Garden office build specs.

jonnypron · 18 May 2017

Hi there,

Firstly I'm new to this forum, so apologies if I've not posted in the correct place etc.

Any advice on the following would be much appreciated:

I'm building a garden office 4.8w x 2.4d x 2.5h. The concrete base is already done (about 80mm on top of hardcore, mot and dpm, no sand but hope it's not punctured).

The main thing I cannot find much info on is the timber floor specs. I was intending on doing;

Query 1) 4x2 every 400mm along 2.4 depth of base and central 4x2 noggins for strength, with 50mm celotex supported by batons so there's about 30mm air space between underside of insulation and concrete base with holes in this area for airflow. On top of this frame would be either OSB 18mm and engineered wood or 25mm solid wood planks & no OSB.

QUES 1: do I need a vapour barrier in-between insulation and OSB or is foil backing sufficient on insulation?
QUES 2: also I've read you need vapour barrier on top of concrete base to stop moisture getting to timber base /floor joists and rotting, but if I have the dpm underneath concrete and one on top before timer, how is the concrete going to dry out?
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Query 2) for the wall timbers I will probably use 3x2 every 400mm or 4x2 every 60mm with middle noggins half way up. It looks like I should probably use rock-wool to help sound insulation at 100mm thickness, then the wall would be from inside out;

Plasterboard, vapour barrier, rock-wool insulation and 4x2, breathable membrane, marine ply panels cladding (painted black with tar paint). OR

Plasterboard, 3x2 with celotex 50mm (leaving 20mm airflow gap behind), breathable membrane, marine ply as above.

QUES 1: should I be using OSB for strengthening, or can I use the ply on the outside directly onto the stud for this? I presume it's ok not to have batons if there's already 20mm airspace in-between the insulation and the membrane? Also do you not need airspace with rock-wool as it's breathable anyway?
QUES 2: should you have vapour barrier over the 3x2 inside even with foil backed insulation? also should you foil tape over the 3x2 to create a seamless flow of vapour control? Someone mentioned foil backed plasterboard???
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Query 3) for the roof I was intending on plasterboard, 5x2 joists with 100mm celotex, OSB 18mm, then EDPM roof.

QUES 1: Do I need any vapour barrier here or is the celotex fine again? I presume you don't need a breathable membrane here as the EDPM is waterproof anyhow and doesn't let moisture in or out???

Sorry for the long post and numerous questions but really want to get this right without any nightmares later on.

Thanks in advance for any advice on the above.
Jon

tomfe · 18 May 2017

All this information is readily available both on this forum and through google, have a search and a read around it will answer much of what you ask.

For the floor why are you using a suspended timber floor over a concrete slab?

The walls need a void to dry they also need anti racking support.

Are you putting 100mm between your roof joists or are you doing a warm roof?

jonnypron · 18 May 2017

Hi - I've looked and have found a lot of the information but not specifically about the points I've made, i.e. dpm above concrete before subfloor timbers. Also the specifics about the use of ply externally as strengthening directly onto membrane with gap behind. Everything else just shows stud work, OSB, membrane, batons and then cladding.

re timber floor on concrete. I've not put insulation into the concrete so need to insulate. The advice was to put timber subfloor for insulation and finishing floor onto that. The questions I asked are still relevant for me and cannot find much info on these specifics.

Thanks

tomfe · 18 May 2017

OK, there have been a number of builds on here is the 'Your Projects' forum they are a good place to start I guess.

1.1 No
1.2 I would make sure no timber stands in water so if that's a dpc/dpm or detailing.

2.1 Ply is as strong as OSB, you might get away with ply on battons to give you anti racking however the normal way is studs -> 11mm osb ->membrane ->batons -> cladding
You still need an air space to let the wood dry
2.2 No the foil is the vcl. Though normal pb and a sheet of polly is cheaper.

3.1 Yes if it's a warm roof. No if it's a cold roof.

jonnypron · 18 May 2017

tomfe said:
OK, there have been a number of builds on here is the 'Your Projects' forum they are a good place to start I guess.

1.1 No
1.2 I would make sure no timber stands in water so if that's a dpc/dpm or detailing.

2.1 Ply is as strong as OSB, you might get away with ply on battons to give you anti racking however the normal way is studs -> 11mm osb ->membrane ->batons -> cladding
You still need an air space to let the wood dry
2.2 No the foil is the vcl. Though normal pb and a sheet of polly is cheaper.

3.1 Yes if it's a warm roof. No if it's a cold roof.

Much appreciated. I didn't post in the 'your projects' as it didn't say anything about advice but will consider that if I need anymore info.

So would an air gap in the stud wall between the insulation and the membrane be enough, with the ply flush against the membrane or does the gap need to be more on the outside, i.e. between the cladding and the membrane?

Thanks again for your advice.

J

tomfe · 18 May 2017

jonnypron said:
Much appreciated. I didn't post in the 'your projects' as it didn't say anything about advice but will consider that if I need anymore info.

Sorry I mean look through the project section as there are many builds the same as yours on there.

The membrane might trap the water against the back of the ply, if you loose lay the membrane it might work however you have to detail the bottom well to shed any water away from your tf and slab.