The easiest way to insulate a flat roof e.g one found on a single storey extension and garages is to install a warm deck roof with the insulation on the outside (above) the building.
1. Either remove the old roof to expose the joists and install the firrings to create a fall of not less than in 1 in 80 towards your rain water gutter.
2. Lay a bead of continuous (water resistant) non-setting gun-grade mastic sealant across all joists.
3. Lay your plydeck (FGF Plydeck) onto the joists, measuring carefully so that edges of plydeck sheets are supported along all edges & trim as required. If this proves problematic then install additional noggins. Sheets should be staggered like bricks to increase their strength.
4. All fixings should penetrate joists by 35mm and no less than 10mm from edges and 50mm from corners.
5. There should be an 25mm insulation upstand along the sides abutting buildings, at least 300mm continuously.
6. You are now ready to install the waterproofing element of your roof including the lead flashing.
Advice is given here is given in good faith but all situations are different and you should seek expert advice. Councils operate differing interpretations of building regulations and you should check that you comply. This is your responsibility.