Introduction to Floor Levelling
When it comes to installing floor finishes like tile, wood, or carpets, having a smooth, flat subfloor is extremely important. Uneven surfaces will lead to cracks, hollow spots, and other flooring failures down the road. So before installing your finished floor, you need to assess the subfloor surface and take steps to level it out if necessary.
What is a Screed?
A screed is a material that can be applied to help level out uneven concrete or wood subfloors. Screeds act almost like a thin concrete topping that adheres to the subfloor and creates a new flat surface for flooring installation. Here are some key things to understand about screeds:
Screed Composition
Screeds are generally composed of cement mixed with sand and water to make a flowable material that can easily be spread thin and smooth. Other ingredients like latex polymers may be added to improve adhesion, strength, and flexibility. The consistency is similar to a thick pancake batter.
Thickness of Screeds
Since screeds contain cement, they can be applied in thicker layers to fill bigger dips and depressions in the subfloor. Screed mixes are typically designed to be applied anywhere from 1/4 to 2 inches thick in a single layer. If there are large valleys deeper than 2 inches, it may take multiple passes to build up the surface.
Screed Drying Times
An important consideration with cement-based screeds is dry times. Screeds undergo hydration which leads to hardening and strengthening over time, similar to concrete. Most screeds take 24-48 hours before they can be walked on, and 5-7 days to reach full hardness and load capacity. The dry times allow all moisture to evaporate so flooring adhesives will properly bond.
What is Self-Levelling Compound?
Self-levelling compound, often referred to as self-levelling underlayment, is another popular material used to level floors before installing flooring. Here’s an overview:
Composition of Compounds
Self-levelling compounds contain gypsum, Portland cement, polymers, and calcium carbonate mixed with water or latex. The compound has a flowable, pancake-batter-like consistency. Compared to screeds, compounds generally contain less Portland cement and more gypsum.
Thinner Applications
Compounds can self-level into a smooth, flat surface, but are designed for thinner 1/8 to 1/2 inch applications typically. Because they contain less cement, they are more limited in thickness capabilities compared to screeds.
Drying Times
An advantage of compounds is they dry faster than cement-rich screeds. Compounds are walkable in about 2-4 hours, and achieve their hardness and full strength within 24 hours in ideal conditions. This makes them the preferable option when fast flooring installation turnaround is needed.
Key Differences Summary
Now that we’ve compared their compositions and performance, here’s a quick summary of how screeds and self-levelling compounds differ:
- Screeds contain more Portland cement and can be applied thicker to level deeper subfloor valleys. Compounds have less cement content and are designed for thinner applications.
- Compounds dry faster, often within 24 hours, while screeds take 5-7 days to fully cure and strengthen. This makes compounds better for fast-turnaround flooring projects.
- In terms of cost, screed materials are typically cheaper per coverage area because they utilize more affordable cement. Self-levelling compounds tend to be specialty mixes that cost more.
When Should You Use a Screed?
Now that we understand what screeds are and how they perform, in which situations should you utilize a cement-based screed? Here are some of the top applications:
Leveling Uneven Concrete Floors
One of the most common uses for screeds is leveling out rough, uneven concrete slabs in basements, garages, or older foundations. Since screeds can be applied over 1 inch thick, they are ideal for filling pits, cracks, and differences in height that would telegraph through flooring. Screeds adhere well to clean, prepared concrete so they create a durable new surface.
Leveling Wood Subfloors
In addition to concrete leveling, screeds work well to fill low spots or contours across plywood and OSB wood subfloors. The cement mix bonds tenaciously to wood and adds stiffness while bringing the floor flat and smooth in preparation for tile, laminate flooring, carpet and more. Screeds over wood may require the use of a primer before application.
Ramps, Walkways, and Incline Applications
Need to create a sloped surface or ramp? Simply shape the screed layer to the desired pitch. The cement formula flows easily to hold its formed shape. Unlike self-levelling compounds that flatten naturally, screeds lock into inclines and custom designs. They make great sloped shower bases, ramp transitions, and decorative contours.
Garages, Driveways, and Heavier Loads
Finally, screeds are the better choice for garage floors, driveways, patios, and other areas that will experience heavy vehicle loads and abuse. Their durable cement composition and thickness is best for holding up to extreme pounds per square inch loads without crumbling or losing bond. Compounds are too thin for vehicle applications.
When Should You Use Self-Levelling Compound?
In contrast to screeds, self-levelling underlayments make the most sense in these types of floor prep applications:
Thin Floor Irregularities
As we learned earlier, compounds have limited thickness capabilities, typically 1/2 inch or less per pour. But this makes them perfect for feathering out minor dips and bumps across large expanses of subfloor. Their efficient self-levelling properties spread easily to create flat, smooth surfaces with little labor.
Rapid Flooring Installations
In situations when the subsequent flooring needs to be installed ASAP, self-levelling compound is the clear winner over traditional screed. Compounds dry extremely fast – in as little as 2-4 hours for foot traffic and 24 hours for flooring installation. Screeds take days before ready for covering. For occupied buildings on a tight timeline, compounds accelerate turnaround.
Moisture-Sensitive Flooring
Want to install wood floors, vinyl plank, or moisture-sensitive laminates? Self-levelling compounds form an excellent substrate because they dry fast and emit less moisture vapor than screeds. Compounds contain gypsum which cuts down on shrinkage and helps maintain a dry base for flooring adhesives to stick well.
Crack Isolation Layer
Compounds make an ideal thin crack isolation layer prior to tile flooring. As the compound dries, it develops small internal fractures similar to the cracking in dried mud. These micro-fractures prevent cracks in the substrate from telegraphing up through the tile. Thin applications of compound essentially hide subfloor flaws.
Conclusion and Recommendations
As we’ve explored in this guide, both screeds and self-levelling compounds have their advantages and preferred applications when it comes to subfloor levelling and prep work. Hopefully you now have a stronger understanding of which material fits your specific project requirements.
For thicker pours over 1 inch to level concrete or wood, cement-based screeds provide the best suited solution, albeit slower drying compared to compounds. For thinner, faster levelling of minor dips and flaws, consider self-levelling compounds instead. Combine this with moisture testing and proper subfloor prep for best results.
Frequently Asked Questions
What thickness of screed should I use?
For most residential levelling applications, a 3/4 to 1 inch thick screed will sufficiently cover dips, cracks, and uneven areas in concrete slabs. Over wood subfloors, 1/2 inch is common. In commercial projects with heavier expected loads, screeds may be specified thicker at 1 1/4 inches or more.
Can I apply flooring directly over screed or compound?
In most cases you should not install flooring directly against the dried screed or compound surface. It’s recommended to apply a primer and minimum 1/8 inch thick cement backer board first to allow for better adhesion and prevent cracks from telegraphing through the finished flooring.
Is it better to use screed or self-levelling compound under tile?
For most ceramic, porcelain, or natural stone tile installations, a self levelling compound makes an excellent substrate and crack isolation layer.