Self build timber garage help

[Lumpybd]

Hi all,

At the moment I have a 40 year old prefabricated detached garage at the back of my house. It's falling apart and is generally in a poor state of repair, so I'm planning on removing the old building and constructing a new building in its place. I'm no bricky, but I'm reasonably handy with a good eye for details so I am going to build a timber studwork replacement.

I was just hoping you could sanity check my design, and answer a few specific questions around points that I'm uncertain on. Please keep in mind that I'm trying to keep the price down where possible, so cost is a consideration in my choice of materials and methods.

As it is just an outbuilding, I'm not too concerned about thermal performance, however I would like to make sure I tick the right boxes as far as weather-proofing and condensation prevention are concerned. Additionally I'll be adding a bit of insulation so that it is tolerable to work in when temperatures are extreme.

Once I've taken down the existing garage, I will have a 4.6m square concrete slab to build off of. So I'm planning on using 2x4 timber studs to build the wall frames and 11mm treated feather board for cladding. I'll board the exterior of the frame and roof with plywood sheets, and then the external roof will be coraline corrugated roofing panels.

The frame wall and ceiling joist cavities will be filled with insulation (I'm thinking maybe 75mm polystyrene sheets cut to fit the gaps between the stud members) followed by an inner wall of plywood again.

So in a nutshell, is that sensible?

With regards to specific questions, I have a few:

• 1. How do you suggest I fix the bottom of the stud walls to the concrete slab? It seems like using a powder actuated tool to fix a treated 'sole plate' to which I can attach the base of the wall is the best way, but are their simpler or more cost effective alternatives? As I don’t have a powder actuated tool, could I simply drill into the slab and install bolts that I can then bolt the sole plate to?
  
  2. Bearing in mind that I have no history of the slab construction, do I need a dpc? I've seen rolls of 100mm wide dpc sheets... would I just run this under the sole plate?
  
  3. Regarding membranes and vapour barriers, I think I need to add a vapour barrier between the external plywood boards and the cladding/roofing. Is that right? Do I need it anywhere else such as under the interior plywood boards?
  
  4. How do I fix the external vapour membrane to the plywood board? Can I just staple it to the board, or do I need to create a gap using battens? Which side of the battens would the membrane go?

Thanks in advance.

 

[foxhole]

OSB is a cheaper alternative to ply , I fit timber to the edge of each sheet and top and bottom with a centre timber then you can simply screw these 'wall boards' together start at corner and they free stand until walls are up then you can screw thru to the base. If economy is required let half the timber overhang on the wall run so you can share the 4x2 between panels and leave one timber short of edge at corners.
Polythene sheet as you mention to the outside then apply feather edge board.You can add rockwool for insulation monies allowing, this is held in place by board inside and poly sheeting outside.
I find the coroline sheets slump heavily when wet and leak if not well supported so need a sheet of osb below them or you could use felt?.
Dpc , or even better a single brick base would help keep it away from damp ground little bricky skill required.

 

[Lumpybd]

Thanks.

Just a few more details though... It's a mid terrace property and the current garage takes up the full width of the property and actually forms the boundary between the 2 adjacent properties, so I need to erect the 2 side walls in a fully completed state as I wont have access to their external faces once erect... so I kind of figured that I'd need to install a sole plate, build the wall on the ground and then lift it and install it to the sole plate.

With that in mind, can anyone advise on my options for:
a: fixing the sole plate to the ground and
b: fixing the wall to the sole plate

Also, is there any difference between poly sheeting or vapour barrier or a breathable membrane? I see the terms used a lot here and I'm just trying to understand the differences if any.

 

[Harbourwoodwork]

I like to keep timber above ground level three brick courses does the job ,Dpc under sole plate,
building can be held down with restraining straps try screwfix
layers from the inside out plaster board, vapour barier,insulatioin such as celotex between studs, air gap,boarding,breathable membrane such as tyveck,batten out,exterior boarding such as feather edge.

 

[Harbourwoodwork]

I like to keep timber above ground level three brick courses does the job ,Dpc under sole plate,
building can be held down with restraining straps try screwfix
layers from the inside out plaster board, vapour barier,insulatioin such as celotex between studs, air gap,boarding,breathable membrane such as tyveck,batten out,exterior boarding such as feather edge.

 

[foxhole]

I find plasterboard in an unheated building gets damp and fails easily. I does not take the shelving you need in a garage.