Information and CPD for Architects and Specifiers
Architects & Specifiers
CPD doesn’t have to come from, be accredited by, or approved by the RIBA to count.
Should you read this article and learn a little, you may award yourself a CPD point.
An accreditation is a commodity that can be purchased, like any other commodity. The supply of accreditations is a business and not an independent assessment free of commercial interests.
When considering the true value of an accreditation, the reader should always look for scientific information that can be compared and tested, rather than unqualified expressions of opinion.
In the area of Floor Screeds, the BBA Certificate is automatically hailed as a mark of quality in the UK.
So does BBA Agrément Certificate 91/2678 for Isocrete K Screed, for example, provide comparable scientific data or just express an opinion?
The strength and stability of a screed is an important consideration. We might, reasonably, expect compressive strength to be expressed in terms of N/mm².
By testing the compressive strength and providing an actual number, the BBA could have satisfied the requirement for comparable scientific data.
What we actually find in the BBA in question, however, is nothing as straightforward or comparable as an actual figure.
What the BBA say, in Section 6 Strength and Stability
6.1 The product has adequate strength for use on concrete bases and has sufficient resistance to normal loading and loads with light wheeled traffic comparable with conventional sand/cement screeds.
Or, in Section 9 Durability
The product, when correctly installed and covered by a suitable floor covering, should have a life equal to that of the building in which it is installed.
Comments such as these are of little value when comparing screed performances.
As such, can we rely of these traditionally recognized accreditations as measures of performance or are they of little more value than a club membership that can be purchased and worn like a badge?
Technical Data Sheets
Most manufacturers produce technical data sheets, so they are probably the best place to start a search for comparable data when trying to choose which screed to specify.
But sometimes comparable data can be frustratingly obscure.
With all types of floor screed, the time it takes for the screed to dry enough for the application of the floor finish is valuable information and possibly, the main reason for paying extra for an enhanced or specialist screed.
A 50mm thick traditional sand and cement screed without any performance enhancing additives would take about 2 months to dry enough to receive floor finishes.
Why pay extra for an enhanced screed that performs the same as plain sand and cement?
Surely, we would want something that is more than just comparable to a plain screed.
We are informed in the British Standards, that a screed must have a moisture content of less than 75%RH before a floor finish may be applied, so the time it takes to dry is of great importance.
An importance that is readily understood by anyone who has witnessed ballooning sheet vinyl that had been glued to a screed that hadn’t fully dried as required.
A newly installed screed must first be allowed to cure before commencement of the drying process.
Curing is affected by covering the freshly laid screed with a curing membrane, normally a sheet of polythene these days rather than dampened hessian sacking. This slows down the drying process to prevent rapid shrinkage and subsequent cracking or lifting and curling at bay edges.
Screeds are normally cured for 5-7 days, depending on atmospheric conditions, before the curing membrane is removed and the screed allowed to dry.
Consequently, a basic understanding of the difference between curing and drying and knowledge of both the curing time and the drying time is of equal importance when selecting a screed.
Some screeds have fast curing times but slow drying times so manufacturers sometimes take advantage of any confusion between curing times and drying times, highlighting one without mentioning the other.
On the innovation prize winning Isocrete Flowing K data sheet, for example, it says the following under the heading of Drying Time in black letters to stand out on a white background.
“Moisture sensitive floor finishes can be installed when the screed is dry to 75%RH as per BS 8204. After 24/48 hours curing (without draughts) ensure the area has sufficient ventilation to dry”.
Considering the title “drying time” careful reading is required to note that the drying time which is actually, up to 3 weeks, subject to drying conditions, isn’t mentioned at all. The only time stated is, notwithstanding the title, just the curing time.
In fairness to Flowcrete, they do also state a qualified drying time elsewhere, albeit in small green print on a green background.
Typically 14-21 days, depending on drying conditions.
To calculate how long this screed has to be left before the floor finish may be applied, we need to add the curing time of up to 2 days to the drying time of a further 2-3 weeks but then, noting that these times are dependent on drying conditions, add a few more days, so a total of about a month.
All manufacturers want to sell their products so will tend to emphasize their positive attributes.
In the Technical Data sheets for Gypsum based screeds, emphasis will be focused on the fact that they may be walked on after just 24 hours, implying that they are dry and good to go. Whereas, in fact, they take 1 day per mm of thickness for the first 40mm then 2 days per mm of thickness above 40mm to dry to the required 75% RH.
Technical data sheets are not independent assessments and the data displayed has often been obtained by self-testing in laboratory conditions.
They should, therefore, always be read with a degree of caution as they are a marketing tool as much as a source of information.
The Technical Data Sheets for Retanol products, produced by PCT-Chemie are refreshingly straight forward.
For example, the Retanol Xtreme Pro 1 Technical Data Sheet just states that the screed should be left for 48 hours for tiles and 72 hours for the installation of all other floor finishes, from the time it is laid.
Most manufacturers offer a warranty against defective materials.
Manufactured products are often installed by approved installers.
So, what happens when there is an issue on site?
Unfortunately, in the event of an issue, this sort of warranty invites the installer to blame the material and the manufacturer to blame the installation, leaving the Client to resolve.
Only PCT-Chemie guarantee the performance of their products.
That fact alone should give the specifier enormous confidence in the selection of PCT-Chemie products on their projects.
Types of Floor Screed
If we ignore the more specialist varieties of screed, these fall into two main types.
- Liquid Screeds.
- Semi-dry screeds.
Liquid screeds are also of two main types.
- Gypsum based.
- Cement based.
Semi-dry screeds are, similarly, also of two main types.
- Traditional sand and cement screeds.
- Enhanced sand and cement screeds.
Which type is best for my Project?
There are many factors to take into account when selecting the most suitable floor screed for a particular project, these include:-
- Performance requirements and screed properties.
- Health and Safety.
In the most empirical of terms, the less sophisticated the screed, the cheaper it is.
It follows that the least expensive screed is basic sand and cement without any additives.
But specifying a traditional screed isn’t always the cheapest option.
There are other elements of cost to consider in the value engineering exercise, including:-
- Minimum thickness requirements.
- Speed of installation.
- Site set-up costs and minimum area requirements to maintain installation time savings.
- Curing and drying times required before installation of floor finishes.
- Additional preparation and treatment costs.
- Additional requirements such as fibre or D49 reinforcement, isolating or curing membranes.
- Specific performance requirements in service.
- Other factors, such as design or site condition considerations etc.
Basing screed selection on one element of cost without due consideration of all elements of cost could easily result in the most expensive option being selected where the least expensive was sought.
For example, it might first appear that a Gypsum based liquid screed will provide the most economical option as large areas can be poured faster than a traditional screed can be troweled.
But, if the site programme doesn’t lend itself to the exclusion of all other trades from large areas to permit those areas to be completed in a single pour, the resultant standing time or return visit costs could well mean that the selection of such a screed becomes the least economical.
Similarly, if a screed is selected because it can be installed quicker in the anticipation of an earlier handover, any saving in time would be wasted and the cost benefit lost if that screed had then to be left for a month before the floor finish could be installed, delaying actual handover.
Another common mistake is the specification of a cheaper type of enhanced floor screed to save a small sum on the installed rate. This can prove to be a false economy if, for example, an expensive epoxy DPM has to be subsequently installed to allow the floor finish to be installed to programme.
It often proves more economical in the long run to choose a better performing screed, albeit at a slightly higher rate than the cheaper option. This would certainly be the case where the requirement of a DPM could have been obviated by selection of a faster drying screed.
As is often the case, with screeds, you get what you pay for. Or, more accurately, you either pay a little more now, or pay a lot more later.
The level of programme detail required to enable a specifier to select the best option at tender document production stage, will, however, be seldom available.
So, to avoid the accidental specification of an uneconomical product, it will always be safer to specify the most versatile product in the first instance and allow the natural process of value engineering to take place if and when the specialist finds that an opportunity arises to offer a better alternative.
Not all screeds are suitable for all requirements.
Very few screeds are, for example, suitable for wet environments or external conditions in service as they might dissolve or be subject to erosion in the presence of water.
Even fewer screeds can be installed in wet or external conditions.
All liquid screeds and most semi-dry screeds, must be protected from water ingress.
When most manufacturers are asked for a screed suitable for external or wet conditions, they can only offer far more expensive polymer or waterproof screeds.
Liquid screeds are also not suitable for installing to falls or cross-falls due to their self-levelling function.
Where falls are required, a semi-dry screed is required.
The requirement for steps at door thresholds to accommodate different floor finish thicknesses is another common requirement and one that would require temporary formwork and sectional installation to achieve with a liquid floor screed, but not with a semi-dry type of screed.
A similar situation arises where the formation of matwells might be required.
Again, semi-dry screeds will be more versatile in such situations and might well save more time in real terms than any time saved by the speedier installation of liquid screeds.
A more versatile screed will always be a better option where the floor must be laid to falls, steps to accommodate different floor finish thicknesses, in matwells, dishing around drainage outlets, to stair treads and landings or any number of other design considerations.
Another suitability consideration might arise, for example, where there are height restrictions that require a floor screed to be installed to a finished floor level that might require a screed thickness below the minimum thickness permitted for an otherwise ideal screed type.
One screed type may be suitable on a project for some areas, but not for all areas.
In those areas, a more enhanced screed with better compressive strength may be required such as in a plant room, to a loading bay, or to a garage area, so a screed that is suitable for both external conditions and have a higher load bearing capacity might be required.
Where different screed specifications are required to suit different areas on site, there is an elevated risk of contractor error.
The requirement of different screed types for smaller areas also reduces economies of scale and adds to delivery costs and the carbon footprint.
The solution is to select a screed that is versatile enough, such as Retanol Xtreme Pro 1, to suit all conditions, wet or dry, internal or external and all service requirements.
The performance characteristics of a screed are of vital importance.
Floor screed failures are both costly and highly disruptive.
In the case where a hospital ward, for example, has had to temporarily close whilst the sheet vinyl is removed and the underlaying screed re-placed or repaired due to collapse under point loading might even have consequences that are greater than mere inconvenience.
The load bearing capacity, or compressive strength of a screed is, therefore, of prime importance when considering the performance characteristics of a screed.
All cement-based products tend to shrink and crack as they dry from the upper exposed surface downwards loosing moisture through evaporation. The forces exerted in this process also cause lifting at day joints and curling at bay edges.
To minimize the effect of such forces and reduce the extent of such shrinkage cracking, lifting and curling, additional enhancements might be recommended by the manufacturer.
These could include a waterproof membrane for curing, D49 steel mesh reinforcement or polypropylene fibre reinforcement.
The requirement of such additional enhancements to attain the level of performance required as well as the additional time involved, must be understood and taken into account.
Other more innovative screed enhancements may, however, modify the drying process so that the screed dries evenly throughout the matrix rather than from the upper surface downwards. This type of advanced drying technology eliminates lifting and curling and reduces the tendency to crack upon shrinkage to such an extent that no reinforcement is required.
This advanced technology also obviates the requirement for curing and the associated cost of a curing membrane, saving both time and money.
Curing time and drying times have already been mentioned as elements of cost appraisal.
The speed of cure and time required for drying also relate to the attainment of essential performance requirements for the continuity of the build programme.
The percentage of full strength attained at an earlier time may well be a crucial factor.
Similarly, a screed that is capable of withstanding wet conditions may have to be selected for use as a roof screed, particularly in the UK where it rains, on average, every other day.
Falls are usually formed by stepping insulation boards rather than installing screeds at a thickness to achieve the required slope for roof drainage.
This method enables the designer to create the required fall for drainage, reach the required level of insulation and adhere to design load restrictions.
The selected screed then installed over the stepped insulation boarding may well be exposed to the elements for a significant time before the waterproofing layer could be added, so a screed designed to perform in such conditions is essential.
Site conditions may prevent the completion of the roof waterproofing prior to the requirement for the screed to be installed in internal rooms, leaving those internal areas subject to possible water ingress or even flooding.
Such circumstances are extremely likely in the UK and selection of a screed that may not be exposed to water would result in delays to the programme.
Such delays could be avoided by selecting a screed that is designed to perform in both wet and dry conditions.
The specifier requires an understanding of the performance requirements of the screed to be named in the specification, including the following:-
- Minimum thickness, bonded, un-bonded or floating.
- Requirement for fibre or steel reinforcement to reduce shrinkage, lifting and curling.
- Requirement for isolating membranes.
- Suitability for wet or external conditions.
- Impact and abrasion resistance.
- Load bearing capacity and compressive strength.
- Flexural strength.
- Jointing requirements.
- Suitability for use with Under Floor Heating Systems, time required before commissioning, thermal conductivity, minimum pipe cover requirements, need for reinforcement.
- Speed of cure and requirement of a curing membrane.
- Speed of drying to 75% RH.
- Compatibility with subsequent floor finish and requirement for further treatment such as abrasion to remove surface imperfections or substances.
- Suitability for falls, cross-falls, steps, treads and risers, matwells or other construction details.
- Special preparation requirements such as waterproofing the substrate to receive a liquid screed.
- Minimum access requirements.
Product selection through specification, like design, often requires a compromise.
The screed that may be installed the fastest, for example, may not be the most suitable in wet conditions. There are any number of similar examples of such situations.
As it may well be an impossible task to find the best solution without risking the selection of an unsuitable product, the answer is, again, to specify the most versatile product available.
The most versatile screed available in the UK today is Retanol Xtreme Pro 1.
No other screed can meet to all programme and service requirements, in all conditions.
Health & Safety
All manufacturers produce Safety Data Sheets in conformity with current regulations.
How often, however, is the information contained on these data sheets properly considered before a product is specified?
Would it surprise you to learn that the screed most widely specified on NHS projects contains ingredients that can cause lung cancer in humans?
Ingredients to watch out for include carbon black and respirable crystalline silica-containing dust.
The British Medical Journal, BMJ have concluded that both current and cumulative exposure to carbon black have a deleterious effect on respiratory morbidity.
The International Agency for Research on Cancer, the IARC, have classified carbon black as a possible human carcinogen.
Of equal concern is exposure to respirable crystalline silicates.
The inhalation of silica dust may cause silicosis, a nodular pulmonary fibrosis.
The IARC have concluded that crystalline silica, inhaled whilst handling materials containing this substance, can cause lung cancer in humans.
Floor screeds and floor screed additives that are safer to use are available, but how do we know which they are?
The recognized standard in our industry is TÜV Rheinland certification.
Only products that have attained that standard may be classified a free of components that are a risk to human health.
Only Retanol products, manufactured in Germany to this exacting standard and supplied exclusively in the UK via PCT-Chemie UK Ltd have attained this level of health and safety certification.
The Carbon Dioxide equivalent, or C02 e of a product is the usual measure of sustainability.
This is an important consideration as for every ton of cement produced, 622Kg of C02 is emitted into the atmosphere.
It follows that, on face value, screeds of lower cement content, such as Gypsum based screeds have a lower C02e.
Gyvlon, for example, claim a reduction in emissions of 35%.
Use of Factory Produced Composite Cement would also reduce emission by approximately 25%.
The amount of ordinary Portland cement involved in screed installation is, however, miniscule in comparison with that used by the Construction Industry in the production of concrete.
As with most things, taking a view on face value can often lead to choices that have the opposite effect to that intended.
The production of polythene sheets, for example, involves the production of 6,000 Kg of C02/Ton of polythene produced, nearly 10 x that released in the production of cement.
Before reaching any conclusion as to the screed with the lowest carbon footprint, numerous factors have to be taken into account.
Are polythene sheets required to tank the base to contain the liquid screed?
Are polythene sheets required to seal windows or for temporary protection against water ingress or draughts that could spoil the installation or affect the cure?
Are polythene sheets required as an isolation membrane for an un-bonded or floating screed?
Are polythene sheets required to provide a curing membrane?
How is the material delivered to site?
How is it mixed, pumped or delivered on site?
Are plastic sheets required to protect the installed screed? Is plastic involved in the product packaging?
Does the minimum thickness required mean that more product is required?
Do any performance factors affect the carbon footprint of a screed selection, such as the maintenance of temporary site services whilst awaiting curing or drying times?
These are just some of the many factors to take into account when selecting the most sustainable floor screed option.
One good indication of sustainability is BREEAM certification.
All Retanol products, manufactured by PCT-Chemie GmbH and supplied in the UK by PCT-Chemie UK Ltd have attained BREEAM certification.
The conclusion reached in all of the considerations required in screed selection is the need for versatility.
There is only one screed available in the UK that meets all possible service requirements, standards and performance parameters.
Retanol Xtreme Pro 1
Standard specifications in either CAWS or Uniclass formats are available as open Word documents for adjustment by specifiers to job specific specifications upon request.
About the Author.
This article was written by Mike Williams, an independent consultant of over 45 years’ experience in floor screeding in the UK and Europe.
Mike’s career highlights:-
Initially an Economist that qualified as a Quantity Surveyor to enter the Construction Industry, he soon specialized in floor screeding after working for various Civil Engineering and Construction companies and PQS practices before joining specialist sub-contractor John Cothliff, an original Isocrete Licensee, in 1977.
Joined the Liverpool branch of the BEC, Chaired the Membership Committee, became a National Committee Member and BEC Parliamentary Liaison Spokesman, hosting Ministers for Merseyside, Tom King and Michael Heseltine.
Established Multicoat with Paul Fields, then of Conren Chemicals that had been established since 1972. Multicoat completed some of the largest resin flooring projects in the UK and Europe.
Sold Multicoat to Roadcoat GmbH and was appointed MD of the Roadcoat Group in the UK.
Established Interflow Group plc with Paul Fields that later became part of Flowcrete Group plc.
Interflow completed many of the largest resin flooring and car park projects in the UK including the Millennium (now the Principality) Stadium and the Underground Car Park at Hyde Park.
Worked with Flowcrete to create a car park waterproofing division that became Deckshield.
Purchased Isocrete Roofing from Flowcrete to establish Deckshield Contracts that became Deccon.
Established William Lea Ltd with Billy Lea, now one of the UK’s major floor screeding companies.
Established Deckmaster as Deccon’s in-house resin product brand.
Completed several major resin flooring and car park projects throughout the UK with Deckmaster.
Sold Deckmaster to David Tomlinson and Resdev, the co-developers and sole manufacturers of Deckmaster products.
Sold William Lea Ltd to Colin Jones and the Lea family.
Established Car Park Coating Ltd and completed several major car park projects, winning the BPA’s Best New Build Car Park award with Vinci for the Royal Liverpool Hospital Car Park.
Sold CPC to a management team headed by Anthony Gregory and Roadgrip Ltd for whom CPC had completed numerous large car parks throughout the UK including Westfield Shopping Centre.
Now retired and spending his time between his bungalows in Ness on the Wirral and Portocristo in Mallorca, Mike works part time as a consultant for PCT-Chemie.
“I want to support PCT-Chemie’s range of fantastic products that perform like nothing I have ever had the pleasure of working with.
Retanol Xtreme Pro 1, in particular, is the best floor screed that money can buy and the one I would always recommend without hesitation or reservation”.
Michael Victor Williams 2022