What is the proper cement-to-sand ratio for exterior Portland cement plaster applications?

What is the proper cement-to-sand ratio for exterior Portland cement plaster applications?

• A. 5:1
• B. 3:1
• C. 7:1
• D. 4:1

Answer: (B)

Exterior Portland cement plaster needs a cement-rich mix to form a durable, weather-resistant coating. A three-to-one cement-to-sand ratio (three parts cement to one part sand) provides enough binder to bond to the lath and resist outdoor conditions, while the sand supplies bulk and helps prevent excessive shrinkage and cracking. If there’s too much sand, the plaster loses strength and bonding; if there’s too much cement, it becomes too stiff, cracks more easily, and costs more. So the standard exterior mix is a 3:1 cement-to-sand ratio.

Exterior Portland cement plaster needs a cement-rich mix to form a durable, weather-resistant coating. A three-to-one cement-to-sand ratio (three parts cement to one part sand) provides enough binder to bond to the lath and resist outdoor conditions, while the sand supplies bulk and helps prevent excessive shrinkage and cracking. If there’s too much sand, the plaster loses strength and bonding; if there’s too much cement, it becomes too stiff, cracks more easily, and costs more. So the standard exterior mix is a 3:1 cement-to-sand ratio.