Garden office floor

[primarycause]

Hi All,

Have started a thread in "Your Projects" to document the build of my garden office. I've got a lot of the research done, but there's a few areas I either can't find enough info on (spent hours on here and the web researching) or need opionions on as I'm undecided on the best course of action.

I'm building a garden office garden office approximately 4m X 4m with timber framed construction (cedar cladding/tyvek or similar/ OSB/ kingspan/4X2/ foil backed PB).

I have cleared the area for the base and given that it's on sloping ground, have built a small retaining fence. The ground is roughly levelled and I'm down to undisturbed clay:

http://media.diynot.com/212000_211318_69233_51107390_thumb.jpg

I have taken delivery of 4 tons of MOT type 1 and 1 ton of sand and I will be hiring a whacker plate this weekend. I will lay 4in of sub base, compact, then blind it, add 1200 Gauge DPM and will pour a 6" solid slab for the raft foundation. I'm also considering throwing in some steel mesh as well for a belt-and-braces approach.

I have a couple of questions:

1) Is the steel mesh overkill or a necessity?

2) I don't think I need an expansion joint, would you agree?

3) Can I set anchor bolts in the slab, lay DPM and attach my wall plates using the anchor bolts and do a floating insulated floor? Or would it be better to lay bearers, then joists, insulation, OSB floor and then build the wall plates off the OSB floor and joists?? (I'm specifically worried about vermin ingress and insulation/floor damage with the bearer method).

I would much appreciate any advice from someone more experienced.

Regards,

 

[themiddleagedun]

I would forget the raft.. Use strip foundations, 2 course of concrete block to ground level, then 2 or 3 course of engineering brick, 150mm MOT, sand blind, visqueen, insulation, 100mm concrete slab.
DPC onto brickwork, treated 100x50 wallplate screwed to bwk then build your walls.
Mesh shoudn't be needed. No exp. joint.

 

[primarycause]

Thanks for taking the time to respond middleagedun. I'm currently weighing up your idea. My only worry is my brick/ block laying experience is zero.

For me, this build is about me doing every bit of the build, and I have to take into account my own limitations when formulating a plan. The raft seemed like a good idea due to the clay ground and the raft is pretty easy to do as long as I get my levels right.

Just to confirm- do you mean dig the perimeter down enough to have 2 courses of block bring me back to ground level, do the perimeter with 2 or 3 courses of engineering brick then fill in the middle of this to create the floor as described (MOT, Blind, DPM, Insulation, slab)?

 

[themiddleagedun]

In reply to your last paragraph; yes that's exactly what I meant.
If you're doing all the work yourself, a slab would be easier, cheaper & quicker too, but you would see an unsightly? upstand to the edge of your slab, as it would be presumably be 150mm above ground level, Also the timber would be sat on the concrete. Which could leave "rain-splash" problems, unless you make the slab the same size as the GO.
You'd also have the faff of a floating floor.
If using a decent thickness of insulation would this not be above the height of the doorstep..

 

[noseall]

Agree with Middleman, build a brick footing then build your garden office so that when it is cladded, the cladding material overhangs the brickwork.

You will end up with a far superior weathering structure and none of the nightmares that are attributed to concrete sectional garages etc, that are built on a slab.

 

[primarycause]

Thanks for reinforcing Middleagedun's viewpoint noseall.

I can definitely see the advantages to doing a brick footing and that's now got me wondering about making the office from brick/block- I've got 1600 facing bricks in the garage that perfectly match my house.

Couple of newbie follow up questions:

1) What lines the bottom of the trench to build the first course from? Hardcore/Concrete? If so, how much.

2) I've already dug down 0.3m (5m3 in total) by hand to level the ground, am I right in saying that I would need to dig an additional 600mm down and 450 wide all around the perimeter? That could be a nightmare!

 

[themiddleagedun]

I would be more inclined to stick with your original plan to build an insulated timber structure with the cedar cladding...

Q1 No hardcore, only concrete, about 150 thick
Q2 Yes more digging