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Room within a Room with a dropped Slab

914 Views 3 Replies 2 Participants Last post by mhutchins, May 21, 2021

batlin27

Discussion Starter · May 18, 2021

Finalizing foundation plans. I will be dropping the lab 8" so that I can walk in elevated and step down to the front row or step up to the back row.
Question on Room within a Room construction with the dropped slab

Option A: Drop Slab, framing starts at 8" high and the bottom 8" of my room is slab

Option B:
Drop Slab and leave enough space for framing down to the floor

Thanks!!

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mhutchins

· #2 · May 18, 2021

There are several details that option B allows that will improve your sound isolation through decreased flanking via the main/elevated slab. Design the drop slab as a separate pour from the main slab with a gap around the perimeter of the drop slab to prevent any contact between the main and drop slabs. Prepare a compacted sand bed for the drop slab as opposed to compacted gravel or earth. Build all of the interior room as a separate structure contained on the drop slab, including ceiling joists. No part of the interior room framing should contact the exterior room framing. Use steel strap or wood diagonals in a “V” or “X” pattern on the outer face of all the interior walls for racking resistance which will allow you to forgo the usual resilient connections between the inner and outer walls when this step is skipped. Don’t forget to add any plumbing and wiring conduit (both for line voltage and for low voltage / signal wiring) for the drop slab before the pour.

In this manner you can achieve the best possible sound isolation with very little added cost over a traditional drop slab.

Mike

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batlin27

Discussion Starter · #3 · May 20, 2021

mhutchins said:
There are several details that option B allows that will improve your sound isolation through decreased flanking via the main/elevated slab. Design the drop slab as a separate pour from the main slab with a gap around the perimeter of the drop slab to prevent any contact between the main and drop slabs. Prepare a compacted sand bed for the drop slab as opposed to compacted gravel or earth. Build all of the interior room as a separate structure contained on the drop slab, including ceiling joists. No part of the interior room framing should contact the exterior room framing. Use steel strap or wood diagonals in a “V” or “X” pattern on the outer face of all the interior walls for racking resistance which will allow you to forgo the usual resilient connections between the inner and outer walls when this step is skipped. Don’t forget to add any plumbing and wiring conduit (both for line voltage and for low voltage / signal wiring) for the drop slab before the pour.

In this manner you can achieve the best possible sound isolation with very little added cost over a traditional drop slab.

Mike

Thanks for the advice but unfortunately making a separate pour didn't get pass the WAF

You mention running wiring in the slab.. is it ok/acceptable to run conduit in the slab or should I just hardwire a few 110 to the floor of the slab and then run my riser plugs off that?

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mhutchins

· #4 · May 21, 2021

It's not necessary to run electrical in or under the slab, but in some circumstances, it can be a solution for running wires from A to B. The vast majority of theaters do not require this and accomplish their wire runs through the walls and ceiling. Plumbing, on the other hand, can be a necessity. The main point was to get you thinking ahead.

With regards to a separate pour, it doesn't necessarily mean a separate pour days or weeks later, it just means that the concrete itself is not contiguous with the rest of the slab. In some construction methods, especially with expansive soil, it is a requirement that the slabs are separate from the foundation walls.

Mike

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