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Heavy Timber-Frame Outbuilding On Concrete Slab - Advice Sought


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#1 jellicorse

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Posted 26 April 2016 - 02:17 PM

Hi all,

I am planning the construction of a timber-framed garden room. It is to be used partly as a music practice room and I therefore would like to reduce sound transmission outwards as far as possible.

The site where I plan to build the construction is a reinforced concrete slab, measuring 6.8m x 3.7m and at least 20cm thick (it had been used as a garage at some point in the past and contains an inspection pit). The slab appears to have been poured straight onto the (clay) soil, as when digging around the perimeter I have not come across any hardcore or rubble.



The plan is to construct a shed within a shed - i.e. two, separate frames.

1. Block perimeter around existing slab (1 block high), 100mm wide. On this will sit the external frame.

2. External frame (made from 100 x 47mm timbers) will be clad in 22mm OSB, then 25mm larch cladding. Inside, it will have 1 layer of 15mm plasterboard.

3. Roof to be constructed from 225mm joists, resting on external frame. On upper side, 2 layers of 22mm OSB and/or Moisture resistant chipboard, then a tin surface.

4. Internal frame (75 x 47mm timbers) will sit on joists placed onto existing slab, over neoprene pads. 1 layer of 15mm plasterboard on exterior of this, and 2 x 15mm soundbloc plasterboard on interior. Ceiling - possibly Resilient Channel suspended from joists, 2 layers of 15mm soundbloc plasterboard.


My concern is about whether the weight is too much for the blocks and slab. Very rough calculation of mass of exterior frame would result in weight over 200-250kg per linear metre bearing down on slab, through blocks…

Assuming frame is of quite rigid construction, does anyone have an idea if the slab/blocks would be likely to withstand this weight around perimeter, especially under high wind gusts (long edge of building is south-facing)?

I thought I would put my idea out here to see if anyone with experience and knowledge of construction would be able to tell me if this is immediately obviously a bad idea, or otherwise. I have attached a couple of drawings to illustrate the idea. Any help would be much appreciated.

Attached File  Shed in SHed 2.jpg   32.67K   8 downloads
Attached File  Diagram of shed.jpg   23.11K   7 downloads

#2 jsharris

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Posted 26 April 2016 - 06:07 PM

Without doing detailed calcs it sounds reasonable to me. It might be easier to do a block on side foundation layer to spread the load into the slab a bit better; blocks have the same compression strength both ways, but you'll lower the bearing stress a lot by using block on side as the base layer. Using block on their side also means you have more meat in the blocks to take the frame anchor bolts

I'm guessing that you're looking at using the inner plasterboard layer on the outer frame to help with the sound adsorption, but I think you'd probably find it easier to line the inside of the outer frame with a dense fibreboard, MDF is surprisingly good, is easier to work with and a fair bit stiffer than plasterboard.

I like the idea of two structures to reduce acoustic coupling, should work quite well. I think your biggest problem is making it all well sealed, as more sound is likely to leak out through thin gaps and openings that through the structure.

Remember to add a vapour control layer right on the inside, both to help with airtightness and to control vapour transmission through the structures. Also have a think about how you're going to ventilate the building yet still stop sound getting out. Some simple acoustic silencers, like the ones used as duct silencers might work, and can easily be fabricated up from MDF and cheap acoustic foam lining.

#3 jellicorse

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Posted 26 April 2016 - 08:48 PM

Many thanks indeed for your reply, jsharris. I will certainly consider putting the blocks on their sides.

Yes, my aim is to make both structures as airtight as possible, sealing all small gaps to prevent noise escaping that way.

Ventilation is something I have only just started researching after having roughly designed the structure... I will have to do something like you say with a duct silencer.

#4 ProDave

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Posted 26 April 2016 - 09:37 PM

My guess is that slab probably once had a sectional concrete garage on it. I am sure your proposed building will have a lot lower loading on the slab than a concrete garaeg would have.

#5 SteamyTea

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Posted 27 April 2016 - 06:23 AM

Welcome.
As your 'shed' is almost as large as my house, have you thought about the acoustics much.
I know that my tiny 1W radio can sound louder when it is in another room. My house it a timber frame place, 1987 vintage.

Also, and I know little about this, if you soundproof too much, don't you run the risk of 'flattening' the sound. I went to see a workmate play at the Mean Fiddler once, it was a good set, but then I saw him a second time at a local pub, he sounded flat, quiet and dreadful. His sound engineer said that he could do nothing about it as the place was 'too solid'.

Also what are you doing about the roof, both acoustically and weather proofing wise?

A mate of mine, who is a drummer, uses headphones when he practices, quite odd to watch him but not have to listen to the din.

Edited by SteamyTea, 27 April 2016 - 06:29 AM.


#6 jellicorse

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Posted 27 April 2016 - 08:05 AM

I'm not sure there was ever a concrete garage there - at the moment it is just a dilapidated timber shed, with a large door at the end for cars. I am lucky it is there, as the site is within 1m of a boundary, but the max height of the current shed is over 2.5m (and it's been there far, far more than 7yrs), which gives a bit more leeway in terms of headheight and insulation...


I can't say I have thought about the acoustics, really - my main concern is to make a space where practicing aloud is not going to interfere with anyone else. This plan is obviously going to considerable length to do this - first of all it's outside in the garden, and second it will be as soundproof as I can get it. I know that the properties of a room can massively affect the acoustics but it is only going to be used for practicing, not recording, so isn't critical.


The roof is what I am least certain about, and I'm still looking at different ideas. It has a span of about 4m. Plan is to have a monopitch roof (same as current shed), insulation between rafters (225mm joists), couple of layers of 22mm water resistant chipboard above, then two or three layers of 40mm celotex, a layer of OSB, then metal sheeting... Similar to this: http://www.greenbuil...e-unvented-roof

I think it has to be an unvented roof, because as soon as you open up ventilation there, an extremely weak link is created for sound to escape and you may as well not bother going to great lengths with the rest of the structure.

Acoustically, I am currently considering two optioins. the first is Resilient Channel to hold up 2 layers of 15mm soundbloc plasterboard. The second is staggered rafters - lower ones attached to interior frame; upper ones to exterior frame. Then 2 layers of 15mm soundbloc plasterboard on underside of lower rafters. Obviously no holes to be made in plasterboard of ceiling (or walls, for that matter): all electrical fixings and lights to be surface-mounted and cable run in trunking...