Concrete is one of the most commonly used building materials in construction projects both large and small. It provides strength and durability for structural elements like foundations, walls, columns, and floors. While concrete itself is extremely versatile, often an additional layer called a concrete screed is applied on top to create an even, smooth surface ready for flooring materials or leaving as a finished floor.
But what exactly is concrete screeding? What types of screeds are used and why are they an important step in many concrete floor installations?
What is Concrete Screeding?
Screeding concrete refers to the process of spreading and leveling a layer of wet concrete mix on top of a concrete slab or subfloor. The screed layer is generally 1-2 inches (25-75mm) thick and it serves to fill in any low spots as well as scrape off any high points in the underlying concrete floor.
This leaves a flat, smooth surface ready for additional concrete finishing steps, or for the installation of flooring materials like tile, wood, or carpeting. The purpose of concrete screeding is to correct any imperfections in the slab so the end result is a perfectly straight and level floor.
Screeding is an essential step to achieve floors with proper flatness and levelness. The process is performed by concrete workers using specialized hand tools or power screeding machines, depending on the size of the floor area.
Once the concrete screed layer has been placed and smoothed out, it is left to dry and cure fully before applying any coatings or floor coverings on top. This ensures the screed has adequate strength and stability to support the finished flooring.
Key Takeaway: Screeding concrete involves spreading a layer of wet cement mixture over a concrete slab to create a smooth, level surface before final finishing and flooring installation.
Why is Concrete Screeding Necessary?
There are a few key reasons why adding a screed layer is an important process when working with concrete floors:
- Achieves a Level Surface: Freshly poured concrete rarely cures to a perfectly level surface. As the concrete starts to dry, low spots and high points become evident. Screeding fills in any low areas and scrapes off extra concrete from high spots to create a flat, even surface.
- Smooths Out Rough Textures: A screed helps smooth out the somewhat coarse, gritty texture of concrete into a more refined finish ready for flooring materials that require a smooth subfloor.
- Strengthens the Slab: In addition to leveling, a cement-rich concrete screed adds extra impact resistance and strength to support floor coverings.
- Insulates Floors: A thicker floating screed can provide thermal insulation and soundproofing between floors.
- Covers In-Floor Heating: Screeds are commonly used to encase radiant heating pipes installed directly in concrete floor slabs.
Overall, screeding is a simple but indispensable step for any concrete floor installation where proper flatness and long-term durability are desired.
Types of Concrete Screeds
There are several different types of screeds used in concrete floor construction. The three main categories are:
A bonded screed is applied directly onto the hardened concrete slab. Before spreading the screed mix, a bonding agent is brushed or rolled onto the concrete subfloor to help adhere the two layers together firmly.
Bonded screeds provide a smoothed, strengthened surface and are commonly used when flooring needs to be installed soon after the concrete has cured. They are also suitable for areas with continual foot traffic, wheeled loads, or other weight bearing requirements.
Common thicknesses range from 1/4 to 1 inch. Thicker layers are not necessary since the screed is fully bonded to the concrete beneath it.
Unlike a bonded screed, an unbonded screed has a separation layer between it and the subfloor concrete. This is usually a damp proof membrane or plastic sheeting.
The unbonded screed is not attached directly to the concrete, which allows it to move independently. This helps minimize cracks from concrete shrinkage as it dries and cures.
Unbonded screeds provide acoustic insulation and added impact resistance to the floor. Thicknesses typically start around 2 inches. They can be used with in-floor heating pipes since the membrane offers protection against condensation and corrosion risks.
A floating screed sits on top of a separate layer of insulating material over the structural concrete slab. This allows the screed to “float” independently from the substrate. The insulation helps isolate the floor from moisture and provides soundproofing, thermal insulation, and cushioning underfoot.
Floating screeds with floor heating systems encased inside often require a thickness of 2 inches or more. Thicker screeds generally take longer to dry fully, but provide greater sound and thermal insulation.
How is Concrete Screeding Performed?
The basic process of screeding concrete by hand involves these steps:
- Cover floor with screed mix: Shovel and rake the prepared screed concrete evenly over the slab to the desired thickness. For large floors, the mix may be pumped into place.
- Smooth and level: Work the screed guides back and forth, pressing down to fill low spots and scraping off excess from high points until the surface is flat and smooth.
- Edge and shape: Use edging tools to profile edges flush with walls and trim the overall shape as needed.
- Compact surface: Carefully tamp the surface to bring cement paste to the top and consolidate it for maximum density and hardness.
- Finish: Once smoothed and compacted, the screed can be left as-is or lightly finished with a float to further densify the surface.
For industrial floors needing extreme flatness, laser-guided power screeding machines are used to place and level the screed layer semi-automatically. But the same basic principles apply – adding, smoothing, and compacting the screed material to create a uniform base for floor coverings.
Proper curing is critical for a strong, durable screed. After placement, the screed is left to air dry for at least 7 days and protected from heavy traffic during this time.
Why Use Screed Instead of Just Concrete?
At this point you may be wondering – if a concrete slab floor already exists, why not just use that as the final floor instead of adding another concrete-like layer on top? Here are some of the key advantages screed provides:
- Finishing Capabilities: Concrete alone has limited finishing possibilities beyond a basic broomed or troweled texture. Screeds offer a blank canvas to apply polished finishes, coatings, stains, or other treatments not possible directly on raw concrete.
- Strength: While concrete provides the heavy load-bearing capacity, screeds possess higher cement content and density allowing for greater surface strength and impact resistance.
- Thin Layers: Applying a thin screed ensures the most uniform finish since concrete slabs can be uneven when poured thick.
- Moisture Protection: Unbonded and floating screeds provide protection against ground moisture and humidity compared to concrete laid directly on a vapor barrier.
- Insulation: Only screeds can be conveniently combined with insulation layers to control sound transmission, heating efficiency, and comfort underfoot.
- Aesthetics: Screeds offer a wider range of colors, textures, and decorative options compared to basic concrete.
In summary, screed and concrete work together to create an ideal floor – concrete handles the heavy lifting while screed puts the finishing touches on the surface.
Key Applications for Concrete Screeds
Here are some of the most common uses for concrete floor screeding in construction projects:
- Residential Flooring – Under tile, vinyl, wood flooring
- Commercial Flooring – Beneath carpet, linoleum, laminate
- Polished Concrete Floors
- Industrial/Warehouse Floors
- In-Floor Heating Systems
- Sound Insulation Between Floors
- Leveling Uneven Slabs for Flooring
- Strengthening and Reinforcing Slabs
- Garages, Basements, Laundry Rooms
Virtually any concrete floor that needs to be smooth, flat, and strong enough for foot traffic, wheeled loads, and flooring materials will benefit from screeding. Both residential and commercial sites rely on screeds to create the ideal substrate.
Pro Tips for Screeding Concrete Floors
Here are a few professional tips to achieve top-quality results when screeding concrete:
- Use the proper screed thickness – too thin and it may crack, too thick and drying takes much longer. Typical range is 1 to 2 inches.
- Only screed over sufficiently cured concrete – screeding onto green concrete can lead to delamination and cracking.
- Consider divided bays and pour joints in large floors to minimize uncontrolled cracking as the slab cures.
- Adding fiber reinforcement improves tensile and impact strength to help minimize cracks.
- Gently tamping and compacting the fresh screed forces paste to the surface for maximum density and hardness.
- Use dedicated smoothing and compacting tools to avoid overworking the surface before curing.
- Allow ample drying time – at least 7 days for thin bonded screeds and up to 4 weeks for thicker floating screeds.
- Test moisture levels throughout the depth before applying floor coverings or coatings.
Following these tips helps ensure a screed installation that is flat, smooth, strong, and ready for whatever finish flooring you have planned.
Common Problems With Concrete Screeds
When proper procedures are not followed, screeded concrete floors can sometimes exhibit issues like these:
- Cracking: Early age shrinkage cracking or cracks from slab movement under the screed. Can allow moisture under the flooring.
- Delamination: Bond failure causing the screed and concrete slab to separate. Results in cracks and hollow sounding areas.
- Pitting/Bubbling: Insufficient concrete curing or overworking the surface causes weak spots in the screed.
- Footprints/Indentations: Indicates under-compacted screed with low surface strength and density.
- Curling: Screed edges or corners lift off the base slab due to improper curing or adhesion. Allows moisture access.
- Excessive Moisture: Water trapped under the flooring from screed drying too quickly or lack of vapor barrier.
The good news is these issues can be prevented by following proper installation procedures and allowing adequate drying and curing time before covering the screed.
How is a concrete screed different than a concrete topping?
Screeds and concrete toppings serve a similar purpose of creating a smooth floor over a subbase, but differ in their thickness and installation method. Screeds are thinner at 1-2 inches, while concrete toppings are usually a minimum 2 inches thick. Screeds also utilize a wetter, more workable concrete mix that is spread, smoothed, and compacted into place rather than poured like a topping.
Can a concrete screed be left as the final floor surface?
Yes, screeds that are properly hardened and cured can serve as finished flooring without any additional coverings. Often a sealer, polish, or protective coating is applied to improve durability and aesthetics. For commercial projects, specialized industrial screed mixes allow for heavy loads and foot traffic.
What thickness of concrete screed should I use?
A typical screed thickness is 1-1.5 inches for a bonded screed over a concrete slab. For an unbonded screed, 2 inches is recommended minimum, and floating screeds with insulation average 2-3 inches. Your specific flooring needs will determine the ideal thickness – consult manufacturers for recommendations.
How soon can I install flooring on a concrete screed?
Adequate curing time is crucial before applying floor coverings. A general guideline is to allow at least 7 days for a thin bonded screed, 14 days for a thicker unbonded screed, and 30 days for a thick floating screed to fully dry and gain strength. Test moisture levels throughout the depth to confirm it is ready for flooring.
Can concrete screeds be used outdoors?
Outdoor screeds are possible for patios, walkways, and pool decks, but the mix designs must be modified for freeze-thaw resistance. Polymer-modified or fiber-reinforced screeds are better suited for exterior exposures. A concrete sealer and slope for drainage are highly recommended for outdoor screeds.
Concrete screeding is a simple yet vital process on any construction project involving a concrete floor slab. Adding and leveling a relatively thin layer of cementitious material ensures the concrete below it has a smooth, flat surface critical for proper flooring installation and function.
There are several types of screeds used in different applications, with the main differences being their thickness and how they interface with the substrate. When done correctly, this intermediate layer of screed provides moisture protection, strength, insulation if needed, and an ideal canvas for finished floor coverings.
Understanding what screeding entails, why it matters, and how to do it properly gives contractors, builders, and homeowners the confidence that their concrete floors will perform well for many years to come.