Flow Screed?

I’m Andy Parkin, Managing Director of the Multi Award Winning Speed Screed.

We are here today to talk about flow screed. Flow screed goes under a few different names, liquid screed, flowing, screed pump, pump screed, and calcium sulphate amongst others.  If we stick to the calcium sulphate version rather than the cement based version, I would say 90% of the market is calcium sulphate when you’re talking about flow screed.

It’s normally mixed at a ready mix plant, and then supplied in a truck mixer, delivered to site and then pumped into where the screed needs to go. You are able to use it in all different applications:

  • Over underfloor heating
  • Bonded
  • Unbonded  (on top of a membrane)
  • Floating construction (on top of either acoustic insulation or thermal insulation)
  • It is suitable for residential and commercial properties

Some of the key features are:

Very low shrinkage

Compared to standard cement its shrinkage is significantly less

Does Not Curl

When used on floating construction you will not see the edges curling as it would be possible with a cement based screed.

Minimal Cracking

With less shrinkage comes less cracking

Less Movement Joints

The requirement for movement joints is greatly reduced.  You can actually lay up to 1000 square metres, providing it is a two to one aspect ratio, and no restraints there wouldn’t be the requirement for a single joint.  If it is a heated screed then you would be looking at 300 square metres, again based on a two to one aspect ratio and minus any restraints.

Foot Traffic

The product can generally be walked on after 24 hours

High Productivity

You can typically pour up to 2000 square metres so if you have the right project, if it is open areas, you can get the truck service, the plant isn’t too far away from the project.

Reduced Thickness

In certain applications, against a traditional sand and cement you can actually reduce the thickness. With a 65 mm domestic sand and cement on insulation, the depth could be reduced to 35 mm floating with a typical flow screed. The reduced depth does mean that we have a reduced weight as well.

Drying

After seven days of laying flow screed can actually be force dried, this is something that with a cement based product is not recommended.  With calcium sulphate after seven days the reaction has taken place, it is a crystallisation process. All you need to do then is to take the moisture out.

When force drying, it is very important to do it gradually when you are first heating the slab. Follow the British standard, if it iss under floor heated or if you are using external dehumidifiers.

Under floor heating systems, you can be turned on after seven days. A reduction from traditional sand and cement which the British standard states 21 days.

Thermal Conductivity

The efficiency of underfloor heating with a flow screed means that it will heat up quicker, and cool quicker.  Being almost twice as thermally conductive,  over a long period there will be obvious cost benefits relating to the heating.

We mentioned there is virtually no shrinkage, and movement joints generally only have to be in areas in excess of 1000 square metres, or 300 with underfloor heating. You should also consider putting movement joints typically across doorways, which are normally the weakest points.

With underfloor heating systems you may want to put joints in where you have got different heating zones and they are running at different temperatures and you want to control the heat separately in those two rooms. If you don’t, there is the possibility of thermal movement, and thermal cracking, because you have got two different temperatures between them. So you may put joints between those two areas to reduce the risk.

Compaction

In terms of compaction, with a traditional sand and cement you would hand compact. Flow screed is self compacting, although it wouldn’t be self levelling, but it is self compacting.

So there are no voids, and the need for compaction is effectively eliminated.

Wearing Surface

In terms of its own durability, it isn’t a wearing screed, as with most screeds.  It will require a floor covering, tiles, woods, laminate, etc over the top of it to become the wearing surface.

Generally it is not suitable for areas that are permanently wet, so if there is any possibility that water will get under the floor coverings and into the screed then generally you would probably avoid those areas. So a typical home shower for example, you shouldn’t have any issues but if you’ve got communal shower areas, then generally you wouldn’t be looking at using a calcium sulphate screed.

Falls

The other thing that you could look at is ideally you’re not looking to put falls in this product as well. If there needs to be falls in the screed, then flow screed would not be suitable, you would need a sand and cement to form falls.

Pumping

Flow Screed is pumped into place, to give you an idea it takes 20 to 30 minutes approximately to pump six cubic metres.

The screed is then dappled with a rounded aluminium bar twice, the first is to remove any air bubbles that’s present and then the second one is a light bounce on the surface, which just gives you your final level finish.

The floor coverings that are to be laid onto the screed, must only be laid when you are sure that the screed is dry, and so a screed moisture test would always be recommended.  You are looking for impermeable coverings, you would be looking at half a percent moisture, and one percent for breathable ceramics and carpets.  Always check with the manufacturer of the adhesives and floor coverings to confirm the required moisture content of the screed.

Primer

When applying cement based adhesives to the surface, you would need to prime the surface. That is because calcium sulphate and cement based products are not compatible, so they will not adhere to each other, and they would need a primer in between.

Minimum Depth (standard calcium sulphate)

  • 20mm bonded
  • 30mm unbonded
  • 35mm floating domestic
  • 40mm floating commercial

Maximum Depth (standard calcium sulphate)

  • Maximum 80mm

There is a specialist calcium sulphate product that you can actually go down to 15 mil bonded/unbonded,

Over heating pipes assuming the heating pipes are 15 mm the generic calcium sulphate will go 30 mil above the pipes, but there it a specialist product that will go down to only 20 mm above the pipes. If your pipes are 15 mm then you are looking at 45 mm and 35 mm respectively depending on which product it is.

So I hope that’s been of help, if you’ve any further questions please don’t hesitate to get in touch.

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