Traditional sand and cement screed is by far the most common type of screed and what most people think of when discussing screed. However, there are other types of screed that are becoming more and more popular such as flowing liquid screed and self-leveling screed, which have some advantages over traditional screed.
Throughout this guide, we will be focusing on traditional sand and cement screed. We have already briefly discussed it, now it is time no go into more detail.
According to BS 8204-1, to produce good quality screed the ratio of cement to sharp sand should be 1:3-5. If the specification requires the screed to be stronger then you can also add in fibres into the mix which acts as a reinforcement.
When using a screed pump, the screed gets pumped to the designated area in many small piles. The screeder then uses a straight edge to spread and level the screed to the specified level. Once the whole area has been levelled, the screed then either gets hand or power troweled to correct any imperfections and to improve the density of the screed. This should leave a smooth finish and ready to receive a final floor finish once it has finished drying.
This is an important factor when planning to apply screed. If it is a large project with tight deadlines, adding additives into the mix is not an option. This is because screed typically dries at 1mm per day up to 40mm and then at 0.5mm per day after that for example, 75mm screed would take 110 days to fully dry. There are many additives on the market, an example would be Ardex A35. This additive allows for light traffic after 3 hours of the screed being paid and is ready to receive a floor finish after 24 hours. Without any rapid drying additives, you would have to wait up to 28 days to apply the final floor finish which as you can imagine, is not always an option. Therefore, the rapid drying additives have been helping drastically in keeping projects within their timelines.
Screed thickness will depend on a few factors, the first one being on the type of screed:
- Bonded Screed – The thickness will most can be between 25mm and 40mm but it can go up to 200mm if needs be.
- Unbonded Screed – This will vary from 50mm-70mm and can also go up to 200mm.
- Floating Screed – The minimum thickness should be 75mm and can be higher.
- Modified Screed – This type of screed will vary between 55mm – 200mm.