02/06/2026
What should be done if it suddenly starts raining during roof slab casting?
Answer:
Rain during roof slab casting can cause several issues, depending on the intensity of the rainfall and the stage of the casting process.
Possible Problems:
Washing away of the cement paste: If rain starts during the early stages of casting, the smooth cement layer on the surface may be washed away, resulting in a rough and uneven finish.
Reduced concrete strength: If the rainfall is heavy and occurs before the concrete has begun to set, the water-cement ratio may be altered, weakening the concrete.
Crack formation: If rainwater accumulates on the slab and is not properly removed, it may lead to cracks in the concrete.
Recommended Actions:
Immediately cover the slab with polythene sheets or tarpaulins as soon as rain begins.
After the rain stops, remove any standing water from the slab surface.
Ensure proper drying and curing conditions for the concrete.
Additional Recommendations for the Rainy Season:
During the rainy season, consider arranging two sets of labor and concrete equipment so that casting can proceed more quickly.
Do not wait for rain to start before taking precautions. As casting progresses, cover completed sections with polythene sheets to protect them. Even a small amount of rainwater can be harmful to freshly cast concrete.
Aggregates (stone chips) often do not remain in SSD (Saturated Surface Dry) condition during the rainy season. Therefore, consult the site engineer and adjust the concrete mix design accordingly. In some cases, changing the type of cement or using specialized concrete admixtures may be necessary.
Important Warning:
If unexpected rain causes the cement-sand mortar to wash away or separates the cement paste from the aggregates:
Never attempt to repair the affected area by simply adding a mixture of cement and sand on top.
Even if it results in additional labor, time, and cost, the damaged concrete should be removed and recast with a properly prepared concrete mix to ensure structural integrity and durability.
