SMET advises on trouble-free screeding with UFH

Trouble-Free Screed Laying in Combination with UFH systems

BS EN 13813 – ‘Screed material and floor screeds – Screed material – Properties and requirements’

BS 8204-1 – ‘Screeds, bases and in situ floorings. Concrete bases and cementitious levelling screeds to receive floorings – Code of practice’

BS 8204-7 – ‘Screeds, bases and in situ floorings. Pumpable self-smoothing screeds – Code of practice’

DIN 18560 – ‘Floor screeds in Building Construction’

 

The specification of a floor screed (by a contractor) will vary depending on the requirement for its load-bearing capacity, durability, surface levelling requirements, final floor covering, time-frame and consideration of environmental aspects.

 

Screed can be installed using three main methods: Bonded, Unbonded and Floating (heated or unheated) Screed Construction

 

Bonded screeds are described in BS 8204-1, BS 8204-7 or DIN 18560 Part 3. These are fully bonded to the base. The strength of a bonded screed depends on the type of use.

Unbonded screeds are described in BS 8204-1, BS 8204-7 or DIN 18560 Part 4. These screeds are laid on a separating layer i.e. not bonded to the base, as typically a DPM is placed under the screed. Specification of the screed thickness should take account of BS 8204-1, BS 8204-7 and DIN 18560 Part 1.

Floating Screeds to BS 8204-1, BS 8204-7 or DIN 18560 Part 2 an insulation layer is used to enable installation of thermal or acoustic insulation. The screed is laid on top of insulation to create a thermally efficient floor. Floating screeds are commonly used where underfloor heating systems are provided or thermal or acoustic insulation is required. The thickness is determined by the binder type, projected live loads and final floor finish.

Heated screed is a floating screed to BS 8204-1, BS 8204-7 or DIN 18560 Part 2 where the floating screed layer is installed over underfloor heating pipes. The screed serves to conduct the heat evenly across the floor surface, avoiding hot or cold spots. In order that heat propagates only in the required direction of the room to be heated or cooled, the elements are inserted above insulating panels.

 

BS 8204 Part 1 mainly applicable to Sand & Cement and fine concrete heated screed constructions

Heated levelling screeds are generally laid in conjunction with proprietary underfloor heating systems as floating screeds over thermal insulation. The heating pipes or cables should be secured in position; their installation details should be provided by the manufacturer of the heating system. Cementitious levelling screeds should be laid at the thicknesses recommended in 6.4.3c (75mm minimum or 65mm for domestic applications) unless otherwise specified by the manufacturer of the proprietary system.

 

BS 8204 Part 7 applicable to cementitious and calcium sulphate based self-smoothing heated screed constructions

The thickness of the screed should be as detailed in section 6.4 (40mm minimum for commercial and 35mm minimum for domestic applications) but also ensuring a minimum cover over the heating pipes or cables of 25 mm for a calcium sulphate based screed or 15 mm for a cement based screed. Cement based screeds should not be heated until they are cured and dried because of the increased risk of cracking.

 

DIN 18560 Part 2 Distinguishes between three heated screed constructions

For Construction A, the screed thicknesses determined using DIN 18560 Part 2, Tables 1–4 should be increased by the external diameter of the heating pipe. For flexural tensile strength class F4, the minimum pipe cover totals 45 mm for cementitious screeds and 40 mm for self-levelling calcium sulphate screeds where natural stone and ceramic coverings are installed.

 

Example: 1. Cementitious screed (CT-F4) t = 45 mm

Heating pipe dia = 15 mm ➥ 60 mm total screed thickness

Example 2: Self-levelling calcium sulphate screed (CT-F4) t = 40 mm

Heating pipe dia = 15 mm ➥ 55 mm total screed thickness

 

Avoiding failures in heated screeds

Historically, floor screed failures have regularly resulted from the interaction of heated screed constructions with a wide range of coverings. Failure by the contractor to check on residual moisture and consequent installation of coverings on an overly wet screed may have serious implications for the durability and longevity of the flooring. The maximum permissible residual moisture (measured using CM tester) is 2% for cementitious screeds and 0.5% for calcium sulphate screeds.

 

UFH Systems Commissioning Times

Apart from measuring the residual moisture in the substrate, the contractor should also examine the commissioning report for the floor heating system and inspect the screed for possible cracking. The commissioning procedure serves to check the performance of the floor heating system and screed. DIN 4725, Part 4 provides for the heating up of

  • cementitious screeds after 21 days
  • calcium sulphate screeds after 7 days
  • rapid-set screeds (e.g. Sopro Rapidur® B5) after 3 days

 

Rapid-Set Systems allow fast-track screed laying in conjunction with UFH

However, it’s inevitable the lengthy drying times required for traditional screeds are frequently at odds with tight construction programmes and deadlines. Depending on their coat thickness, standard cementitious screeds need between four and ten weeks to achieve the prescribed 2% CM residual moisture content. The drying process may be accelerated through the use of specially designed binder mixes suitable for the production of cement screeds.

 

 

Available from SMET, are new screed products such as Rapidur® B5 rapid-set screed binder and Rapidur® FE 678 self-levelling screed (ready-to-use dry mix), which can be used to produce screeds that dramatically reduce the time of drying.  These allow early flooring installation complementing  UFH systems, while exhibiting all the properties of and delivering the same performance as standard cement screeds.  See the comparison table below:

 

 

The use of these latest high-performance screed products mean tiles may be installed in as little as 24 hours (Sopro Rapidur® FE 678) or 3 days (Sopro Rapidur® B5) after screed laying.  This has the result of speeding up the entire screed laying process, allowing quick access to follow on trades.

If you are a flooring contractor and wish to discuss these ‘next generation’ screed solutions or you’ve a job to lay in combination with UFH, fast-track or otherwise, call the screed specialists at SMET: contact: Smet Building Products Ltd 

 

T: +44 (0) 28 3082 5970

info@smetbuildingproducts.com 

www.smetbuildingproducts.com

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