Floor preparation for gypsum screed

For a gypsum (anhydrite) screed floor to be strong, even and durable, the surface must be properly prepared before pouring.

1. Gypsum screed on concrete.

If you pour onto concrete, seal all cracks and holes with foam so that the screed cannot leak through anywhere. Then coat the surface 2x with primer. We recommend Knauf Haftemulsion primer. Also apply primer to the bottom edge of the walls up to the screed height. With gypsum screed, the primer keeps the substrate from absorbing water and prevents it from drying too fast.

On concrete, you can also lay plastic film instead of primer. Lay the film with an overlap and tape the joint.

2. Gypsum screed on wood (OSB, boarding, plywood).

If you pour onto a wooden surface, cover the wood with vapour barrier paper. Lay the paper with an overlap and tape the joint.

3. Thermal and sound insulation

If needed, thermal or sound insulation boards are installed. EPS or impact sound insulation.

The boards must be even and laid tightly, without gaps.

Lay plastic film on top of the boards.

4. Edge insulation

Install edge insulation strip along walls and structures to prevent sound and cold bridges. If you do not want to use edge insulation strip, run the plastic film up the wall slightly higher than the screed height.

We recommend an edge insulation strip with a plastic film flap attached. Tape the strip’s film flap and the film on the floor together. This gives the best corner finish. The corner stays neatly square and the screed is not weak along the wall. When taping, make sure the strip’s film is not stretched tight. Otherwise, the weight of the screed can pull the edge strip away from the wall during pouring.

5. Heating pipes and cables

Heating pipes, electrical cables and other services must be installed and fixed before pouring.

Gypsum screed does not need reinforcement. Fix the heating pipes with clips directly onto the film on the insulation. The clip spacing should be no more than 40cm.

If you use reinforcement, keep in mind that it lifts the pipe off the floor, which in turn needs a thicker screed. Never lay the reinforcement with overlaps. Also keep in mind that the reinforcement must be fixed to the substrate, otherwise the gypsum screed will lift it to the surface together with the heating pipe. The gypsum screed instructions say the screed should be 35mm above the top of the pipe. In practice, this requirement has proved to be more of a recommendation. A slightly thinner layer is also enough.

6. Check before pouring

Check that all pipes and cables are fixed.

Make sure the edge strips and films are in place.

If there are openings in the floor, for example a stairwell or similar, build formwork in front of them.