Does my roof need a new screed before it is insulated?

The 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. is the cement layer on top of the slab that gives a roof its falls towardsFalls (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 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.. 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. and waterproof coatingsElastomeric waterproof coatingA thick, rubbery waterproof paint for flat roofs, parapets and roofs. It stretches without cracking as the building expands and bridges hairline cracks. follow the shape of the surface below them: they don't fill hollows. If water pools on the roof before the work, it will pool afterwards too, unless the surface is corrected.

On a roof with a solar water heater, a new screed was first pumped up and spread over the whole surface. Once it had dried, a 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. went on and then the foam was sprayed.

On another roof, next to a tiled roofTiled roofA pitched roof covered with clay tiles, on a timber frame or a concrete slab. The real waterproofing sits underneath, in the layer below the tiles., the old torn membraneBitumen membraneWaterproofing that comes in rolls of reinforced bitumen. It is torched onto the roof in sheets that overlap at the seams. was stripped and then a new screed went down with the solar panels left in place. It was spread first in the awkward spots under the panels and around their bases, and primer and foam followed.

A screed can also go on top. On a bare slab the foam was sprayed first, a coat of tarTarA black bitumen-based material spread hot or as a liquid emulsion. It seals the pores of concrete and keeps water out. went over it, and then came the new screed, levelled with a straightedge so no hollows were left. There it protects the insulationThermal insulationA layer of material that lets heat through only slowly, so a home keeps its warmth in winter and stays cool in summer. and gives a hard surface to walk on.

Keep in mind: a new screed adds height and weight. Thresholds, drains and the height of the parapetsParapetThe low wall around the edge of a flat roof or balcony. The joint between it and the floor is one of the most common places for leaks. are checked beforehand, and on older buildings an engineer's view may be needed. Whether a particular roof needs one is decided on site.