How to waterproof a flat roof, terrace or balcony with a liquid membrane
To waterproof a flat concrete roof, terrace or balcony with a liquid membrane, start by fixing the slab rather than the coating. Clean it back to sound concrete, let it dry completely, seal moving joints and cracks, then prime the porous surface. After that, roll or brush the membrane on in one direction at the rate the manufacturer gives. With Akfix EM600 Waterguard, Akfix give 1.0 to 1.5 kg per square metre on flat areas, so one 20 kg bucket (KSh 20,900) covers roughly 13 to 20 m². Most leaking slabs fail at the edges, joints and outlets, not in the open field, so spend most of your time there.
Why flat roofs leak
A concrete slab moves as it heats in the afternoon sun and cools at night. Over time that movement opens hairline cracks, and joints between the slab and parapet walls pull apart. Water ponds where the fall to the outlet is poor, sits on those cracks and works its way through. Typical places to look:
- the joint where the slab meets the parapet or upstand wall
- around rainwater outlets, gutters and downpipes
- expansion joints and joints between precast panels
- around pipes, tank stands and anything bolted through the slab
- low spots where water stands for days after rain
A coating will not cure standing water. If the roof ponds badly, correct the falls with a screed first, or the membrane will be sitting under water for much of the rainy season.
What you need
| Job | Product | Our price |
|---|---|---|
| Waterproof coating for roofs, terraces, balconies and gutters | Akfix EM600 Waterguard, 20 kg bucket | KSh 20,900 |
| Moving joints in the slab, expansion joints, slab-to-wall joints | Akfix P635 PU construction sealant, 600 ml | KSh 1,450, or KSh 16,200 for 12 |
| Metal gutters, downpipes and sheet-metal flashing | Akfix P645 PU metal sealant, 310 ml | KSh 950, or KSh 10,800 for 12 |
| Hairline cracks in parapet walls and ceilings that will be painted | Akfix AS606 siliconized acrylic, 300 ml | KSh 450 |
You will also need a stiff broom, a scraper, a roller or brush (see roller and brush sizes; EM600 can also go on by airless spray), and backer rod for deep joints.
About Akfix EM600
EM600 is a one-component, water-based acrylic membrane. It contains no solvent, goes on by brush, roller or airless spray, and cures to a seamless elastic film. Akfix list it as UV resistant and water-vapour permeable. They rate the cured film at 600% elongation or more, and it can be painted over or tinted with water-based colour pastes. Akfix publish it for balconies, terraces and roofs, for roofing details such as gutters and chimney edges, for wooden surfaces, and for asphalt and bitumen floorings.
Step by step
1. Clean and repair the slab
Akfix require the surface to be clean, dry and free of grease. Scrape off loose screed, flaking paint, moss and old failed coatings, then sweep and wash the slab and let it dry out fully. On a slab that has been soaking all rainy season, that can take several dry days. Break out and patch any hollow or crumbling concrete before you seal anything.
2. Seal joints and cracks
Treat joints that move with a sealant that can take the movement. P635 is Akfix's low-modulus polyurethane for expansion joints and floor joints, indoors and out, including pedestrian areas, with 25% movement capability. Akfix specify a joint about twice as wide as it is deep, with backer rod setting the depth so the sealant bonds to the two sides only. Their table gives about 5 metres of a 15 × 8 mm joint, or 3 metres at 20 × 10 mm, from one 600 ml pack.
For metal gutters, downpipes and flashing, use P645, which Akfix publish for gutters, spouts and sheet-metal seams. It is over-paintable and rated for −40 °C to +90 °C in service.
AS606 is for low-movement cracks in walls and ceilings that will be painted. Akfix say not to use it on joints permanently exposed to water, so keep it off the slab itself, where water stands.
Akfix's product text does not say whether EM600 can be applied over P635. If a sealed joint will end up under the membrane, message us and we will check the data sheets for you before you start.
3. Prime
On porous surfaces such as concrete, cement screed and plaster, Akfix call for a primer coat of EM600 before the main coat. The data sheet on the product page gives the primer mix. Use that figure rather than guessing.
4. Apply the membrane
Stir the bucket. Apply EM600 undiluted (only the primer is mixed), working in one direction, between +5 °C and +35 °C as Akfix specify. Work the edges, outlets and upstands first with a brush, carrying the coating up the parapet wall rather than stopping at the corner. Then roll the open areas. Do not coat a damp slab, and do not start if rain is likely before the coating has dried. On a hot roof, avoid the hottest part of the day so the film does not skin too fast.
5. Inspect
Once it has cured, walk the roof and look for pinholes, thin patches and missed edges, especially around outlets and pipes, and touch them up. If you want a colour finish, EM600 can be painted over.
How much membrane you need
Using Akfix's 1.0 to 1.5 kg per m² for horizontal surfaces (about 0.75 kg per m² on walls), and rounding up to whole 20 kg buckets:
| Roof or terrace area | EM600 needed | Buckets | Membrane cost |
|---|---|---|---|
| 15 m² balcony | 15 to 22.5 kg | 1 to 2 | KSh 20,900 to 41,800 |
| 30 m² terrace | 30 to 45 kg | 2 to 3 | KSh 41,800 to 62,700 |
| 50 m² flat roof | 50 to 75 kg | 3 to 4 | KSh 62,700 to 83,600 |
| 100 m² flat roof | 100 to 150 kg | 5 to 8 | KSh 104,500 to 167,200 |
Add the parapet upstands to your area. Rough or very porous concrete will use towards the top of the range.
Common mistakes
- Coating over a damp slab or over loose, flaking material.
- Skipping the primer on bare concrete.
- Stopping the membrane at the corner instead of carrying it up the parapet wall.
- Leaving cracks and joints unsealed and expecting the coating to bridge everything.
- Thinning the main coat with water to make the bucket go further.
- Ignoring a roof that ponds; fix the falls first.
Where to buy
EM600 and the joint sealants are in our sealants and Akfix collections, with Akfix data sheets attached to the product pages. For other sealants see which silicone to use and adhesive vs silicone vs foam. Collect from our Nairobi trade counter at Royal Business Park, opposite Complast, Old Mombasa Road, Monday to Saturday, 8:00 to 17:30, or order for countrywide delivery at a flat KSh 350, free on orders over KSh 15,000. Contractors buying regularly can request a trade quote on our trade page. Send photos of your roof on WhatsApp and we will help you work out quantities.
Common questions
Can I apply EM600 over an old coating?
Only if the old coating is sound and well bonded. Akfix require a clean, dry, grease-free surface. Anything loose, blistered or flaking has to come off first, or the new membrane fails with it.
Can I walk on the roof afterwards?
Akfix publish EM600 for balconies and terraces, so light foot traffic is expected. For a heavily used terrace, or one that will be tiled over, ask us before you buy so we can check the data sheet for your case.
Will it work on an iron-sheet roof?
EM600's published uses are concrete roofs, terraces and balconies, gutters and roofing details, wood, and asphalt and bitumen. For leaking laps and seams on metal sheets and gutters, use P645 PU metal sealant.
Can it be coloured?
Yes. Akfix say EM600 can be tinted with water-based colour pastes and painted over once cured.