Concrete block vs plasterboard; and flat roofs...

[badass_mcfunk]

I'm still finalising my design plans and will post for comments over the next few weeks, but have a couple of questions.

I'm building an outside 'room within a room' in my garden, all constructed on a 5" thick concrete pad (with fibres added). The pad size is 5.4m x 4.8m

I was initially thinking to erect 2 stud frames for the outer and inner walls, and mounting boards on the outer side of the outer frame / inner side of the inner frame. I'm now thinking that using concrete black for both walls may be more effective in terms of mass and therefore sound reduction, with cost difference being minimal. I cant find any STL ratings for concrete block (I'll be using 100mm thick blocks). Any views?

Secondly, I'm wondering how I attach a flat roof and maintain the integrity of the outer mass whilst also creating a gentle slope? I'm thinking that a small gradient will suffice in ensuring that pools of standing water dont form, and I will be attaching firing to the joists to give a rise. In doing this I'm not sure how to fill the edges of the roof that run at a gradient. If the roof was completely flat and square, I'd obviously just take by block or other materials right up to the roof so that there was mass consistently covering all of the exterior. With a gradient I dont understand how to do this. Any thoughts?

Many thanks

James

[Soundman2020]

I'm building an outside 'room within a room' in my garden, all constructed on a 5" thick concrete pad (with fibres added). The pad size is 5.4m x 4.8m

That sounds like you have an excellent basic plan. How high do you plan to have the ceiling? Try to get it as high as you can, within reason.

I'm now thinking that using concrete black for both walls may be more effective in terms of mass and therefore sound reduction

True, but how much isolation do you need? That's the key for determining the best type of construction. You should put a number to that, in terms of how many decibels of isolation you need.

I cant find any STL ratings for concrete block

I think you mean STC, not STL? But don't rely on STC ratings: STC is a single-number rating of isolation for typical home and office sounds, such as speech, small radios and TVs, telephones, typical office noises, etc. It is no use at all for dealing with full-spectrum music, because STC does not take into account the bottom two and a half octaves of the spectrum, where drums, bass, keyboards and etc. put out a lot of energy. It also doesn't take into account the top two and a half octaves, but that isn't so important.

So don't rely on STC numbers to figure out how to isolate your room.

Instead, use "transmission loss", which tells you how many decibels of reduction you get across the entire spectrum.

Secondly, I'm wondering how I attach a flat roof and maintain the integrity of the outer mass whilst also creating a gentle slope?

Have you considered a "beam and block" roof? That would maintain your mass (if you do go with block walls), and can be angled any way you want, or just left flat, if done properly. Google "beam and block roof" to find out more about how to do that.

I'm thinking that a small gradient will suffice

Better make sure you get the slope right! Just "thinking" that something will work doesn't mean that it actually will work! Better to find out for sure what gradient is good for where you live, for whichever type of roof you choose. If you live in the middle of the Atacama desert, where it never rains, then any angle would be just fine, but if you live in an area prone to thunderous downpours or heavy snow, then you'd want to get a pretty large angle. Check your local building code to find out what is permitted, and check with local contractors to find out what the common practice is in your area.

With a gradient I dont understand how to do this. Any thoughts?

with beam-and-block it is easy: Angle the rood as needed, then use mortar as needed to fill the gaps.


- Stuart -

[badass_mcfunk]

Many thanks Stuart. A builder I've been using for the pad suggested that there doesn't need to be a gradient at all, but I'd feel more comfortable with one, and 1:40 seems to be what flat roofs in our area often have.

I've been looking at beam and block options - will look into the costs in due course - thanks.

[stevev]

I'm building an outside 'room within a room' in my garden, all constructed on a 5" thick concrete pad (with fibres added). The pad size is 5.4m x 4.8m

When you say 'fibres added' do you mean fibreglass reinforcing that comes in bags and is added to the mix instead of putting steel reinforcement in the slab? If so then get that idea certified by an engineer. The kind of weight you'll be putting on that slab with block walls, roof and the rest of the construction may be too much for fibre reinforcing alone to handle.

steve

[badass_mcfunk]

I'm building an outside 'room within a room' in my garden, all constructed on a 5" thick concrete pad (with fibres added). The pad size is 5.4m x 4.8m

When you say 'fibres added' do you mean fibreglass reinforcing that comes in bags and is added to the mix instead of putting steel reinforcement in the slab? If so then get that idea certified by an engineer. The kind of weight you'll be putting on that slab with block walls, roof and the rest of the construction may be too much for fibre reinforcing alone to handle.

steve

Hi Steve

Yes, its been endorsed by the firm thats supplying the concrete as ample for my needs given that its single storey.