First published in 2010
Note that this only applies if your building does not require Building Regulations approval. If it needs to meet the regs, the base will need to be properly designed according to the local soil conditions and tree locations etc.
The concrete should be laid on:
-clean compacted hardcore, from 75 to max. 150 thick
-sand blinding (for the protection of the DPM)
-1200 gauge DPM
Note the golden rule of walls: The Vapour Barrier goes on the warm side of the insulation!!!!. The vapour barrier in this drawing is the OSB, which is full of glue and therefore highly resistant to the passage of moisture.
Key features of this design are the brick plinth and the 25mm air gap between the frame and the back of the cladding. In the roof, it is essential that there is a 50mm clear ventilated void above the insulation, and that there is a continuous 25mm gap at the eaves (with insect mesh) to provide air movement. These features are essential to prolong the life of the building, and to keep everything inside dry.
You can omit the floating floor if you wish. My own workshop simply has the concrete slab as the floor.
I imply no structural calculations for the roof! Each roof should be designed individually, and I am always happy to help with that. I would suggest min. 150mm rafters so that you can fit 100mm of insulation in without restricting the airflow, but with some roofs the rafters will need to be much deeper for structural reasons.
The boarding can easily be replaced with render, so long as the airflow behind is maintained, and this is made easier by using a backing of building paper behind the EML (mesh) so that the render doesn't get pushed through too far into the cavity. Note that with boarding there should only be one nail per board per stud position, and that that nail should be situated about 30mm up from the bottom edge of the board.
And now a variation: with Timber Suspended Floor
I like this less, particularly because of the large step up and the resulting taller building (or reduced headroom). This isn't the only way to do this, but the principle is to have the insulation hard up under the flooring, with a continuous ventilated void below the joist.
The surface of the reduced ground level below the floor should either have a geotextile membrane or a layer of lime laid on it, or it should be treated with a weed-killer. Avoid using this design in wet/ boggy areas because of the reduced ground level below the floor.
I hope this helps.