Laying a level floor screed on a concrete base is one of the most common building practices in the construction industry.
A screed is a thin layer of material applied to a concrete subfloor to provide a smooth and flat surface for flooring. The material composition is slightly different. A screed is made from sharp sand, cement, and water in a ratio of 1:3 or 1:4 cement. Additives may also be added to the screed.
Tools Required To Lay A Floor Screed
- Tape Measure
- 100mm X 50mm timber battens
- Spirit level (four-foot level is best suited)
- Wheelbarrow or buckets
- Piece of excess timber or a straight edge
How To Prepare Bonded and Unbonded Floor Screed
Before you can choose between using a bonded or an unbonded type of screed, you might need to weigh their pros and cons or understand why they are used.
An unbonded screed is usually made to sit on top of plastic sheets instead of sitting directly on the concrete. This helps to protect the floor from moisture present in the subfloor. It could also help to protect the main structure from other issues that might arise. However, there’s a high probability of the unbonded screed eventually curling. To solve that problem, it is usually reinforced. The unbonded screed should be at least 50mm thick at all points.
A bonded screed, on the other hand, a bonded screed reduces the risk of cracking and curling. This type of screed is best suited for thin layers of screed because the concrete will remain strong and uncracked. If the concrete base is flat enough, a 35mm screed is ideal. If not, use a 40mm thick screed.
Preparing the unbonded screed
- Clean the concrete base by removing all dust and grease before you start.
- Lay down PVC or polyethene sheets to separate the screed layer from the concrete. Ensure the sheets are overlapped by at least 20cm, then tape them together. You can then fold the sheets up to 10cm.
- Line the walls and pillars with a compressible material to protect their surfaces from damage.
- Reinforce the creed by either:
- Mixing polypropylene fibres into the screed mix before adding water, or
- Position crack control steel mesh over the subfloor.
Preparing the bonded screed
- Roughen the surface of the concrete with a chipping hammer or shot blaster to expose the aggregate in the concrete. This will ensure it bonds with the screed.
- Remove all dust and grease stains.
- Apply a bonding agent to the concrete by:
- Dampen the cement a day before, let it stand, remove all excess water and let it dry.
- Mix sand and cement grout of equal quantity and add enough water.
- Stir continuously and apply within 30 minutes of mixing. If the grout dries due to delay, the screed will not bond.
Procedure to Floor Screed
After preparing the screed, the next step is to lay the screed. Follow this procedure:
- Purchase a ready-to-use screed mix
- Divide the floor into sections using a timber batten by:
- Lay down about 3cm of screed to keep the battens in place.
- Ensure the battens are wet so they can be removed easily after screeding.
- Use the battens to divide the room into strips of 3 to 4 meters.
- Use a spirit level to ensure each batten is level along the top and with adjacent battens in the room.
- Apply the compact layer of screed to the floor: Do this by dumping enough screed to fill 60cm length of the strip at the farthest section from the entrance. You can then spread it out with a trowel and compact it immediately by chopping it down with a straight-edge screed board. You should also tamp the edges with a hand tamper.
- Level the screed with a straight edge: Push the straight edge across the screed surface in a side-to-side sawing motion. You should tilt the lumber slightly so the corner can fork a cutting edge to make the screed level.
- Repeat the process: Trowel, compact, and level until the first section is filled and then move on to the next section. Once the second section is filled, remove the batten and fill in the gap. Repeat this process till you complete the screening of the whole room.
- Finish the concrete: Float the screed immediately after to remove imperfections, and then float a second time once the concrete has stopped bleeding and excess water has evaporated. You can then create a textured or smooth finish.
- Cure the screed: Place under a polythene sheet sealed at the edges and leave undisturbed for at least 7 days.
- Wait for the floor to dry.
Although laying a screed is usually a professional’s job, it can be done as a DIY. It is okay not to be able to complete the screeding in a day. As long as you follow the procedures and precautions, you are good to go.