RC Flat Roof Waterproofing.
Exposed roofs, plant decks and roof terraces on reinforced concrete slabs. The system carries foot traffic, plant and sun for the life of the building. This page sets out the six layers, the order of work and the test we carry out before handover.
The build-up.
Six layers over the structural slab, in the order we lay them.
From the slab up
- Base screed to fallCement and sand (1:4), 25 mm average, laid to drain into the gutters and around every outlet. Kept damp for seven days so it cures without cracking.
- PRIMER W, then three coats of CHEMFLEX R-100A water-based acrylic primer, then a micro-reinforced synthetic rubber membrane, fully bonded. The coats go on three to four hours apart.
- HYDROFOAM HD insulation, 50 mmHigh-density extruded polystyrene boards, laid straight onto the tested membrane.
- Separation sheet500-gauge polythene between the insulation and the protection concrete.
- Protection concrete, 65 mm averageGrade 25 with BRC A5 mesh, power-floated and kept wet for seven days, then saw-cut at the surface into 2 m by 2 m panels. Every cut is sealed with POLYSEAL 1PUA polyurethane sealant.
- DURACOTE HB, two coatsPolyurethane coating with a high solar reflectance index, where the roof stays exposed.
How the job runs.
- We inspect the slab firstThe concrete must be at least 28 days old, clean and dry, with the outlets, pipes and upstands fixed in position. Cracks, voids and high spots are repaired or ground down before anything is applied.
- Base screed to fall, then the filletsThe screed drains the roof to its outlets. A 25 mm by 25 mm mortar fillet goes into every internal corner so the membrane can be dressed around it.
- PRIMER WOne coat by roller, brush or broom. It dries in 30 to 60 minutes in normal conditions.
- Three coats of CHEMFLEX R-100Applied three to four hours apart. The membrane is taken 300 mm up every parapet, kerb and upstand and 75 mm into every pipe sleeve.
- Then we pond testThe outlets are plugged and the roof is flooded and held for 24 hours. Where a leak shows, we repair it and test that area again.
- Insulation and separation sheetHYDROFOAM HD boards go straight onto the tested membrane, followed by one layer of 500-gauge polythene.
- Protection concreteGrade 25 concrete over BRC A5 mesh, power-floated and kept wet for seven days. It is then saw-cut into 2 m by 2 m panels and the cuts are sealed with POLYSEAL 1PUA.
Protection concrete, cut into panelsA roof in Sungai Buloh - DURACOTE HBTwo coats on the cured concrete, two to three hours apart, where the roof stays exposed.
Edges, pipes and outlets.
- Parapets and upstands
The membrane turns up at least 300 mm above the finished roof level, over a 25 mm by 25 mm mortar fillet at the corner.
- Pipes
Every pipe through the slab passes through a sleeve with a collar, and the membrane is taken 75 mm into the sleeve.
- Outlets and gutters
The base screed runs into the gutters and around each outlet. An extra strip of membrane goes around every outlet, vent pipe and upstand beam.
- Falls
The screed is laid to fall and checked to within 3 mm under a 3 m straight edge.
- Cracks and voids
Repaired before the membrane goes down. High spots are cut off or ground down.
- Movement joints
A structural expansion joint is a separate system. See expansion and construction joints.
Materials and data.
- PRIMER W
Water-based acrylic primer. One coat at 0.125 to 0.167 litres per square metre.
- CHEMFLEX R-100
Water-based acrylic, micro-reinforced synthetic rubber membrane. Three coats at 0.5 kg per square metre each. Tensile strength 2.5 N/mm² minimum and elongation at break 400% minimum, both to ASTM D412.
- HYDROFOAM HD
High-density extruded polystyrene insulation board, 50 mm. Compressive strength 0.60 N/mm² to ASTM D1621, thermal conductivity 0.029 W/mK, density 40 to 42 kg/m³.
- Separation sheet
Polythene, 500 gauge (0.125 mm).
- Protection concrete
Grade 25, 65 mm thick on average, reinforced with BRC A5 mesh. The thickness varies from site to site. Where the specification sets one, we follow it.
- POLYSEAL 1PUA
Polyurethane sealant for the cuts between the concrete panels.
- DURACOTE HB
Polyurethane coating with a high solar reflectance index. Two coats at 0.2 litres per square metre.
These products are made by CHEMIND, and the figures are from its specification. Where a project specifies another manufacturer's system, we apply that instead.
Roofs in our register.
Twenty-five entries in our job register name a roof in the title, from 1985 to 2016. Five of them:
- 1985
Tun Hussein Onn Eye Hospital, Petaling Jaya
- 1986
Komtar, Phase 1D, Penang
- 1990
Hostel blocks, International Islamic University, Petaling Jaya
- 2010
Measat Global administration building, Cyberjaya
- 2016
Malaysian Flying Academy student centre, Melaka
Most entries record the job as a waterproofing package without naming the zone. See the full register.
Questions we are asked.
- How long is the roof flooded for the pond test?
- We plug the outlets, flood the roof and hold the water for 24 hours. If a leak shows, we repair it and test that area again before the insulation goes down.
- How many coats does the membrane take?
- One coat of PRIMER W, then three coats of CHEMFLEX R-100 at 0.5 kg per square metre each. Each coat needs three to four hours to dry before the next.
- What must be ready before you start?
- The slab should be at least 28 days old, clean and dry, with the outlets, pipes and upstands already fixed in position. Cracks and voids are repaired first.
- Why is there concrete over the waterproofing?
- The membrane and the insulation are covered with Grade 25 concrete, 65 mm thick on average, so the roof can carry foot traffic and plant. The concrete is cut into 2 m by 2 m panels and every cut is sealed.
- Can you waterproof a roof that already leaks?
- Yes, with a different system that goes over the existing roof. See re-waterproofing.
Talk to us about a roof.
Phone and WhatsApp016-332 7534
Emailinfo@hydrocon.com.my