005 12:42 pm

Hi Doug,
In Australia we have a "Timber framing manual" This gives you all the legal load limits for any type of timber, like Joists, rafters, etc. It includes their load catagory (type of timber), size, spacing, etc. I'm sure there would be an eqivalent in your neck of the woods. All architects, structural engineers and builders/carpenters will have a copy of this.
Anything you use from this guide can be safely calculated without an engineer. (building inspector will double check span load capacity anyhow). However, if you want to modify existing load bearing systems, like beefing up your floor with a different size of timber, you will need a structural engineer. Obviously, estimating this kind of thing can be very dangerous. You also don't want to waste money on materials that an engineer may tell you are not needed. - Think of the consultation cost as insurance!
Having said that, I did a quick check here and it looks to me like you will exceed/overload your floors limit with this amount of weight being added. I could work this out more acurately for you but my answer could only comply to Australian building regs, not really much use for you.

Cheers,
Jas

z60611 · Post by **z60611** » Tue Jan 04, 2005 1:35 pm

Eggman:

There are some on line joist span tables

The best two I think are:
http://www.awc.org/calculators/span/cal ... cstyle.asp
and
http://www.awc.org/calculators/span/rev ... secalc.asp

Although these were interesting:
http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/
http://www.efunda.com/formulae/solid_me ... niformload

And for straight tables:
http://www.southernpine.com/spantables.shtml
http://www.ci.lincoln.ne.us/city/build/home/joist.htm

But the tricky part, and why you need a structural engineer and not me for this, is trying to figure out what the live load and dead load on an existing floor are. Adding up the sum of (weight of the joists, weight of the floorboards, weight of the carpet, weight of your furniture) is easy. But what about a party -- what's the live load when you have fourty people in a 10'x10' room dancing to disco? What's a realistic load when you stack everything you own on the living room floor in boxes as tall as your shoulders when you are moving? What if the next occupant has a aquarium fetish ?

My joists are about the same span as yours are, and the same size joist, but are closer together (16" oc), yet I'm only allowed to add 2 layers of 1/2" gypusm. Therefore your limit should be less than that. I would guess that it doesn't matter much if anything if the load is at the top or the bottom of the joists.

Eggman · Post by **Eggman** » Wed Jan 05, 2005 11:26 am

Z60611 & Jas.

Thank you for the info. It is clear I need to beef up my joist system before adding any more weight.

Are there any tables that will give me allowable spans on doubled up 2x8s? I think I would like to see if that could be a reasonable solution.

Steve...could you add your $.02?

Thanks,

Doug

knightfly · Post by **knightfly** » Sat Jan 08, 2005 6:43 am

Sistering your joists might work; I'm not positive, but I think you'd be safe figuring for 4x8's when using doubled 2x8's - you might check out the demo of this

http://www.builderswebsource.com/softwa ... amchek.htm

I've not played with it more than a few minutes, but it's different than the typical span tables that only do 2x lumber.

2x8's on 24's will barely span 10 feet at 40/10, and the dead load (the 10 part) INCLUDES frame as well as all building materials, furniture, etc - so I agree with Z's assessments on caution.

I think that if you need max isolation you should do a separate ceiling frame if possible; what's your existing ceiling height, and do your existing joists have cross braces in place? These are necessary, so you'd need to work around them... Steve

PhiloBeddoe · Post by **PhiloBeddoe** » Sat Jan 08, 2005 8:32 am

Steve,

I'm not sure if this is the place to make suggestions, but I think it would be nice to add a warning statement to the ceiling construction section of the SAE manual cautioning to check joist capacities before adding several layers of drywall. Maybe a caution about the load imposed by a floating floor/room should be added as well.

Not everyone is as wise as Doug is to ask the question before proceeding.

Doug,

I am very nervously undertaking my first attempt at a studio, so I really enjoy hearing success stories like yours. Best of luck with the rest of the project.

Marc

knightfly · Post by **knightfly** » Sat Jan 08, 2005 9:10 am

Hey Marc - good idea, except for one thing; John SOLD that site lock, stock and barrel to SAE some time back, as I understand it; I'm not sure who (if anyone) to contact re changes, or I would have had them change the Helmholtz calculator too (the one on the SAE site is wrong, as are MOST of the ones on the web) -

I try to caution people who post here to ASK FIRST, and for the most part they do; I can't save the world from stupidity though, and in my mind building things that will be ABOVE you without knowing for sure that they will STAY there qualifies for nomination to the Darwin awards... Steve

Eggman · Post by **Eggman** » Sat Jan 08, 2005 12:15 pm

Thanks, I appreciate your help on this one.

I am happy with the isolation I got from the sheetrock between the joists,RC and 2 more layers method that I used on the other room. I think I will built this room the same way, as opposed to interleaving new ceiling joists between the old. By doubling the 2x8, I figure I might as well take advantage of the extra beef it will give my living room floor.

My wife gave me a camera for Christmas, so I will try to document the build of this room and post on the forum.

Doug

knightfly · Post by **knightfly** » Sat Jan 08, 2005 12:51 pm

2x8 #2 and better, Doug fir joists on 12" centers will span up to 11'8" and still give 60/20 loading; this should be similar to putting DOUBLE 2x8's on 24" centers - although my gut feel is that actually PUTTING them on 12" centers (instead of doubling them) would lower the resonance of the upper floor, which would improve the TL a few dB - hmmm... the downside is smaller cavities to insulate, and if using rigid insulation, twice as many pieces to trim to size.

Tough call... Steve

z60611 · Post by **z60611** » Sat Jan 08, 2005 1:08 pm

Eggman:

Pictures will tell the tale.

However, speaking for speaking's sake, assuming there is no cross bracing (silly of me), if you go room-in-a-room, with 2x6's in the space between the existing joists, then that makes the load calcs a lot simpler, and certainly improves TL a couple of db due to decoupling. If one 2x6 at 24"oc isn't enough you could use two between each set of existing joists, either 2x6's together or separated by 8" or 12".

knightfly · Post by **knightfly** » Sat Jan 08, 2005 11:03 pm

Please don't remove, or recommend removing, any cross-bracing; it's there to keep the floor joists from twisting and weakening the structure... Steve

z60611 · Post by **z60611** » Sun Jan 09, 2005 3:51 am

knightfly:
I didn't mean to imply that one could or should remove cross bracing. I don't think I did.
Cross bracing does two good things:
a) as you said, it prevents twisting, and twisting would weaken the structure
b) it distributes the live load (such as walking when weight is all on one foot) from one joist to the two on either side.

knightfly · Post by **knightfly** » Sun Jan 09, 2005 4:05 pm

Yeah Bob, I just have to make sure that EVERYONE keeps this in mind; if I didn't specifically SAY this was a no-no, there would be at least one "lurker" who went away and removed the cross braces because I didn't specifically TELL everyone NOT to, and 6 years down the road the floor caves and takes out an innocent kid or something; it's just easier for me to sleep knowing I WARNED everyone reading this...

Just in case you think I'm being overly cautious, re-read this

http://www.johnlsayers.com/phpBB2/viewt ... c&start=60

Check out the pix, then read the next few posts (the part about not going on the roof til the frame has siding fastened on...

This could have been a LOT worse, and wouldn't have happend AT ALL had I not ASS-U-ME'd that "everybody knows that" -

Anyway, after that one I try to second-guess everyone's interpretation of what I say, and then say it in a way that is difficult to mis-enterpret (hence the long-windedness

Speaking of which, shut up Steve (OK, it's time for bed anyway) So which part of "shut up" didn't you get? I'm sorry, I can't hear you; I'm asleep...