Waterproofing a roof is four-fifths preparation and one-fifth coating. The sequence that works: inspect and find every crack and weak point, clean back to sound concrete, repair cracks and detail the joints, then coat the whole surface without gaps. Most "failed waterproofing" we are shown was coating applied over skipped preparation.
Before you start: diagnose
Walk the roof after rain, or hose it down. Look for:
- Visible cracks, especially where the slab meets the parapet wall
- Areas where water stands long after the rest has dried
- Drain mouths — blocked, cracked or sitting higher than the slab around them
- Old coating that is blistered, peeled or chalking
- Beneath: damp patches on the top-floor ceiling that map to points above
If the damp inside does not sit under any visible defect, the water may be entering somewhere else and travelling — that is worth a professional eye before any material is bought. Photographs are usually enough for a first opinion; send them via the roof waterproofing page.
Step 1 — Clean back to sound concrete
Sweep, scrape and wire-brush the surface. Loose plaster, moss, laitance and old flaking coating must come off — a coating bonds to what it touches, and if it touches dust, it is bonded to dust. Wash the surface and let it dry properly.
Step 2 — Open and fill the cracks
Hairline cracks are cut open into a V so filler can key in, then filled with a crack-fill compound. Filling a crack "as found" leaves a skin over a void. Structural cracks — wide, running, or with displacement — are a repair job first and a waterproofing job second.
Step 3 — Detail the weak points
Leaks concentrate where surfaces change: the slab-to-parapet junction, around drain mouths, at pipe penetrations and expansion joints. These get treated individually — junctions with a coved fillet so the coating turns the corner on a curve rather than a sharp angle, drains sealed where the outlet meets the slab. A roof coated wall-to-wall but not detailed will leak at the details.
Step 4 — Prime and coat
Prime if the system calls for it, then apply the waterproof coating over the entire surface, taking it up the parapet walls to above the waterline, never stopping at the floor line. Apply the number of coats the product specifies, letting each cure before the next; the label of the specific product governs quantities and recoat times, which vary by system. Building Guard 400 is our coating for this work.
Step 5 — Cure, then test
Let the final coat cure fully before the roof sees water or foot traffic. Then the honest test: the first heavy rain. Check the ceilings below during and after it. A small ponding test — a few centimetres of water held for a day over a suspect area — tells you more than any visual inspection.
The mistakes that cause repeat leaks
- Coating over dust, moss or old flaking material
- Ignoring the parapet junction and drain mouths
- Filling cracks without opening them first
- Stopping the coating at the wall instead of turning up it
- Coating in the monsoon, onto damp concrete
- Treating the damp patch on the ceiling instead of the entry point above
Frequently asked questions
When is the best time of year?
Dry weather, well before the monsoon — the surface must be dry to coat and the coating needs time to cure before the first heavy rain.
Can I waterproof over an old coating?
Only if the old coating is sound and compatible. Flaking or blistered coating comes off first; otherwise the new layer fails with the old one.
Do new roofs need waterproofing?
A newly cast slab is the cheapest moment to waterproof — the surface is clean, empty and crack-free. Waiting for the first leak means paying for preparation as well.
Should I do it myself or call someone?
A sound, simple slab with no active leak is a reasonable self-job for a careful owner. An already-leaking roof, standing water, or complicated detailing deserves experienced applicators — we carry out this work where we cover your area, and supply the chemicals everywhere in India either way.