Kumar IronUpdated
In this guide
Before you start
- Check the slope. Water should run toward the outlets. Fix low spots with screed before coating, because a coat cannot fix a pond.
- Pick a dry window of several sunny days. Coats need a dry base and time to cure.
- Find out what is under the old finish. If old coat has failed in patches, plan to strip those areas, not just paint over them.
- Read the data sheet of each product before you buy. Note the coverage, number of coats, recoat time, mesh and curing time. Dr. Fixit, Asian Paints, Birla and UltraTech all make chemicals in these families. We do not rank them here. Read each sheet and use the one that matches your roof.
Step 1: clean the surface
Remove dust, algae, oil, loose plaster, cement splash and any peeling old coat. A wire brush and a scraper do most of the work. Wash the slab, then let it dry fully. A damp slab is one of the most common reasons a new coat peels within a season.
Step 2: fill cracks and holes
Open hairline cracks into a small V groove and fill them with polymer modified mortar or a flexible crack filler, as the product sheet says. Patch honeycomb and hollow spots with a repair mortar. Let all patches cure before you coat over them. Cracks that keep moving need a different treatment, which is covered in our guide on crack repair and expansion joints.
Step 3: cove the parapet and seal the joints
Make a cove of polymer modified mortar where the roof meets the parapet wall. A cove is a curved fillet, commonly about 75 mm each way, so the coating does not turn sharply at the corner. Seal the drain mouths, pipe collars, water tank base and any expansion joint in the same pass. Joints that move need a flexible sealant, not just coating.
Step 4: primer and first coat with mesh
- Apply the bonding primer or SBR latex slurry that the product sheet specifies. Follow its rule for a dry or damp base.
- Brush on the first coat. While it is still wet, press open weave glass fibre mesh into it at the parapet, drain mouths and any crack line. One contractor specification we checked uses a strip about 150 mm wide at junctions. Overlap the mesh at joins.
- Keep the mesh flat and pressed in. Mesh left on top of the coat does nothing.
Step 5: second and third coats
Let the first coat dry to touch, as the sheet says. Apply the second coat at right angles to the first, so that a pinhole in one coat is covered by the other. Use a third coat if the sheet calls for it, which is common where water stands or the roof is heavily used. Keep each coat even and do not dilute unless the sheet allows it.
| Layer | What it does | Check before the next step |
|---|---|---|
| Clean base and crack filling | Gives the coat a sound surface | Dry, dust free, patches cured |
| Cove and joint seal | Stops water at the weakest corners | Sealant and cove set |
| Primer or bonding slurry | Helps the first coat grip | Tacky or dry as the sheet says |
| First coat with mesh | Main layer, reinforced at joints | Dry to touch, mesh flat |
| Second coat (and third if needed) | Builds thickness and closes pinholes | Full coverage, even colour |
| Cure | Lets the coat gain strength | Full cure time from the sheet |
Step 6: cure, then protect
Cure the coat as the sheet says. Some product sheets call for about a week before a water test, and many ask for light water spraying during curing. Keep foot traffic and loose materials off the roof until then.
Protect the coat before you lay a screed, tiles or a tile bed. Lay the protective layer only after cure, and follow the sheet for any separation layer or gap. Do not drop bricks, sand or tools on a fresh coat.
Step 7: the ponding test
Block the drains and fill the roof with about 25 to 50 mm of water. Mark the water level on a wall or a nail. Leave it for 48 to 72 hours and check the ceiling below for damp. If the level holds and the ceiling stays dry, the coat is working. If a damp spot appears, mark it, drain the roof, and repair that spot before you protect the surface.
Mistakes that bring the leak back
- Coating over damp or dusty concrete, or over old peeling coat.
- Skipping the parapet cove, drain mouths or pipe collars.
- No mesh at the junctions, or mesh laid on top instead of pressed into the wet coat.
- Thin coats or watering down to stretch the pack.
- Walking on the coat, or laying tiles and screed before it has cured.
- Mixing products from different systems without checking the sheet for compatibility.
- Coating over a crack that is still moving.
Frequently asked questions
- How many coats of waterproofing does a terrace need?
- Most sheets for cementitious and polymer coatings call for two coats at least. Many add a third in high exposure areas. Follow the number printed on your product's data sheet.
- Where does the fibre mesh go?
- Mainly at the parapet, drain mouths, joints and crack lines, pressed into the wet first coat. Some systems also use it across the whole roof. Check your sheet.
- How long before I can tile over the waterproofing?
- Wait for the full cure time on the sheet, then pass the ponding test, then lay the protective layer. Tiling too early is a common cause of damage.
- Why does the roof need a slope even after coating?
- A coat keeps water out, but standing water still adds load and finds any weak spot over time. A slope of about 1 in 100 toward the outlets keeps the roof draining.