AUC Plot 2 Roof Waterproofing
The Plot 2 roof at the American University in Cairo was waterproofed with hot-spray polyurea in 2021.
01 · Project overview
What is this project?
The American University in Cairo left its downtown campus for a new one in New Cairo, and that campus is now one of the landmarks of the city built in the desert. Plot 2’s roof sits over teaching space that is occupied on every day of term.
In 2021 that roof was closed with pure polyurea. A university does not empty a building so a crew can spend a fortnight on recoats. The system had to be fast, then durable in hard sun.
| Place | American University in Cairo, Plot 2 |
|---|---|
| Year | 2021 |
| Scope | Plot 2 roof |
| System | Hot-spray pure polyurea |
02 · The challenge
What did this structure ask of the membrane?
New Cairo light is direct, and a teaching building collects vents, parapets and outlets. The deck moves every afternoon. A crack over a classroom is not a stain you schedule for the summer. It is a leak in term time.
03 · The solution
Which Krypton system was trusted here?
Polyurea Rayston is sprayed to a usual 1.5 to 2 mm and is tack-free in seconds, so the flat roof and its turns are closed in one visit. An aliphatic topcoat takes the sun on the exposed face. The polyurea underneath keeps its strength when an aromatic film would otherwise shift in colour.
04 · Application
How is this system built?
Concrete is prepared before any resin goes down. Polyurea Rayston, Krypton’s hot-spray pure polyurea, wants a support that is flat, compact and cohesive. Pull-off strength should be at least 1.4 N/mm2. Cracks are cut out and filled first. High-pressure sand or abrasion opens the pores, and the dust is vacuumed off. Moisture in the concrete must sit under 4 percent. Concrete temperature belongs between 10 and 40 degrees C, and the air should be under 85 percent relative humidity. If damp is suspected behind the slab, two coats of epoxy primer go on, the second one blinded with quartz sand. Metal is cleaned and primed with a polyurethane primer. That bond is what stops a blister when the structure heats up.
An Egyptian roof takes full sun, a wide swing of temperature between afternoon and night, and dust on the wind. The membrane has to stay elastic while the slab moves, and an aromatic film left bare will fade even if it keeps its strength.
Spraying is hot and fast. Both components are brought to about 25 to 30 degrees C before they are loaded. In the machine they run hotter: about 65 degrees C on the resin, 75 degrees C on the isocyanate, and 65 degrees C in the hose. Pressure at the gun is at least 140 bar. The usual build is 1.5 to 2.0 kg/m2, which leaves a film about 1.5 to 2 mm thick. Gel time is about 4 seconds at 25 degrees C. The surface is tack-free in a matter of seconds, not hours. Hardness climbs from roughly 28 Shore A at five minutes to about 88 Shore A after four days. One pass should make the thickness. Where a corner needs a second pass, it should follow within about two hours.
Speed is the practical gift on a crowded site. Upstands, outlets and internal corners can be closed before the next trade arrives. The film itself is elastic. Typical elongation is 324 percent and tensile strength is 16.2 MPa, with tear strength around 69 N/mm. It bridges cracks instead of snapping when concrete moves.
On a roof the sun is part of the design, not a footnote. Polyurea Rayston is aromatic, so light will shift the colour even while the film keeps its strength. Where the membrane stays exposed, an aliphatic polyurethane such as Colodur or Impertrans is the coat that holds the appearance. Elongation around 324 percent is what lets that film follow a deck through an Egyptian afternoon and the cool night after it. Water uptake is very low, about 0.002 kg/m2 per square-root hour, which is how a thin membrane still keeps a ceiling dry. The product carries CE marking to EN 1504-2 and European Technical Assessment 16/0148. Those are assessments of the product, not a measurement taken on this roof.
Polyurea Rayston technical data sheet
After the spray, the job is the details nobody photographs. Outlets have to be part of the film, not a collar of mastic added the next day. Turns at parapets need the same thickness as the flat bay. Light traffic can wait a few hours. Full hardness is a matter of days, not a lunch break. An exposed aromatic film wants its aliphatic coat before the first hard summer. Skip that and the colour fades. Skip the thickness at the outlet and the ceiling finds out first.
05 · Results
What does a job like this ask us to stand behind?
Plot 2 has to keep its teaching space dry, and the spray has to fit around a living campus. Thickness at the outlets, and a topcoat that can take the desert light, are the two things that decide it.
06 · Products
Which products carry this duty?
Polyurea Rayston
Hot-spray pure polyurea. Typical build 1.5 to 2 mm. Elongation 324 percent. Root resistance tested to CEN/TS 14416.
07 · FAQ
Frequently asked questions
How fast does the polyurea set?
Polyurea Rayston gels in about 4 seconds at 25 degrees C and is tack-free in seconds. Full hardness still takes days.
Will it bridge a crack?
Typical elongation is 324 percent and tensile strength is 16.2 MPa. The film is meant to stretch with the concrete, or with a metal panel, instead of tearing at the movement.
How thick is a normal spray coat?
The usual rate is 1.5 to 2.0 kg per square metre, about 1.5 to 2 mm. That is guidance from the datasheet, not a reading taken on this job.
Does an exposed roof need a topcoat?
Yes, if it will sit in the sun. An aliphatic polyurethane keeps the colour. The polyurea underneath keeps the waterproofing.
What moisture can the concrete have?
Support moisture should be under 4 percent, with concrete between 10 and 40 degrees C. Wetter concrete gets an epoxy primer blinded with quartz.