Preparing the Surface

The 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 only as good as the surface it sticks to. That's why, on a roof, much of the work happens before anyone picks up the spray gun.

Cleaning

Dust, dirt, lichen and soil stop the foam from bonding. On an old metal roofMetal roof sheetingMetal sheets, flat or profiled, used for roofs and sheds. Light and quick to fit, but they get very hot and rust at the screws and cut edges. the sheets were black with dirt and were pressure washedPressure washingCleaning a surface with a strong jet of water from a machine. Before waterproofing, insulation or painting, it removes dust, dirt, lichen and loose material so the new layer bonds well.. On a roof with old paving the whole surface was washed, joints between the slabs included, right up to the railings. After washing, the surface has to dry before spraying.

Whatever isn't holding comes off

An old membraneBitumen membraneWaterproofing that comes in rolls of reinforced bitumen. It is torched onto the roof in sheets that overlap at the seams. that has come unstuck is no base for anything. Spray foam on top of it and the foam will only hold as long as the membrane does.

On one roof the joints of the old membranes had opened and pieces had lifted, letting water in underneath. They were all stripped, and only then did the real state of the slab show. On another, with solar panels, the torn membrane was stripped even from under the panels.

Falls and a new screed

Foam follows the shape of the surface. It doesn't change the falls and it doesn't get rid of hollows: if water pooled before, it will pool afterwards.

On some roofs a new 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. was pumped up first, spread and levelled, and the work carried on once it had dried. On one of them the solar panels stayed in place and the screed was spread around their bases, starting with the hardest spots under the panels.

Primer

On many of the roofs 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 first. It binds the remaining dust and helps the foam stick firmly. To do its job it has to reach everywhere: under the solar water heaters, between the pipes, at the foot 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. and in the narrow passages around a rooftop room.

Covering what must stay clean

Spraying leaves a fine cloud of droplets in the air, and anything left uncovered gets marked. Before work starts, walls, windows, doors, awnings and air conditioning units are covered in plastic sheeting.

On ceilings the light fittings are wrapped or taken down, with their wiring protected. In one pilotisPilotis (open ground floor)The open ground floor under a block of flats, standing on columns with no walls, usually used for parking or as the entrance. the walls, openings, air conditioning unit and floor were covered, and in one room everything was wrapped in plastic from ceiling to floor. If there are cars nearby, they are moved. In a block of flats it's a good idea to let the building manager and the neighbours know beforehand.