Building Garden Office - advice on methods/materials please!

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Hello everyone

After doing a lot of reading on here and elsewhere I've got a plan on how to build my garden office which I'd really like DIYnot input on. Sorry this is a bit of a long post. Here goes:

For a variety of reasons the only space for the office is in an area 7m deep x 2.5m wide which is between the side of the house and the boundary wall. I'm planning on making the office approximately 3.6 m x 2 m. It needs to be well insulated and easy to heat / keep cool as I work from home. Speed is also of the essence as I’ve got to get this built before the beginning of August.

I've rung building regs and planning and I don't need to involve either providing I make the top of the office roof no higher than 2.5m and I should make the wall nearest the boundary out of non combustible materials. Unfortunately, if I were to attach the office to the house wall I'd invoke building regs. The electrics will be installed and signed off by an electrician.

My plan is to level the ground and use a plastic grid system with builders sand and weed barrier beneath it to act as the foundation and extend it beyond the floor area and fill that bit with pea gravel so it also acts as a french drain, : http://www.shedbase.com/probase/. Cost about £160

Then use 4x2? timber to build the base, put a damp proof membrane over the lot, fill the gaps between the joists with 100mm celotex, then nail tongue and groove floorboards on top (I already have the floorboards). Am I missing anything here re layers of materials to use?

I'm getting some sliding patio doors from ebay and some upvc windows likewise which I'll build the walls around.

I was thinking of building the long walls as a series of 3 frames approx 120 wide by approx 230 high and the short walls as one frame. (the sliding door will go in one short wall) I thought I’d use 4x2s spaced 40cm apart at the centres to make the frames and then nail these "frames/panels" together and to the floor, with a top plate around the top to keep the whole lot rigid. (I’ll be working on my own so I figured making and raising the walls as individual frames would be easier? Or would it be simpler to toe in the studs one by one and then fix the top plate to the wobbly lot to make it stable?)

I'm thinking the walls will be filled with rock wool and then internally lined with a thin layer of celotex and then plasterboard. So is this the right order of materials from the inside?:
Plasterboard,
vapour barrier (plastic sheeting or use foil backed plasterboard),
20mm Celotex to cover any cold spots from the frame,
4x2 timber frame stuffed with rock wool (cheaper - but would ideally use celotex),
11 mm? OSB,
Tyvek breather membrane
vertical timber straps to allow a gap for ventilating the wall
Cladding - inspired by fmck’s project //www.diynot.com/forums/your-p...with-garden-room-inc-store-room.361658/page-2 I'm thinking versapanels for the low maintenance (and fire resistance). I might put cedar cladding on the shorter walls (which will be seen).

If the panel building route makes sense, I’m not sure whether I should try to attach the cladding etc before raising the walls (due to the access issues to the sides of the office) or whether that will just make them too heavy/I won’t be able to get the detailing of the corners correct. Will mixing cladding materials be a problem at the corners?

The roof will be at an approximately 10 degree angle and is planned as 18mm OSB covered in an EPDM (quote £300) with 100mm celotex and plasterboard underneath and with guttering around the lot taking the rain water to a barrel.

Would using SIPS to speed up the build process be an option or would it be really expensive?

Does this approach mean that the office will breathe enough?

Opinions on construction method and materials suggested please!

Once I've got the materials/approach etc finalised I can then get all the quotes in.

thanks very much in advance

Tamsen
 
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I don't like that Probase, how do you level the base? its OK for garden shed but not for any thing you are going to invest time and money in.
On another point, in their slide show they show a shed made of planks on edge. What they do not say is that with a window and door like that the door will always rub as the short plank between the door and window are not fixed, like wise the planks between the door and the corner are subject to being moved about by the planks "round the corner".
Caveat Emptor !
Frank
 
OK I think I understand your point about the rubbing. But I think the idea is that you level the ground before laying the grid (using pea shingle etc if necessary). I was basing my research on rmwebs who posted his project here //www.diynot.com/forums/your-projects/the-man-cave-a-garden-office-aka-fancy-shed.321919/page-2 and who hadn't had any problems a year or so on: http://buildgardenoffice.blogspot.co.uk/2012/05/base.html#comment-form

Can you see a work around with this method? I'm not keen on building a concrete slab (which would have to be an exact fit so that the rendering could go over it. Any thoughts?

thanks again
 
What about building the walls? Is it best to build as separate panels and raise them individually or best to toe in each upright as I go along and then clad afterwards?

thanks for any thoughts

Tamsen
 
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I would be thinking strip foundations and walls etc but that's because I'm a builder.

If you are going to build it all off a slab then do it robustly.

Cast the slab so that the top of the slab is level with the external ground level. Make the dimensions of the slab the exact size of the building footprint and no bigger. Use 1200g DPM.

Build your timber walls so that they are positioned on the edge of the slab. This is so that when you add the external cladding material, it is proud of the slab and is weathering it.

Allow for 80mm of Celotex plus 20mm of deck material when setting out your sole plate. Don't forget to add another layer of DPM directly beneath the insulation.
 
Thanks Noseall and thanks for the sketch too!

So in any and every case I should build a sole plate and make it the same depth as my floor materials? If I used strip foundations would I then just build a suspended timber floor?

What do you think about raising the walls as panels or toe in the studs? Or is it down to my preference?

IF I were to use the plastic foundation (I know, it does sound flaky) then should I do the same thing and make it the same dimension as the base or should I try the French drain approach (which presumably should deflect rain splash back?

Thanks again for your thoughts
 
French drains have a lifespan. Quite a short one if you have plenty of trees shedding leaves to clog the thing up. Don't rely on this sort of thing.

You can build the structure in modules or panels if you wish as long as you have the room and the weather etc. Once you have a corner up and braced then you can work single handed.

Building strip foundations means that your options for design are increased. I personally wouldn't bother with suspended timber floors because of venting issues.
 
Thanks noseall and sorrt for late response

Trees are a problem so that woukd suggest no french drains. Coukd you point me to a good buikding a strip foundation post?
I've never used concrete before - im pretty handy at DIY - should i be ok if i take my time and follow the indtructions?

If suspended timber floors arent ideal then what woukd you suggest?

Are my proposed wall materials (insulation, etc) ok?

Anything else i should look out for?

Thanks again

Tamsen
 
Thanks for all the input so far. I've gone the concrete route and am having a foundation installed as I write: 6 piers of concrete (cast in the ground) and a 10 cm thick reinforced concrete slab on top with DPM underneath.

Looking at Noseall's drawing, should the DPM underneath the concrete slab come up the sides of the slab too or should it just sit between the hardcore and the concrete?

Second question - I'd really like to clad the wall panels on the longest sides (where I have no access) before I raise them into place. I'm not sure about making the corners watertight with the shorter walls to which I do have access. Should I leave the e.g. breather membrane long and then wrap it round the corners with the shorter walls once they are raised?

Would mixing cladding materials be a problem at the corners? e.g. cedar and versapanel?

thanks again for your help with the project. I'll upload some photos soon.

Tamsen
 
Ok hoping my project hasn't got lost in the new forum .... and that someone has some good advice.

Concrete foundation is now laid but slightly larger than I wanted and not square :(. DPM goes up the sides of the slab. How should I best build on to it if I want to make sure water doesn't pool on the excess concrete and affect the sole pate. Should I just make the shed larger than I wanted (and overhang the sole plate where the sides are not square)?

Thoughts really apprecaited!!

thanks
 
Please? Anyone? Should I overhang the soleplate directly onto the concrete or add a one brick high wall for the sole plate to sit on or something else entirely??
 

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