Ceiling Support

[bloeff]

Hi all,

I'm converting a mobile home bedroom into a recording studio. I'm building a room within a room for sound isolation. Right now, I'm down to studs and subfloor.

My plan is to attach 2x4s to existing top plates and studs at opposite ends of the room to act as ceiling rim joists for the new lower ceiling. I'm going to run either 4x4s or sistered 2x4s to act as ceiling joists with each end hanging in a 4x4 joist hanger. Which would be better?

The joists will be perpendicular to existing trusses. The span is 13 feet. At midpoint of span the joists will be attached to what appears to be a 2x2 that is part of an existing truss to prevent sagging. I will also cross-brace all joists to improve rigidity.

Two 1/2" layers of drywall will be hung on the joists. I would go with bigger lumber for joists, but I'm already cramped for headroom (floor will also be raised).

I couldn't find much data on such a setup. Will this structure support the drywall? How much of the load will the center truss bear? Will it be excessive?

To handle all the extra weight of the room, I will install perimeter blocking under all the outriggers under the room and also block floor joists under walls running parallel to floor joists.

Any advice would be greatly appreciated!

Thanks,
Bruce

[Kelly_Bramble]

Look at local building codes - and build accordingly.

[Hudson]

Using this site to get engineered plans puts members in a bad position because we cannot know all of the conditions in a construction project from your brief question. So this is not an engineering recommendation. It is a description of a couple of the steps that might be taken during the evaluation of your project. It will be up to you and whomever you hire to make an appropriate evaluation.

An old rule of thumb, (See page 4.5, Building Construction Illustrated by Francis D. K. Ching), is that the permissible joist span = 24 x joist depth. Now this will work for a lot of normal floors and ceilings in homes. We note that 16” joist centers were common when this rule in common use. Today there are readily available joist tables on the internet.


Using the old rule and your 13 feet, you’d pick a joist with 6.5” minimum depth or the next biggest size, a 2 x 8.

Too deep and too expensive you say? And probably over kill as part of the roof loads have been handled in the design of the existing mobile home.

The roof structure of your mobile home was probably designed to resist snow loads normal for the area in which it was sold. It also held up some kind of ceiling materials originally. We can ignore the roof loads then? What is needed for the dry wall?

We could approach it from another direction. Joists are usually sized based on two things, allowable deflection and allowable bending stress. Typical limits are 1/360^th of the span for deflection and about 1200 psi stress. Usually at least a 40 lb load per square foot is allowed and often more for a residential floor.


Dry wall weight can vary but let’s say one ½” piece is about 2 lbs per square foot. Two will be 4 lbs per square foot. That is 1/10^th of a typical live load joist design limit.

Deflection varies with the cube of the depth and stress with the square.

As a start, let’s compare the rule of thumb 6.5” joist depth to the 3.5” depth of a 2 x 4:

(6.5/3.5)^3 = 6.4 So, for the 40 lb/sqft load the deflection is roughly (we used a rule of thumb remember) 6.4 times as large in a 2 x 4 but your load is only at tenth as large as large as the 40 lb/sqft limit so the deflection will be roughly 6.4/10 or 64% of the customary limit based on these assumptions. Again, know that rules of thumb do not give precise deflection and stress predictions so the 64% has a considerable margin for error.


While adding the 2 x 4’s doesn’t look bad initially, there is no guarantee or warranty and no evaluation of the support structure. Your spacing was not specified. The design limits vary for different types and quality of lumber.....

[bloeff]

Sorry I'm so late in responding. Thank you so much for spending the time and giving me such valuable info! Due to the entire weight of the project I've decided to go with metal framing with a structure similar to this.



The ceiling joists will be 18 guage steel 16" OC with midspan bracing. The span chart shows this structure will easily support the double drywall. Will the walls also need to be 18 gauge? The thicker the metal, the less effective acoustically. I'd like to frame the walls with 25 gauge, if possible. As far as I can tell, this structure doesn't rely on hangers from above structure. This is perfect for me, creating a totally decoupled system. Am I interpreting the diagram correctly?