How long before waterproofing new concrete? The 28-day rule.

Ask why a membrane cannot go on a three-week-old slab and the answer is usually the standard. Look for the clause and it is not there.

Where 28 days comes from

Concrete strength is specified at 28 days, so the number is on every structural drawing. In the standards FORSPEC works to, it appears as a strength age and as a time for conditioning samples in a lab. It does not appear as a waiting time before waterproofing.

What the standards ask for

Wet areas (AS 3740)

Measure the moisture before the membrane. It must suit the membrane.

Balconies and roofs (AS 4654.2)

Dry, clean, smooth, with moisture to suit the membrane

Tiling (AS 3958)

Guide drying times of about 4 to 6 weeks, and a test where moisture could do harm

So when does 28 days apply?

When the data sheet says so. A membrane maker can set a minimum age for the concrete, and that becomes the rule for that product. The age and the moisture test are two separate checks: a 28-day-old slab under plastic in winter can still be too wet, and a thin topping in summer can be ready much sooner.

If the program cannot wait, send us the slab age and the readings. There is usually a primer answer.