An Introduction to Screed Drying

One of the fastest growing product ranges in our portfolio is the HPW and WB portable electric boiler packages. Although useful for a range of applications including cover during plant heating maintenance, provision of heating and hot water for events and temporary structures, or as emergency cover during fixed system breakdowns, the most popular use for them isn’t exactly the most obvious: screen drying.

What is screed? Although it comes in many different types and forms to suit specific applications screed is, generically speaking, the layer of flooring that sits on top of the concrete and gives it a level, flat surface.

Here we’ll try to give you a brief run down as to why screen drying can be a problem how a portable electric boiler can help.

The Issue

The pace of modern life coupled with an I want it now culture, a greater efficiency (which often just becomes ‘faster’) and pressure to deliver ASAP all set against a backdrop of greater regulation mean that the construction industry is under greater pressure than ever before. In addition to financial pressures, these days construction companies also have to consider: customer satisfaction, time constraints, environmental considerations, changing technology, staying one step ahead of the competition and accommodating a diverse and complicated workforce made up of a number of different trades, many of which can’t be onsite until the previous crew have finished. Delays in construction projects are costly, frustrating and in the worst cases, catastrophic to either the project as a whole of the principle contractor overseeing it. Waiting for Screed to dry is an unavoidable process but it can bring a project to a grinding halt as construction ceases until the workforce can get back into the area to carry on.

The Answer

A portable electric boiler can be used to run an underfloor heating system to carefully, steadily and safely reduce the screen during times which means the construction firm can keep to a dealing and the project on schedule. However, it should be noted that speeding up the drying process requires expertise and experience and should only be undertaken by a specialist provider. If you’re looking at using a packaged hot water system but aren’t quite sure what or how then please feel free to get in touch with us and we’ll recommend one of our specialist expert hire distributors.


To avoid ruining your screed always seek the advice of an expert before rushing into anything. Rapid screed drying is a process that requires a specific knowledge and skillset so you really are best off going to someone like Cross Rental if you’re new to it. Seek advice from a Safe Contractor accredited company with OFTEC approved engineers. Whomever you use though it would be prudent to ensure they are Safe Contractor accredited, have a good reputation, the relevant experience and look to invest in their fleet as technology advances.

What types of screed are available?

There is a great variety of screed types on the market, some better suited to certain applications than others. Some types such as Gyvlon, Supaflo and Truflow; a traditional, granolithic or fast-set such as Mapei Topcem and K-screed are what is known as flowing screeds which can be good if time is limited however the flip-side to that is cost as they tend to be more expensive. For domestic type applications or where ceramic tiles are to be used as flooring you tend to find good old fashioned sand and cement screeds are better suited as the bond is generally stronger than other screeds. Alternatively where the floor will see heavy traffic from both people and handling machinery such as forklifts then a hard aggregate based granolithic screed will normally be used as they are very hard-wearing.

Traditional and Free Flowing Screeds

FeatureTraditionalFree Flowing
Max DepthDeeperShallower
Price ConsiderationNormally more economicalNormally less economical
Curing time considerationSlowerFaster


Unbonded Screed – Not bonded directly to the concrete floor but is instead laid onto a PVC or polyethylene layer that covers the floor.

Floating Screed – As with unbonded a floating screed will be laid over another layer that separates it from the concrete floor. With floating screed this extra layer will usually be a form of insulation, normally thermal and/or acoustic such as expanded polystyrene.

Bonded screed – Laid directly onto the supporting material or structure and bonded to it using something like an epoxy resin, PVA, SBR or even concrete.

Heated screed – Laid over an underfloor heating system made up of plastic pipes which mounted on a thermally insulating material and camp proof layer. As with a conventional radiator this pipework is then plumbed into the cetnral heating system where the boiler provides the hot water.


Modified Screeds and Additives

Hydraulic binding agent or binder – A chemical such as Mapei Topcem can be mixed into screeds to produce a faster drying screen with greater strength and resilience.

Plasticiser – Added to screed to improve curing times by reducing the moisture content. Common brands of plasticiser are K-screed and Truscreed by tarmac.

Aluminium – Aluminium based cement has a faster setting time however that can be offset by the disadvantages, which include cost and lower, quality. Nevertheless, aluminium concrete is well suited for certain applications within the construction industry. The most well known products on the market are Ardex Ardurapid and Isocretescreedfast.

Polypropylene fibres – Added to reinforce, prevent cracking and give a more cohesive screed. Truscreed is a good example of a screed using this type of reinforcement.

Polymers – By adding a large enough quantity of something like PVA or SBR you produce viscous, sticky screed giving it chemical and water resistance properties. It can also allow the floor screed to be slightly thinner than other types.

TypeMinimum Depth – Liquid Screed
Fully bonded25mm
Over Insulation35mm
Over Insulation40mm
Over Underfloor Heating (UFH)30mm from top of pipes


Screed Drying and Setting Times


ProductTypical Setting TimeTypical Drying TimeTypical Strength
Traditional screed (Premixed 1:4)24 – 48 hours1mm per day27 – 30N</p<
Fast – set screed (1:5)1 – 2 hours10 – 15mm per day22 – 27N
K-screed (1:4)6 – 12 hours25mm per week30N+
Mapei Topcem (1:6) Including Tarmac Cem56 – 24 hours5 days27 – 30N
Flowing screeds – Gyvlon/Supaflo etc6 – 48 hours1mm per day25 – 27
Granolithic screeds12 – 28 hours2mm per day40N+
Tarmac Truscreed (1:4)24 – 48 hours25mm per week30N+

In order for screed to set properly it is vital that the environmental conditions are right from the start. Temperature, ventilation and exposure to moisture are probably the most important conditions to be taken into account as they will have the greatest effect on drying times, and how well the screed sets.

If we take a free flow material such as Gyvlon, which is commonly used over underfloor heating systems, as an example, we work on the basis of a drying rate of 1MM per day, up to a thickness of 40MM. At thinesses greater than 40MM the curing rate halves to 0.5MM per day thus doubling the overall drying time.

In order to reduce drying times it is possible to ‘force dry’ Gyvlon using a portable electric boiler or portable hot water package, but only after seven days following installation. When using this method operators will run water through the underfloor heating system at a lower temperature of 20-25°C for 24hrs and increase it 5°C per day up to 55°C, then reverse the process by bringing the water temp down 5°C per day until it gets back down to 20 or 25°C.

Broughton EAP offer a range of portable boilers that can be used in conjunction with an underfloor heating system to help reduce the curing times of screed. The HPW20 and HPW40 can both be programmed to automatically increase water temperatures 5°C per day and can also supply water from as low as 10°C so are ideal for these types of applications.

Most screed will reach maximum strength after 28 days however residual moisture levels must be assessed before laying any flooring over it, to make sure it is safe to do so. Residual moisture testing should only be carried out using a recognised, approved test method.

Using a Boiler to assist with curing a screed floor

If you are interested in using a portable hot water package on your project then an experienced, Safe Contractor accredited company will be able to offer you advice. One of oyur specialist distributors will be able to specify, support and install the right product for your application. As each application comes with it’s own set of technical difficulties then the experience and expertise of the supplier will be essential to ensure the project runs smoothly.

Preparing a screed floor prior to laying flooring

Whatever type of screed you’ve put down you must ensure it’s properly cured before you start laying the actual flooring over it. Under optimum conditions most traditional screeds will take a week for every centimetre of thickness to properly dry, which can add up to over a month on thicker floors. Use of a portable electric hot water boiler can reduce this time without reducing the effectiveness of the finished floor.

As previously mentioned, any screed floor must be checked for residual moisture levels before moving onto the next stage, such as tiling. If the residual moisture level is still too high it can prevent a proper bond forming with the tile adhesive. It also recommended that your screed flooring is lightly sanded and primed to provide a barrier between the screed and any cement based adhesive you might be using.

Common issues with screed floors

By definition the process of drying something out involves the removal of moisture. If rushed or generally not carried out properly then you run the risk of encountering problems:

Cracks – After going to the expense of having a new floor laid the last thing you want is cracks.

Curling – Tends to affect thinner screeds because they dry more quickly than thicker ones. The most likely area’s to see it are internal angles such as the corners where walls meet and day joints.

Shrinking – As you remove water from a screed the overall volume will decrease which unless it’s properly laid can mean you’ll see shrinkage in the screed. In order to mitigate this some manufacturers use something called a super plasticizer to reduce the likelihood of both shrinking and cracking.

Because screeds are so susceptible to environmental factors then it really is best to use an experienced tradesman to ensure it’s done properly. This becomes doubly important if you plan to use an artificial drying process which could possibly lead to, or even exaggerate some of the problems mentioned above.

Common misconceptions associated with screed

Traditional screeds won’t properly encase underfloor heating pipes so I need a free flowing screed.

This is not entirely accurate, a professionally laid traditional screed laid by an experienced professional can also encase pipes. In any case, should the pipes not be fully encased it will have little to no effect on the efficiency of your underfloor heating as heat travels vertically.

Artificially speeding up the drying time causes cracking, shrinking and curling in my screed flooring

Only if it’s not done properly. If you use an expert, experienced company to help with speeding up drying then there is no reason why it won’t be done properly. These new methods are becoming the norm on large building projects where time is critical to the financial success of a project.


Use of a portable boiler or hot water package can be an extremely effective way to help you meet, or even improve on deadlines. They can allow you to get all those other trades in, the guys that can’t really start until the floor is dry days or even weeks before you might be able to if you’re relying on natural drying. Plus it the installation is handled by an experienced, well trained company then there is no reason to fear shrinkage, cracking or any of the other possible problems which you can get on with job.

If you’re planning on adding portable electric boilers or hot water packages to your fleet, or would like advice on who to speak to about renting one in then please feel free to contact a member of our friendly, professional sales team on either 01527 831610 or Alternatively, details on the HPW range of trolley mounted boilers can be found at our website together with information on our range of portable air conditioning units, industrial fan heaters, ventilation fans, cooling fans, dehumidifiers, fan coil units.