A crack repair that lasts is cut open, cleaned, filled and sealed — in that order. The repair that fails is filler wiped over the crack "as found": a cosmetic skin over a void, gone by the second summer. The difference is twenty minutes of preparation per metre.
First: which kind of crack?
- Hairline maps across the slab — thermal movement, the normal ageing of Indian roofs (cause one). Repairable as below.
- Straight lines at junctions — parapet and joint movement; repaired with the junction re-formed, not just the line filled.
- Wide, running or stepped cracks, or any with displacement — possibly structural. These get professional assessment before cosmetics; filling them hides the messenger (the separation guide).
The four steps
1. Cut the crack open
Widen it into a shallow V with a cutter or chisel. Counter-intuitive and essential: the filler needs walls to grip, and the V turns a slit into a socket.
2. Clean the cut
Brush and blow out dust; a dusty groove returns you to mistake territory — filler bonded to powder (mistake two). Prime the groove if the filler system specifies.
3. Fill with the right compound
A flexible crack-fill compound worked into the groove, slightly proud, tooled flush. Flexibility matters: the crack was made by movement, and rigid mortar in a moving joint simply cracks alongside its predecessor.
4. Seal over the repair
The filled line, cured, gets covered by the roof's waterproof coating — as part of a whole-surface system on an ageing slab, or as a banded local seal on an otherwise sound one. Filler alone is a plug; the coating is the raincoat.
After the repair
Note the crack lines and glance at them each pre-monsoon (the checklist habit). New hairlines beside old repairs are the slab continuing to talk — treat them at hairline price.
Frequently asked questions
Can cement slurry fill roof cracks?
It fills them until it cracks with them. Moving cracks want flexible fillers; slurry belongs elsewhere.
The crack keeps returning in the same place. Why?
It is a movement line — likely a junction doing structural duty. It needs re-forming and flexible treatment, not annual refilling.