What It Is and Where It Goes

In many of the jobs you'll see on this site, flat roofsFlat 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., metal roofsMetal 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. and 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. ceilings were insulatedThermal insulationA layer of material that lets heat through only slowly, so a home keeps its warmth in winter and stays cool in summer. the same way: a machine sprays a liquid that turns 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 it leaves the gun and sets into one continuous layer. This guide explains what spray polyurethane foam is, where it fits, how the work goes step by step and what is worth watching out for.

What it is

Spray polyurethane foam is made from two liquid components. A machine pumps them through hoses to the spray gun, where they mix. As soon as they come out, they expand almost at once into a light, rigid foam that sticks to the surface.

Because it goes on as a liquid, it follows any shape: the waves of a corrugated sheet, the steel bars sticking out of a slab, the legs of a solar water heater, a ceiling with light fittings.

One layer, no joints

Insulation boardsExternal wall insulation (ETICS)An external insulation system: insulation boards are glued and fixed to the outside walls, then covered with reinforced render and a finishing coat. leave gaps between them, and membranes in rolls have seams. Foam, by contrast, forms one continuous layer. That has two consequences. There is no seam for water to find, and there are no gaps for heat or cold to pass through. Because the foam also runs up 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 covers the edges of the slab, it cuts down thermal bridgesThermal bridgeA spot in a building where heat and cold pass more easily than through the rest of the wall or slab, such as columns, beams and the edges of a slab., the places where insulation often stops a little short.

The foam closes the way to water, but it is not a finished surface. The sun breaks it down, so outdoors it is always covered. The last chapter explains how.

Where it's used

• Flat roofs: bare concrete slabs, roofs with old torn membranesBitumen membraneWaterproofing that comes in rolls of reinforced bitumen. It is torched onto the roof in sheets that overlap at the seams. once these are stripped, and even old paving once it has been washed.
• Metal roofs: the foam covers the screws and the joints between sheets, which is usually where water gets in, and keeps out the heat.
• Pilotis and room ceilings: it is sprayed from below, without touching the floor of the home above.
• Walls: on a house under construction the bare outside walls were sprayed before the renderRender (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., and on a balcony the walls were sprayed right up to the window frames.

On this pitched roof you can see the difference clearly: in front, the part already sprayed; behind, the bare sheets waiting their turn.

When it needs some thought

Foam isn't the answer for every roof. Before deciding, it helps to know the following:

• It raises the floor. Door thresholds, drains and the height of the parapets need checking beforehand.
• It doesn't fix hollows. It follows the shape of the surface, so if water pools, the falls are corrected first.
• On a roof that gets regular foot traffic, the foam and its top coat need protecting from wear, for example with a 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. on top.
• It needs a sound, clean, dry base. Anything that is loose or crumbling comes off first.

Which solution suits a particular roof is decided once someone has seen it up close.