Room-within-a-room Ceiling

[mikkymo]

I have a 6m x 5m garage which I’m planning to use as a workshop for tinkering on cars, playing music, etc.. To keep noise down, I’m planning on building a sealed, timber framed room-within-a-room, clad with a layer of OSB and a layer of plasterboard. The idea being that noise generated in the inside room won’t be transferred to the outer structure.

It's under 30m^2 so I don't need building control approval, but I'd like to make sure the construction is safe.

The new internal wall will be a 2x4 stud wall, bolted to the floor but my issue is the ceiling.

Normally you would run the internal room’s ceiling joists horizontally between the two top plates of the stud walls, tying everything together and stopping the outer walls ‘bowing out’ in the middle. However, the trusses which make up the ceiling of my garage are raised tie, so there’s sloped sections at the side (900mm horizontally out from the wall, 450mm vertically) and higher headroom in the middle of the room which I need to keep.

Span tables suggest that at 400mm centres, I can use 8x2 joists to span the 5m gap. Is there a way I can make up ceiling joists which have angled sections which can span the gap? Could I join the angled sections to the main horizontal section with 2-3mm thick steel plates and lag bolts?

If I start beefing things up to that extent, will my 2x4 stud wall be able to cope with the weight of the joists, plus OSB and plasterboard?

All help appreciated!

 

[^woody^]

You could suspend the joists with binders and vertical ties dropped down from the rafters or the braces above. Double up the rafters just in that sloping section from the plate to the new joists

 

[mikkymo]

Thanks for the reply!

Unfortunately I can't attach any of the new structure to the old as it will transmit sound (flanking sound). The new ceiling has to be totally detached from the outside walls.

From doing a bit of research, what I'm talking about making is a 'rigid frame' (as opposed to a 'truss' which meets at at a point in the centre) where there would be a horizontal beam with 2 angled beams attached to it using gusset plates. It's a little like the 'Gambrel roof' that is popular in barns in the USA.

So my existing structure is the blue walls and raised tie truss, the new stud wall and 'rigid frame' would run along the top.

As it's an internal ceiling, it won't undergo any snow/wind/dynamic loads. All it will do is support the weight of the OSB and plasterboard ceiling. If I make it in 8x2 with 12mm plywood gusset plates glued and screwed to each side of the joints and mount them at 400mm centres, I think it should be strong enough, particularly since the OSB will resist any shear/twist in the structure.

What do you guys think?

 

[^woody^]

Unfortunately I can't attach any of the new structure to the old as it will transmit sound (flanking sound)

Your concept of complete isolation will not acheive much better acoustic performance than one with a few timber ties in the roof.

 

[mikkymo]

Your concept of complete isolation will not acheive much better acoustic performance than one with a few timber ties in the roof.

I respectfully disagree. If I hammer on the internal wall and it's totally detached from the outer structure, the internal walls and ceiling vibrate and the energy is dissipated. Insulation between the outer and inner structures helps damp the vibrations.

If I have the ceiling attached to the roof with ties, hammering on the wall vibrates the wall, which vibrates the ceiling, which vibrates the outside roof and can be heard as sound outside (thump thump thump).

The totally isolated structure should make a huge difference to the amount of sound energy which makes it outside the building. It's how home theatres and recording studios are built without resorting to fancy/expensive isolating clip systems or resilient channel (which can't support a huge amount of weight).

A normal ceiling rafter spans across the two walls and transfers the weight of the ceiling/roof/snow load vertically down onto the top/wall/header plate. My angled end design would work in a similar way, except if the load was so heavy that the joints start to splay, which would add a side loading and could push the walls out. From a structural point of view, it seems ok to have ceiling joists sistered or joined with gusset plates (glued and nailed) to transfer the load from one member to the next.

Would it work in this application though?

 

[^woody^]

Your sound is going to be airborne. You will make a much bigger difference by selecting wall coverings, than constructing some elaborate self supporting ceiling.

Anyway, yes if the timber sections are big enough then your idea would work.

 

[mikkymo]

Good stuff - thanks for your help. Much appreciated!

 

[tomfe]

I agree however if I was to make the ceiling of a box I'd use a torsion box

 

[noseall]

I agree however if I was to make the ceiling of a box I'd use a torsion box

Eh?

We use plasterboard.

 

[mikkymo]

I agree however if I was to make the ceiling of a box I'd use a torsion box

I had no idea this was possible with ceilings. I'm an aerospace engineer and wings are huge torque boxes. Essentially top and bottom skins are glued onto a grid structure. The bottom skin is in tension and the upper skin in compression to support the load. Similar to a hollow-core door where the honeycomb structure is extremely light and the strength of the door comes from the outer skins. If the glue fails in one section though (e.g. from impact), the door fails after that.

The only issue for me is that to be a proper torsion box, it needs an upper skin as well as a lower skin which is a '2 leaf system' and bad for acoustic isolation. I'll look into it.

Good find though - there's quite a bit online about flat roof design using torsion boxes. Plywood is preferred over OSB as the Ply has continuous fibres under tension rather than random pieces glued.

 

[tomfe]

Glue and screw the ply and it'll be fine.
Still would not bother though

We use plasterboard.

I am not sure how well plasterboard would work. Might be worth a go though