What is screed? Screed consists of sand, cement and is used for a variety of construction applications. It is a thin layer, similar to cement, which is applied over an insulation or concrete subfloor.
Screeds are typically made of sharp sand and cement, with water in a ratio around 1:1 or 1:2 of cement to sharp sand. Additives can be added to enhance certain characteristics. For example, they can improve the drying time or thermal conductivity of underfloor heating.
If necessary, screeds can also be reinforced with wire mesh.
Let’s learn how to screed the floor now that we understand what screeding is!
There are Different types of Screed
There are many different ways to classify screed.
Levelling screed
This type of screed is used to create an even and smooth surface on which flooring materials can be installed. The structural performance of a floor is not affected by levelling screed.
Wearing screed
Wearing screed is also known as high-strength cement topping. It can be used as flooring. The screed can be used for structural toppings.
Bonded screed
Screed used to maximize bonding on substrates that have been mechanically prepared.
Unbonded screed
Screed separated from substrate by a membrane to prevent bonding.
Floating screed
Unbonded screed that is applied to insulation.
How to Screed the Floor
The preparation steps for cement screeding in Auckland differ slightly depending on whether you are using bonded or unbonded screed. Below we have outlined the steps to prepare your floor depending on what type of screed is being used.
How to prepare for the lay-down of unbonded screed
Unbonded screed is usually placed on top of plastic or another type membrane instead of directly on concrete or other substrates. This layer protects the concrete from moisture and settling.
Step 1: Clean your concrete floor
To begin, you should clean the concrete to remove any dust or grease. Dirt or grease particles can cause the mixture to not settle evenly.
Step 2: Spread polythene sheets
After the floor is free of dirt and dust, you can use polythene or another type of membrane to separate the screed from the concrete. Do not forget to tape the sheets together and overlap them by 20-30 cm to prevent leakage on the concrete subfloor. The sheets must also extend up the wall by 10cm.
Step 3: Install insulation foam or edge foam
Around the walls, use insulation such as edging or polystyrene. This will protect the walls from any damage that may occur when the screed shrinks during the drying process.
Step 4: Reinforce Screed
If needed, reinforce the screed by adding fibres. This will help prevent it from cracking after drying and increase its strength. Screed reinforcement mesh can also be laid over the subfloor. When the screed is laid, this mesh should be placed in the upper half.
Step 5: Spread the screed
After preparing the area, it’s time to screed.
How to Prepare for Laying Bonded Screed
The screed is adhered to the concrete floor subfloor.
Step 1: Roughening the surface of concrete
To maximize the adhesive properties of concrete, the first step is roughening the surface. You can use a chipping or pick hammer or even a floor scrabbler. It is important to expose the aggregates in the concrete as they will be what bonds with the screed.
Step 2: Remove dust, debris and sprinkle water
Then, clean up any dirt, dust or debris and sprinkle water on the area. This will reduce airborne particles. Vacuum cleaners are the best tool to use.
Step 3: Apply the bonding agent
Apply your adhesive to the concrete. Most people choose a PVA glue mixture with cement and water, or SBR, which is best for rooms in contact to water. Follow the instructions provided by the manufacturer to prepare your chosen bonding agent.
Step 4: Apply screed as soon as possible
Do not wait too long to add the screed. Otherwise, the screed will not bond with the adhesive. After this, you can lay the screed.
How to Lay Screed
When pouring screed, aim to achieve 70 to 75 mm. This will allow any variation in height of the subfloor.
If the subfloor isn’t flat or level, aim for a higher thickness. However, never go above this, as debonding can occur if it is too thick.
Step 1: Divide your floor into sections.
Working on small sections of floor is easier than trying to finish the entire floor at once. Use timber battens as a guide. Apply a little screed on the floor and then lay the strips of battens across the surface.
Use a spirit-level to ensure that all battens and guides are at the same height. It’s a good idea to wet each batten before you lay it down. This will make it much easier to remove them once the job is done.
Step 2: The screed is now ready to be laid.
You don’t have to walk over the screed in order to leave the room.
Step 3: Use enough screed to cover around 2ft. of the first section.
Use a trowel to spread the screed, then a screed tool or straight-edged instrument to compact it and flatten out. For best results, use a chopping movement. Vibrating screed boards and other compacting tools are also available.
Step 4: After the screed is laid, you can use a piece of wood or screed board to level it.
Move the straight edge in a sawing movement down the length of each section. Keep the straight edge slightly tilted to ensure a sharper edge is in contact with screed.
Step 5: Repeat the screeding process for the next sections after you have created an even and level section.
After two sections are filled in, remove the batten and fill in the space between the sections, making sure it is level and even.
Step 6 until you have screeded the entire floor.
It is now time to finish your concrete with a bullfloat. Repeat the process once the excess water in the concrete has evaporated. You can create a textured finish on concrete by moving a wooden float in curved motions across the surface. A steel trowel can be used to create a smooth surface by pressing it flat on the concrete.
Step 7: The screed must be cured after it has been laid.
Most commonly, this is done by placing a sheet over it and sealing the edges. Let it sit for 7 days.
Step 8: Let the floor dry
After the seven-day period, allow the floor to continue to dry for another three-week prior installing any flooring.
Final Thoughts
Screeding is a vital step in many renovation and construction projects.
screeding creates a level and smooth surface that allows subsequent flooring materials such as carpet, tiles, laminate or vinyl to be installed properly and securely.
screeding improves the thermal and noise insulation properties of a floor. This increases comfort and reduces energy costs.
It also helps strengthen the flooring, thereby increasing its durability. Screeding allows you to correct any imperfections in the subfloor and create a perfect finished surface.
Now that you have a basic understanding of how to screed, you are ready to tackle this project.
