How do you make a roof as soundproof as its walls?

Wurlitzer · Post by **Wurlitzer** » Wed Nov 30, 2005 1:01 am

Here's something that has me stumped.

In a typical 2-leaf soundproof construction, with good solid outer walls, how can one build a roof that comes anywhere near matching the TL of those walls?

My outer walls are made of 100mm thick dense concrete block. There's no windows, and the door will be 90mm thick and carefully sealed around all edges.

As it stands, the plan is to build a pitched roof by laying thick wooden beams from pitch to edge, nailing 3/4" wooden boards on these, laying roofing felt on that, and then laying concrete tiles. There will also be rockwool insulation between the beams.

It's just occured to me (late in the day, I know) that while this should make a fairly solid roof, it won't have anything like the mass of the outer walls. But then, how CAN a roof have that kind of mass, without being too heavy for the walls and foundations, or requiring advanced industrial engineering processes?

Is there anything I can do to improve the soundproofing without changing the basic design. Some thoughts that have come to mind are:

1. adding a layer of plasterboard underneath the wooden boards - ie between the beams and the boards. But my builder says this could cause problems if the plasterboard is crushed by the weight and starts to disintegrate.

2. Using more than one layer of roofing felt. I understand that stuff has pretty good sound isolation - it's been recommended to me for other soundproofing purposes before.

3. Sealing the gaps between the concrete tiles. My builder is very against this idea, as he says it's not how roofs are generally done, and will take ages to do. I'm wondering whether it would make a significant enough difference to be worth arguing with him on this? And if so, what's best to seal them with?

4. Adding some soundproofing material - sheetblock or whatever, between the wooden boards and the roofing felt.

Any suggestions? Also, to what extent is the weaker nature of a roof in comparison to its walls mitigated by the fact that the sound is travelling upwards? Therefore it will not be as loud to somebody at ground level, a certain distance away?

I'm not sure why we didn't sort this out better in advance - I don't think I fullt understood how he planned to do the roof. Anyway, what's my best plan now, to get the best effect possible?

This is leaving aside the issue of the inner leaf, which will be studwork and 2-layers plasterboard all the way round, walls and ceiling. But that side of things seems OK.

Post by **sharward** » Wed Nov 30, 2005 2:09 am

I'm in a bit of a hurry today, so I don't have time to carefully read and respond to each question you have asked... (I may not even be qualified to do so!

) But I did come across this thread on StudioTips again recently, and it's very relevant to your question.

I hope that helps. If not, or if additional questions are raised, please feel free to ask.

Wurlitzer · Post by **Wurlitzer** » Wed Nov 30, 2005 7:50 am

Thanks Sharward. I had a look at that thread but couldn't really relate it very easily to what I'm doing.

One thing I realise I didn't explain very well is that there is no A-Frame in my roof design. The building has low gabled sides with a huge thick wooden beam running across the top in the middle. The other beams will run from it to the walls. My builder assures me it will be strong enough to hold the weight without a horizontal support at the base of the roof. We decided on this design to give us more space for height in the inner room (which will also be peaked), without having to make the external profile of the building too high.

Sorry I haven't got time to do drawings tonight. But basically imagine two rooms each with a peaked top, with the inner room sitting inside the outer one, and its peak extending up into the peak of the outer one.

Because of this, I don't have the option of beefing up the horizontal base of the roof, making that my true second leaf and the actual roof just a subsidiary third leaf. The roof ITSELF has to be the second leaf.

OK I've come up with two more possible solutions:

1. I mentioned that there would be wooden boards over the roof beams, covered by roofing felt covered by tiles. I'm considering now laying two layers of board, with Green Glue between. This should, I would have thought, make a considerable difference over just the one layer, and the amount of Green Glue just for the roof would not be that much - not like doing a whole building.

2. This one's a bit out there. Is there something we could use to fill the spaces between the roof beams - something like a lightweight polyfiller or liquid glue? We could then attach boards to the bottom of the beams as well, and we'd basically have a single structure the width of the beams + two boards.

This material would need to be heavy enough that it counts as mass, not as insulation (since we're trying to beef up the outer leaf, not to create a third leaf), but not so heavy as to make the roof collapse. It would also need to be cheap, since there is a lot of space to fill.

I'm thinking of something like polyurithane expanding foam, but denser. Just dense enough to fill the gaps so they don't count as a spring. I figure the beams themselves plus two layers of board will give the leaf enough weight. Though I don't know if the inconsistency of weight - the heavy outsides and lighter inside - will matter. Will it?

Maybe I'm clutching at straws here. Must go to bed.

knightfly · Post by **knightfly** » Fri Dec 02, 2005 4:10 am

It would be much easier to comment with some sort of drawing; have you had time to do one yet? I'm having trouble envisioning your construction enough to comment... Steve

Wurlitzer · Post by **Wurlitzer** » Sat Dec 03, 2005 11:47 am

OK, as you've been kind enough to reply:

This is only one orientation, so doesn't really show the structure underneath the beams. But if you can imagine that the bottom (yellow) layer of the roof is not atctually a layer as such: it's just beams laid across the other way at 40cm intervals. On top of this sits the three actual layers of the roof: 18mm board; bitumen-based roofing felt; and concrete tiles.

My concern is that it seems to me the TL of this structure would be MUCH lower than that of the outer walls, which are high density 100mm block, with no window and a mega-thick solid soundproof door.

Since my last post, another factor has come up, which is that our neighbours are getting tetchy about the height and don't really want us adding more layers to the top of the roof. I've thought about using sheetblok here, say between the board and the roofing felt, since its thickness would be negligible. But it's so expensive and I don't know how much difference it would make. Also my builder wants to start getting the boards on on Monday so I don't have a lot of time to order specialist materials.

I could just add another layer of board - that would be easy, though would add half an inch or so to the height (should be able to get away with that).

What would REALLY be best from all these points of view, would be to add some more board UNDERNEATH the 150mm beams. This could simply be nailed on under the beam, we could use a couple of layers depending on weight, it wouldn't add to the height and wouldn't have to be done by Monday. However I'm aware that this would make a three-leaf structure: Outer roof - boards under the beams - inner room ceiling. Does this mean it's necessarily bad? I suppose what I wonder is: which would be best, just the two leaf structure as is, with a single layer of board on the outside, or a three leaf structure with the extra board added under the beams?

Alternatively, or additionally, as I outlined above I was wondering whether I could somehow fill the gaps between the beams with some kind of semi-liquid or expanding material that would set into a solid, heavy enough to count as mass but light enough not to add excessive weight to the roof. If I then put board under this, I'd effectively have a single structure spanning the whole width of the beams plus the boards either side.

In all these cases the roof structure connects directly with the solid block walls - it's not vented to the outside at all. but there will be a single continuous airgap running around the walls and ceiling, between the inner and outer rooms.

I'm a bit confused because it seems to me roofs must ALWAYS be to some extent like this, ie lighter than their walls. Short of industrial skyscraper techniques, we're not going to be building roofs out of thick, dense concrete block, are we? So what's the usual way of dealing with the relatively low TL of the roof? Just don't worry about it?

I feel like a bit of an idiot only thinking of this now. But as the bishop said to the actress - it's my first time.

Wurlitzer · Post by **Wurlitzer** » Sat Dec 03, 2005 11:54 am

[Post deleted as I altered the drawing to better show the structure]

Post by **sharward** » Sat Dec 03, 2005 12:07 pm

Your original "Please critique my build" thread contains some good information and background -- those who are following this project should take a look at it as well...

Wurlitzer · Post by **Wurlitzer** » Sat Dec 03, 2005 10:52 pm

Here's a cross-section of the roof from the front, should make it clearer:

The inner room ceiling will be normal timber frame with plasterboard construction, so the part facing up will actually be the timber frame with rockwool between.

Wurlitzer · Post by **Wurlitzer** » Mon Dec 05, 2005 9:41 pm

As we need to get going on the outer roof tomorrow, I've made a decision on that for better or worse, and ordered another layer of board + some MLV dampling barrier. So the outer roof as it stands will now be:

concrete tiles
bitumen-based roofing felt
12.5mm MDF board
5kg/m2 mass-loaded vinyl
18mm OSB particle board

I don't know whether the MLV will be worth it but it was not too expensive for just the roof, and we saved some money elsewhere so I decided to go for it. Two questions now remain:

1. How much weaker will the TL of this roof be than that of the walls supporting it (100mm high density block)?

2. Should I try to make up some of this difference by triple-leafing: attaching further layers of board underneath the roof beams?

Maybe I should make a new thread about the triple-leaf issue as this one has gotten a bit long-winded.

knightfly · Post by **knightfly** » Thu Dec 08, 2005 12:28 pm

Sorry for the delay; you should NOT make a triple leaf, unless you intend to make lots of mid-range noise and NOT play drums or bass...

What will support your inner ceiling, and do you have a detail of that? It seems as if you can make up most of the shortcomings by doing multi-layer inner ceiling with decent air gap (insulated of course) - I'd need the actual dimensions and spacing for those items before I could venture a guess at performance... Steve

Wurlitzer · Post by **Wurlitzer** » Thu Dec 08, 2005 5:07 pm

Thanks Steve. As I said in the other thread, the roof has now changed considerably as we won't be using tiles (the pitch is too low). So it now looks like this:

WALLS:

100mm high density concrete block, rendered and painted.
(90mm two-ply door with MLV in the middle and compression seals. No window).

ROOF (top down):

Three layer felt system, c. 10mm total. (Top two layers bitumen and mineral based torched on. Bottom layer lighter for airation).
18mm OSB
2mm 5kg/m2 MLV
18mm MDF
12mm MDF
150mm timber beams, spaced 40cm apart with rockwool insulation between.

Any idea how these two compare?

The inner ceiling will be one with the inner room: 2x4 timber frame with two layers of drywall on the inside and rockwool between the studs. Do you mean I should make the ceiling of this structure thicker than the walls, and that that would compensate for the outer leaf being the other way around? I though it would be more complicated than that, since the structural resonances in the outer leaf would all go out through the weakest point (the roof).