Room within a room, 2 by 4 for ceiling

braverichard

Finishing a basement in a house with all hardwood floors upstairs and would like to do some basic level soundproofing especially against the noise of people walking on those hardwood floors upstairs.

I've done full soundproofing of basements before (using clips, green glue, multiple layers of 5/8" drywall, etc) but this time I just want to insulate in between the joists and decouple the basement ceiling and walls from the upstairs floor.

My question is: can 2 by 4s handle the weight of the drywall? I'm figuring they should:

-Regular 1/2" drywall (which I intend to use for the ceiling) weighs 57 pounds for a 4 by 8 sheet. The drywall would have knock down texture which I'm just guessing would add another 5 pounds or so to its weight
-My span for each 2 by 4 will be 13' 3"
-The ceiling 2 by 4s will bear on the wall 2 by 4s which will also be decoupled, however this won't be a true room in room layout because the walls in the basement will be attached to the concrete floor
-The 2 by 4s would be done 16 o.c.

What do you guys think? Every load span website I have found won't allow me to calculate my span for 2 by 4s with no live load so they have not been useful to me. I'm still looking though.

tony.g

The only load the 2x4s will be carrying will be the weight of the plasterboard plus the weight of the joists themselves.
Your p/b'd plus finish will be about 2lbs/sq ft; each joist will weigh about 14lbs, and will support 17.6 sq ft of ceiling.
All this means that each joist will be supporting about 50lbs. With that load on that span, the stress in the timber will be very roughly one-third the maximum allowable (depending on species) so you will be fine on that score.

What you need to watch is deflection. Depending on how firmly (or not) the joists are fixed at the ends, they will deflect somewhere between 0.25" and 0.5". The bend will not be significant structurally and the ceiling won't collapse, but might be noticeable from certain angles.

kwikfishron

I wouldn't do it...Lumber's cheap right now, use 2x6.

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mae-ling

Yep 2x4 should carry it but will sag a lot, I would go 2x6

braverichard

Thanks for the response Tony.g. I actually just found this website which allows me to do the calculation with a live load of 0: https://courses.cit.cornell.edu/arch...8.1/index.html

I get the deflection issue - that is one thing ceiling texture can do a fairly good job of hiding.

kwikfishron, use 2x6 just because lumber is cheap? If I am within one third of allowable load on a 2x4 it just seems like the 2x6 would be overkill regardless of cost. And when you factor in cost, even if lumber is cheap 2x4 will be cheaper than 2x6. Granted I know I will get less deflection with 2x6s, but it looks like the 2x4 should be fine.

braverichard

I just checked prices, 2x4 cost 65% what 2x6s cost in my area, might be able to just do 2x6s then.

hand drive

2x6 no doubt. Anytime you can eliminate planned deflection is a plus and overkill for some people is not enough for others

5/8" sheetrock on the ceiling is a good option too..

braverichard

Yep, I will actually end up coming out ahead if I do 2x6s 24 o.c. with 5/8" drywall. I can't span regular 1/2" drywall 24 o.c. as it will sag (the new USG Ultralight 1/2" drywall can be spanned 24 o.c. however it is lighter than regular drywall (44 vs. 57 pounds) and I need more mass for soundproofing not less).

5/8" drywall is 70 pounds for a 4x8 sheet. How much will 2x6s 24 o.c. deflect with them?

tony.g

With the 6x2 @ 24" c/s, supporting that board, deflection will be down to about 0.2", which is nothing (a lot of the timber you buy nowadays is bent more than that to start with).

(Calcs attached; my invoice for SE services is in the post).

Attached Thumbnails

 

braverichard

Awesome work. Nice to see the formula, I didn't know it before. Thanks.