Spraying and the Awkward Spots

Spraying is the quickest part of the job. Its quality, though, is decided where the roof isn't flat: at the corners, the bases and the edges.

How the spraying goes

The machine feeds the gun with the two materials, which turn into 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. as soon as they meet. The sprayer wears a full protective suit and a mask and works strip by strip or section by section. While spraying goes on, anyone not working is best kept away, with the windows nearby closed.

The foam expands within moments. The surface it leaves is slightly uneven, with a grainy texture, not smooth like plasterRender (plaster)A layer of mortar (sand, cement or lime, and water) spread over brick or concrete to give a wall a straight, sound surface ready for paint or tiles.. That is normal.

Corners and parapets

The corner between the floor and the parapetParapetThe 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. is one of the weakest spots on a roof. The foam doesn't stop there: it carries on up the parapet, and on some roofs over its top as well. Floor and wall become one continuous layer, with no open joint in the corner.

Bases, legs and pipes

Anything that passes through the roof or stands on it is a likely leak point: air conditioning bases, the legs of solar water heaters and tanks, vent pipes, chimneys, steel bars sticking out of the slab. The foam wraps them on every side, so they join the rest of the surface with no gap. These spots are hard to reach and take patience.

Edges, ridges and joins with other roofs

The foam also covers the edges of the slab, so 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. doesn't stop short of the rim. On a pitched 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 ridge, where the two 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. meet, was sprayed with extra care because it is the most exposed spot. And on a roof that meets the bottom edge of 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 foam closed the corner between the two, because that is where water runs down off the tiles.

What stays clear

Some spots must not be covered. On ceilings, light points and spotlights are left clear and clean so the fittings can go back. On a roof, 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. must not be blocked or end up higher than the surface around them, or water will collect next to them. It's worth agreeing beforehand how this will be handled.

Thickness

How well the foam insulates depends mainly on its thickness, and the thickness needed varies from job to job. Ask how thick the layer will be and whether it will be the same across the whole surface, parapets and edges included.