# Acceptable joist depth for a second set of ceiling-only joists

Background

I'm installing a new ceiling in the kitchen of the bottom unit of a multi-family home. The house is quite old and uses 2x8 (actually 2"x7.5", not 1.5"x7.5", due to era of lumber) over a 12 foot span, which experience more deflection than is ideal but is certainly acceptable. This does create more transfer of impact noise (foot steps, dropping things, etc). I have isolation clips for the ceiling furring (http://www.kineticsnoise.com/arch/isomax.html) but, now that we're not installing recessed lighting, I'm considering adding a second, separate set of joists to hang the ceiling from.

Problem

What is the minimum joist depth and maximum spacing needed for new joists that support only a ceiling (since there are separate existing joists carrying the load of the floor)?

My calculations for two layers of 5/8" drywall/blueboard and some mass-loaded vinyl come to just over 5 lbs/sqft. There will be additional weight in veneer plaster and a trivial amount in light fixtures. All span calculators I can find accept a minimum of 10 lbs/sqft of live load and an additional 10 lbs/sqft of dead load. This situation seems to me to be around 6-7 lbs/sqft of dead load and essentially no live load over a 12 ft span. Trying to figure out if I can get away with 2x4s or need 2x6s or can cut 2x6s down to 2x5s, etc and whether I can space them every 24" or need to stick with 16". Not so much a matter of cost as of space for other sound proofing materials and considerations. Deflection concern in this case is to avoid veneer plaster cracking.

To clarify, this image is roughly what I'm planning for, where the red joists are to be added and whose dimensions I'm trying to determine.

• To be clear, you're currently having problems with hearing footsteps and objects hitting the ground and that's what you're looking to reduce? Or is it that you're worried that you'll have to re-enforce he existing joist because of the extra 2.25lbs/sq/ft? Commented Jul 8, 2016 at 0:11
• If you want less noise move upstairs :) other than that to just hang Sheetrock I would use 2x6 over a 12 foot span. Older homes have much better lumber than what is available today. Even if you offset the new wood so no contact is made some sound will still be there. Even if the upper floor was supported with TJI's you would still hear dropped items, maybe not people walking as much. The only way I can see to reduce the noise is with 2x? Or TJI's that have no contact with the upper floor and insulation. Commented Jul 8, 2016 at 0:23
• @Chris Yes, reduction of impact noise as you say. This is about the ceiling weight though since I'm talking about adding a new, separate set of joists (staggered between the existing joists that support the floor above) to carry the ceiling weight only and not attaching the ceiling to the existing joists. Commented Jul 8, 2016 at 0:25
• Consider the case of someone looking at your ceiling in 10 or 20 years and not suspecting it is a light duty design. They might reasonably consider hanging their 500 pound cast iron pot-and-pan rack from it. Be careful not to cut safety margins if you can. Commented Jul 8, 2016 at 0:36
• Perhaps you should do both. Add joists to eliminate the bounce, and then suspend a ceiling from them with the clips. IME, (cheap) attempts at sound dampening are never "effective" but every little bit helps. - A couple thousand dollars in spray foam ought to do the trick; no extra ceiling needed ;) Commented Jul 8, 2016 at 2:53

Old I know but worth it: Lee Stan has the right idea but, To answer your question more directly - ****The joists should be AT LEAST 2x6 and can be mounted or hung lower than the rafters using joist hangers.

Note the joist width is entirely dependent on your local codes for your span. California is different than others for example

Also for 5/8 double layer drywall with essentially staggered ceiling Green Glue compound performs better than MLV and saves weight and money** **

• Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, props for taking our tour before posting; few newbies do. Commented Jun 22, 2020 at 2:14

You’re dealing with “impact sound control “ not “airborne sound control “. Stacking 2 layers of 5/8” gypsum board on “hat channels “ is not the best way to control impact sound.

### First

We control impact sound (walking in high heels, dropping objects, etc.) at the source by using

1. soft materials to walk on, (carpet, cushioned vinyl flooring, etc.), and
2. isolate the underlayment from the subfloor and joists with sound absorbing fiber board. If you don’t have access to the floor above your unit, then adding sound control below is difficult...but not impossible.

### Second

Isolating your ceiling from the floor above is correct, but using Resilient Channels, not hat channels. The transfer of impact noises down to your space needs to be isolated....that’s why adding ceiling material to the bottom of the floor joists does not work AND WHY using hat channels does not work as well as Resilient Channels.

Yes, the new ceiling joists (isolated from the floor joist above) helps isolate the ceiling from the floor above, but impact noises travels through the framing. Hat channels do not isolate the impact noise.

1. two layers of 5/8” gypsum board is 6.25 psf,
2. hat channels is about .5 psf,
3. new 2x6 joists at 16” oc is 1.5 psf,

1. rock wool insulation at 4” at 2 psf,
2. 1/2” fiber sound board at 2 psf and misc. (lights, etc.) at 1 psf...

for a total dead load of 13.25 psf. (Even the crappiest grade of 2x6 at 16” oc will support that for 12’)

## I’d recommend:

1. isolating the ceiling joists from the wall by installing sound board between the new ceiling joists and the wall,
2. installing 4” of rock wool in the existing floor joists, in the stud spaces and on top of the new ceiling,
3. don’t install your light in the ceiling (cutting s hole for a wire allows sound into your space,
4. install 1/2” sound deadening board on top of your new ceiling joists (if possible,
5. install 5/8” gypsum board on RC -1 channels on gypsum board on RC-1 channels on the 2x6 ceiling joists,
6. caulk the perimeter with sound control caulk...leave the 1/4” spacing at the wall as per your insert you referenced.

Yes, two layers of gypsum board is better than one, and separating the layers with RC channels is best.

After reading more of what you want I would use 2x6" for the span and 16" centers with hangers this would be enough to support the ceiling and even recessed lighting like this. hung at the bottom this will provide the isolation you are looking for. We use these hangers for supporting overhangs with sheeting and roofing all the time and for decks. I would hand select the boards to get the best quality #2 or better but if you can afford #1's because today's lumber is not of the quality it was 20 years ago because of the smaller trees.

• Thanks for your consideration of this. I agree that 2x6 at 16" will definitely work. Looking to understand what the minimum height (possibly less than 6") and maximum spacing (possibly 24") is and why. Commented Jul 8, 2016 at 20:06
• You may be able to go to 24" centers but I have had trouble with 1/2" sheet rock on 24's 5/8" works better but is much heavier to hang. Considering it will only be 1 or 2 more boards going with 16" OC the cost and ease of hanging the Sheetrock would have me going with 16" OC. Remember you will have a butt seam in the center. I usually put a nailer up for this always on 24" but with a very cheep job on 16's the center nailer is left out. Commented Jul 8, 2016 at 20:55
• Re:16" OC, I can do that for weight, but the steel furring is attached to the joists via clips only every 3-4 ft along the furring and there is 2 ft between the furring strips. Shown here as the 'staggered layout' link. With 16 O.C. can I do 2x5 or 2x4? Do you know of a resource, reference, or standard calculation that can be done to find this? Commented Jul 8, 2016 at 21:11

You can get a very decent sound reduction by using Roxul insulation and being careful about how it is cut and installed so that there are no holes or channels for sound to travel through. Just stuffing the ceiling with Roxul will give you a pretty good idea how much noise reduction you can expect.

I think that two sheets of 5/8" drywall are overkill and not as effective as separate sheets or a single sheet on resilient channel (even the cheaper variety of channel, but you have to be careful with your screws so as not to accidentally provide a solid vibration path)

Having separate joists will help. A lot of inter-party walls in condos are double walled, so two separate sets of 2x4" studs with insulation and 5/8" drywall on the occupied-space side of the walls. These are very good as long as you don't have outlets positioned back to back on the inter-party walls.

I've seen a 2x4 drop ceiling with a 14' span and 1/2" drywall below a bedroom, and that seemed fine, but the deflection calculators online tend to indicate that 2x4's might be marginal, and 2x6 should be used, especially if they are only hung at the ends and no support along the span, as might be needed to reduce sound transfer. Either 16"or 24" centers, whichever is more convenient.

If it were my project would probably do Roxul against the subfloor above, hang new 2"x6" on 24" with joist hangers and bracing between the new joists, then resilient channel and 5/8" or 1/2" drywall. Electrical boxes can be covered by Roxul as well, or more sophisticated sound deadening membrane. Also, any penetrations in the floor above for pipes should be sealed around with something like a latex caulk.

If I was going to make it cheaper, I would just put the channel on the bottom of the existing joists, and stuff the cavity with Roxul.

(Roxul is way better than fibreglass at sound dampening)