A comprehensive guide for construction professionals to save time when installing floor coverings on fresh screed.
How long does it take for sand and cement screed to dry?
Laying screed is an important part of constructing a typical concrete floor. Screed is a mixture of cement and sand (and additives) that is used to create a smooth, flat surface over the concrete slab onto which the flooring layer can be placed. Once a screed has been placed, it needs to be left alone in a controlled environment until it has fully dried. The surface can be opened to foot traffic within, say, 24 to 48 hours, but the rest of the screed will take a longer time to dry.
The time for a screed to dry will vary depending on the type of screed and its thickness. Most traditional sand-and-cement screeds take a day of drying for every millimetre thickness of screed. Therefore, a 30-millimetre screed will take approximately a month to dry fully and develop its full strength. (This is very similar to standard concrete, which needs to be left for 28 days to develop its specified compressive strength.)
Can the process be accelerated?
You will have to allow time in your construction programme for the screed to dry. But there is a way to shave several days off the total time from application of the screed to laying the floor covering.
Here’s how.
BOSTIK HYTEC A370 RAPID is a ready-to-use, one-part, water-based damp-proof membrane that can be applied without needing a primer. It has been developed to suppress residual construction moisture in textured and absorbent cementitious subfloors, concrete, and sand/cement screeds. You can apply HYTEC A370 RAPID over a traditional sand-and-cement screed after about 21 days, before the screed has fully dried. The membrane will trap the remaining moisture and ensure that the curing of the screed continues while you proceed with the next steps of the flooring process.
The exact timing for applying HYTEC A370 RAPID will depend on the type of screed used and the manufacturer’s recommendations. For example, if you use a BOSTIK screed and follow carefully the instructions for its application, you will be able to apply the damp-proof membrane after only 18 days.
Preparation, planning, and application.
Bostik HYTEC A370 RAPID relies on adhesion to a sound, strong substrate. (Note: it can only be applied to cement-based substrates.)
Step 1: Preparation
- Ensure the subfloor is sound, clean, surface dry, and free from contaminants that may prevent adhesion.
- If necessary, vacuum the surface to remove dust.
- Remove all surface water.
- Application temperature +5 to +30 degrees Celsius.
- If there is an underfloor heating system, it must be fully commissioned.
- Warm water underfloor heating systems need to be switched off for 48 hours before application of the HYTEC A370 RAPID and left off for 48 hours after application, following which the temperature can be gradually increased.
Step 2: Planning coverage rates
- You must always apply two coats of HYTEC A370 RAPID.
- The specified coverage rate depends on the moisture level of the subfloor and whether or not there is subfloor heating.
- For unheated screeds where the moisture level is less than 90% RH, the maximum coverage is 40m2 per coat, with half of the contents of the bottle (6kg) used for each coat.
- For unheated screeds with moisture level between 90 and 95% RH, the maximum coverage is 30m2.
- For heated screeds, the maximum allowable moisture level is 90% RH and the maximum coverage is 30m2.
- Thus, moisture testing will determine how soon you can apply the HYTEC A370 RAPID.
Step 3: Application – first coat
- Shake the bottle immediately before use.
- Pre-wet a long hair roller with Bostik HYTEC A370 RAPID. Apply the first coat in one direction, ensuring that the material is worked well into the substrate.
- Allow it to fully dry. The product is blue when applied and then changes colour to a glossy, black when it dries. This will take about 15 to 30 minutes at +20 degrees Celsius.
Step 4: Application – second coat
- Apply the second coat of Bostik HYTEC A370 RAPID in a direction at right angles to that of the first coat.
- Allow the second coat to dry to a tack-free film. This will take about 60 minutes at +20 degrees Celsius.
- Drying times are based on ambient conditions of +20 degrees Celsius and 65% RH. Colder and/or more humid conditions will delay drying.
Step 5: Next steps
Once you have applied the HYTEC A370 RAPID, it is important to keep the membrane free of foot traffic, dust, and other contaminants.
Within 24 hours of applying the second coat, cover the membrane with a minimum 3mm thick layer of a BOSTIK self-levelling smoothing compound. This will provide an absorbent medium for the flooring adhesive.
Note: if you miss the 24-hour window for applying the smoothing compound, you will need to apply another coat of HYTEC A370 RAPID.
Choice of Materials.
Self-levelling smoothing compounds
BOSTIK offers you options for the self-levelling smoothing compounds you can choose to apply over the HYTEC A370 RAPID membrane. Select the one that best matches the conditions of your project.
Floor Adhesives
We recommend the following BOSTIK floor adhesives for use with different types of flooring:
A good floor takes time.
But you can streamline the process.
Install a HYTEC A370 RAPID damp-proof membrane over fresh screed before it has fully dried, and you can cut days off your programme. No primer required, just two coats of the membrane and then a smoothing compound, and you will be ready to apply your adhesive and lay the floor covering.
Use BOSTIK screeds, membranes, self-levelling smoothing compounds, and adhesives that are fit for purpose and best suited to the floor covering and building environment.
Peace of mind. A job well done.
Related articles
Stay aware about our latest innovation and our latest news and sponsoring actions
Discover Our Waterproofing System Using Our 3D Building Tool
The best way to insulate windows
