When insulating a bare roof slab from scratch, how are polyurethane foam, bitumen and pumped screed combined?

When a flat roofFlat roof (roof terrace)The flat concrete roof on top of a building. It is the most common kind of roof in Greece and the part that takes the most sun and rain. is built up from the bare concrete slab — or after a complete strip-outDemolitionKnocking down or stripping out whatever has to go before a renovation: walls, tiles, render, bathroom fittings, old floors or old insulation. — it needs thermal insulationThermal insulationA layer of material that lets heat through only slowly, so a home keeps its warmth in winter and stays cool in summer., waterproofingWaterproofingThe work and materials that stop water getting through on flat roofs, balconies, bathrooms, basements, planters and tanks. and proper drainage falls formed with pumped screedScreedA layer of sand-and-cement mix or lightweight concrete poured over the slab to create falls and a solid surface for the next material..

In the first build-up, polyurethane foamSpray polyurethane foamA foam sprayed on as a liquid that expands on the spot and sets into one seamless layer. It insulates against heat and cold and closes cracks and joints. is sprayed directly onto the bare slab for thermal insulation, a bitumen layer is applied over it, and a concrete pump delivers the new screed on top, which is levelled with a straightedge to protect the insulation underneath and create a walkable floor.

In the second build-up, where an existing roof lacks proper drainage slopesFalls (drainage slope)The gentle slope built into flat roofs, balconies and bathroom floors so water runs to the drain or gutter instead of pooling., the new screed is pumped and laid first to form the falls toward the drainsFloor drainThe outlet in a floor where water drains away on roofs, balconies and in bathrooms, with a grate on top and usually a trap below.. Once the screed has cured, it is coated with black bitumen primerPrimerThe thin first coat that goes on before the main material. It binds dust, seals the pores and helps paint or waterproofing stick firmly. and sprayed with polyurethane foam.