Roof waterproofing systems compared: sheets, liquids and inverted roofs

01Exposed or covered: the first fork

An exposed roof faces UV, heat cycles and maintenance traffic directly, so the membrane must be made for exposure: UV-stable sheets, a mineral-surfaced cap sheet, or a liquid with a UV-resistant top coat. A roof covered by tiles, screed, ballast or an inverted build-up protects the membrane, but it needs a protection layer and a build-up that drains.

In Riyadh’s climate, the sun’s heat on an exposed roof surface is a large part of the load the membrane lives with every day.

02Sheet membranes

• Bituminous rolls (SBS, APP)Modified bitumen torch-bonded with laps, with a long track record on roofs and basements. SBS is more flexible, APP copes better with heat; they usually need a protection layer, and torching calls for fire-safety controls.
• PVC membranesRolled out and hot-air welded into one continuous layer. Light colours reflect part of the heat; they need a separation layer wherever they meet bitumen, and welding is a job for trained technicians.
• TPO membranesHot-air welded like PVC, but plasticiser-free and stiffer, so details take more care.
• EPDM membranesA highly flexible synthetic rubber that copes with UV, ozone and expansion. Large sheets keep seams few; it is adhered or ballasted, and suits exposed roofs, garden roofs and complex shapes.

03Liquid membranes

• Liquid polyurethaneApplied by roller, brush or spray over a primer, it cures into one continuous rubbery layer and excels at details, drains and penetrations. Its performance depends on preparation and the thickness applied; some systems need a UV-resistant top coat.
• PolyureaSprayed with plural-component equipment, it sets within seconds and builds a tough membrane quickly over large areas. It needs specialist equipment, experienced applicators and a prepared, dry surface.
• Clear polyurethane coatingFor sound tiled or stone roofs without removing them, after the joints are repaired. It does not fix hollow or loose tiles.

04Conventional or inverted roof?

In a conventional roof the thermal insulation sits below the membrane, which stays on top, exposed to heat and traffic. In an inverted roof the waterproofing goes directly on the slab and insulation boards — usually XPS, which tolerates moisture — go above it, under ballast or tiles, so the membrane stays at a stable temperature and protected from damage. The price: extra height and weight, and boards made to work in that position.

05At a glance

06How to choose

• Will the roof stay exposed, or be covered with tiles or a protection layer?
• How many penetrations and details are there — AC units, pipes, skylights?
• Does the roof need thermal insulation too? An inverted roof combines both.
• Do the falls lead water to the drains? If not, a screed to falls comes first.
• What state is the existing waterproofing in, and is it compatible with the new system?
• What do the project specification and the consultant’s approval say?

After an inspection we propose the system and the details for each location, and the roof is flood-tested before handover.