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Sand filled roof, is this a stupid idea?
Old 3 weeks ago
  #1
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Sand filled roof, is this a stupid idea?

Question per title really.

I'm nearing the end of the design stage of my studio, I'm hoping the slab will be poured before christmas, but we're at the mercy of the builder's schedule and the weather, so I guess time will tell!

My external walls will be sand filled blocks with facing bricks. I want the roof to match the mass of the walls as much as possible. I know it's not going to be the same.

(This is just the outer roof - I'll be adding a separate stud room inside.)

The roof construction is a warm roof, here's a screenshot from our plans

I'm wondering if the part labelled 'Unventilated air space' could be filled with sand? The roof can take the weight - the roof is over specced, the rafters are at 400mm centres, and the structural engineer has told me that the roof will be able to hold 400kg/m2.

So broadly, I'm wondering if I use plywood instead of the plasterboard layer, fixed with plenty of strong screws, ensure it's sealed, and then fill the gap with sand from the top.

Good idea / crazy idea? Will it be impossible to fully fill that cavity with sand? Is that air space actually needed for some reason?

Thanks!

Chris
Old 3 weeks ago
  #2
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I doubt 12.5mm plasterboard will hold the weight, you would need to replace that with something much stronger like OSB and I think thicker than that.

Also this looks like a 2 leaf system, actually 3 ish if you consider the pockets for the roof shingles...if you are doing a room in a room, why not remove the plasterboard all together from the inside, add that mass and more to the roof decking and then have a proper 2 leaf?
Old 3 weeks ago
  #3
You might want to use Vermiculite instead, lighter but some of the same properties as sand. It is good for dampening and does not weigh as much as sand. FWIW.
Old 3 weeks ago
  #4
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Quote:
Originally Posted by RyanC View Post
I doubt 12.5mm plasterboard will hold the weight, you would need to replace that with something much stronger like OSB and I think thicker than that.
Yep, I did say replace that plasterboard with something stronger like plywood etc.

Quote:
Originally Posted by RyanC View Post
Also this looks like a 2 leaf system, actually 3 ish if you consider the pockets for the roof shingles...if you are doing a room in a room, why not remove the plasterboard all together from the inside, add that mass and more to the roof decking and then have a proper 2 leaf?
Getting this roof makeup was actually an attempt at making a proper 2 leaf. For ages the plans kept coming back with a vented roof void. It's so unbelievably frustrating, but this is what I've got. What needs to change to make it a 'proper' 2 leaf? Is it just removing that layer of plasterboard?

I was actually thinking of not having that (I asked the architect not to draw any in, these plans have been a struggle!). Really I'm trying to figure out how to add as much mass as possible, so whether it's loads of layers of plasterboard or something else. What would you suggest to add the mass? A few / many layers of plywood / OSB? Something else?

Thanks!!
Old 3 weeks ago
  #5
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Quote:
Originally Posted by Thomas W. Bethe View Post
You might want to use Vermiculite instead, lighter but some of the same properties as sand. It is good for dampening and does not weigh as much as sand. FWIW.
Thanks, I've not heard of that, having a google now

Mass is what I need though isn't it? I've overspecced the roof structure in an effort to add as much mass as possible
Old 3 weeks ago
  #6
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Quote:
Originally Posted by Chris1234 View Post
Yep, I did say replace that plasterboard with something stronger like plywood etc.



Getting this roof makeup was actually an attempt at making a proper 2 leaf. For ages the plans kept coming back with a vented roof void. It's so unbelievably frustrating, but this is what I've got. What needs to change to make it a 'proper' 2 leaf? Is it just removing that layer of plasterboard?

I was actually thinking of not having that (I asked the architect not to draw any in, these plans have been a struggle!). Really I'm trying to figure out how to add as much mass as possible, so whether it's loads of layers of plasterboard or something else. What would you suggest to add the mass? A few / many layers of plywood / OSB? Something else?

Thanks!!
Gotcha, sorry getting my coffee going here...

So I guess the question is are you doing a decoupled room in a room build? Or it's all fully coupled?

The higher isolation assembly will essentially be one complete structure (wall and ceiling structural elements) built inside the other- each with its own single leaf. There is also the possibility of decoupling with clips and channel, or drywall hangers...

Having a fully coupled MAM will have a lot less isolation then decoupled.
Old 3 weeks ago
  #7
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Sorry, reading it again that sounded really snarky, didn't mean it to come across like that! Must type careful-er.

It's a totally separate room within a room. Well, sitting on the same slab, but apart from that there will be no flanking paths (all being well!).

the walls are from outside in: facing brick, sand filled blocks, air/fluff gap, separate stud structure, a few layers of plasterboard.

The outer roof is currently the biggest weak bit, so looking to add as much mass as possible to it.

Chris
Old 3 weeks ago
  #8
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Quote:
Originally Posted by Chris1234 View Post
Sorry, reading it again that sounded really snarky, didn't mean it to come across like that! Must type careful-er.

It's a totally separate room within a room. Well, sitting on the same slab, but apart from that there will be no flanking paths (all being well!).

the walls are from outside in: facing brick, sand filled blocks, air/fluff gap, separate stud structure, a few layers of plasterboard.

The outer roof is currently the biggest weak bit, so looking to add as much mass as possible to it.

Chris
No worries! Communication via text certainly has it's flaws. I'm no pro either, just a guy who's learned a lot here and willing to help where I can...

If you have room in a room complete with ceiling framing and plasterboard, then aren't we looking at more then two leaves?
Old 3 weeks ago
  #9
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Quote:
Originally Posted by RyanC View Post
If you have room in a room complete with ceiling framing and plasterboard, then aren't we looking at more then two leaves?
I don't know, I hope not! This is a sketch of my construction

The 'thermal insulation' needs to be there for building regs unfortunately, but I'm stressing to the builders that it needs to be tight, so that the assembly acts as one leaf. The bricks and blocks will be tied together through the insulation too.

Then inside, there's the airgap, and then probably 3x plasterboard layers, all staggered joins and sealed etc.

That's two leaves isn't it? Or have I misunderstood something along the way (entirely possible, that's why I'm here!)

For the internal ceiling, I'm thinking exactly the same makeup as the internal wall, just underneath and almost parallel to the roof (leaving a little void for ventilation ducting). Hence why I'm brainstorming ways to add as much mass as possible to the external roof, as currently it's nowhere near as heavy as the walls. The structure can take 400kg/m2, so trying to maximise that as much as possible, it's just a case of figuring out how to add the mass!

Thanks

Chris
Old 3 weeks ago
  #10
Gear Nut
 

I really wouldn't use sand. You'd never get an even fill and it would settle leaving voids. Very difficult to remove also when you change your mind! We used rendered woodwool slabs in our secondary ceilings which worked well, but ours were laid onto joists, so supported from underneath, not part of a roof structure.
Old 2 weeks ago
  #11
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Quote:
Originally Posted by Andy Le Vien View Post
I really wouldn't use sand. You'd never get an even fill and it would settle leaving voids. Very difficult to remove also when you change your mind!
This is a very good point. I've definitely moved it to the stupid idea category. I think I kinda already knew that.

Quote:
Originally Posted by Andy Le Vien View Post
We used rendered woodwool slabs in our secondary ceilings which worked well, but ours were laid onto joists, so supported from underneath, not part of a roof structure.
So in my case, would that be:
rafters, then woodwall, then roof boards, then insulation, then tiles?

Are there any other ways of adding mass to the roof? Would cement board work? Anything else I could look into?

Thanks all!

Chris
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