Underlayments make new or old floors smooth and level before floor coverings are applied. Traditional underlayments such as cellular lightweight concrete and latex-modified patching mortars must be screeded or troweled. Portland cement- or gypsum-based self-leveling underlayments don't require these time-consuming finishing steps. HOW DO PORTLAND CEMENT- AND GYPSUM-BASED PRODUCTS DIFFER? Most self-leveling underlayments contain portland cement or gypsum plaster. In general, portland cement-based products are two to five times stronger than gypsum materials. They are water resistant while gypsum-based products are not. Many will accept floor covering within a day, while gypsum underlayments take at least 5 days to dry. Gypsum products are less likely to crack, except for rare hairline cracking. Sound control, fire resistance, and energy conservation are improved when there are fewer cracks. HOW TO INSTALL SELF-LEVELING UNDERLAYMENTS If you are installing the underlayment yourself, follow these steps: 1. Clean and prime the subfloor. 2. Mix the underlayment. 3. Spread the underlayment so it's level. 4. Use joints to control cracking, if recommended by the manufacturer. 5. Wait for the floor to set before allowing traffic. 6. Wait for the floor to dry before adding floor covering.