Polyurethane foam or a liquid membrane: what's the difference?

A liquid membraneLiquid waterproofing membraneA waterproofing product applied as a liquid by roller, brush or spray. Once dry it forms one continuous, flexible membrane with no seams. is a thick waterproofingWaterproofingThe work and materials that stop water getting through on flat roofs, balconies, bathrooms, basements, planters and tanks. product applied by roller, brush or spray. It dries into one continuous, flexible layer with no seams, and its job is to keep water out. A white one also reflects much of the sun, but it isn't 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.: it is thin and doesn't, on its own, stop the heat and cold that pass through the slab.

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 on as a liquid, expands on the spot and sets into a thick layer with no joints. So it insulates and closes the way to water at the same time. It has one weakness, though: the sun breaks it down. That is why, on this bare concrete roof, the foam was then covered with a white protective coat.

In practice the two are often combined. On a roof with torn membranesBitumen membraneWaterproofing that comes in rolls of reinforced bitumen. It is torched onto the roof in sheets that overlap at the seams., the membranes came off, foam was sprayed, and a white liquid polyurethane membrane went on top to waterproof the foam and shield it from the sun. On an apartment terraceFlat 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., by contrast, only waterproofing was needed: washing, primerPrimerThe thin first coat that goes on before the main material. It binds dust, seals the pores and helps paint or waterproofing stick firmly., sealing the cracksCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. and two coats of white liquid membrane.

Which one suits you depends on what you want to fix. If the problem is only leaks, a liquid membrane may be enough. If the floor below bakes in summer and freezes in winter, it makes sense to add insulation as well. The state of the surface also matters, as do 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. and door thresholds (foam raises the floor) and any equipment on the roof. So the choice is made after someone has seen the roof.