Traditional Floor Screed

A traditional floor screed system is the only reasonable method to achieve a floor in a wet room to falls.

Systems for Traditional Screeds

There are three main systems for traditional floor screeding, Fully bonded, Partially bonded, Floating.

Fully bonded screeds need to be laid on a shot-blasted / scabbled concrete base, and be bonded using an adhesive such as PVA, SBR, epoxy resin or good old fashioned cement.

PVA glue is a good screed adhesive especially if it is mixed with water & cement, to make a slurry. If the concrete base is primed the day before with PVA & water mixed, then a slurry applied prior to screeding, then it is quite reasonable to expect screed to fully bond at just 25mm thick.

SBR has approximately the same characteristics as PVA, but it is not water soluble after drying, and so is recommended around wet areas, such as swimming pools.

Epoxy resin, such as Isocretes’ M-bond’ allow screeds to fully bond at depths as little as 15mm. It is quite an expensive system, but has many advantages over other bonding agents. One such advatage is that if two-coats of epoxy resin are applied (one 12-24hrs before the second coat & screeding), then the epoxy resin acts as a top end DPM.

Cement makes a reasonable bonding agent if there is reasonable depth of at least 50mm. A good slurry must be formed using water, and must be brushed in thoroughly and any drying spots must be kept damp as screed is applied.  Always a good idea to damp the concrete slab the day before.

Partially bonded screeds are a cheap alternative to doing the job properly, and have the added risk of the floor screed failing, because at depths less that 50mm, the screed can start to break up if the bond to the concrete is lost. This system is only used when people are tryin to save money, and sometimes bonding agents like PVA & SBR are still used, but they are still at high risk from failing due to the slab not being shot-blasted or scabbled.

Unbonded Screeds as the name suggest are not bonded directly to the concrete base, but are intentionally debonded with the use of a DPM (damp-proof membrane). The advantage is that any cracking from the sub-base is not carried through to the screed, drying times can be reduced by omitting the concrete drying time. All walls and pillars must be lined with edging foam or 20mm insulation, such as kingspan, to protect against shrinkage cracking.

Floating Screeds are laid on insulation to provide an insulated floor area. All walls and pillars must be lined with edging foam or 20mm insulation, such as kingspan, to protect against shrinkage cracking.  This system is the most commonly used in house building, especially with underfloor heating systems.  There is very little difference between unbounded and floating screeds.  high-impact insulation such as extruded polystyrene can give the same levels of rigidness on floating screeds as you would expect from unbonded screeds

 Modified screeds

There are three main types of modified screed which are in use today, fast-drying, fast-set, and polymer modified.

Fast-drying screed such as Isocretes’ K-screed’,and Tarmacs’ Truscreed’, use super-plasticisers to reduce the amount of water needed to make the screed mixture plyable, this in turn makes for a fast drying time as there is a higher cement to water ratio.  The drying times for both these systems is around 3mm per day.

Fast-set screed such as Ardex Ardurapid and Isocretes screedfast rely on aluminium based cement to accelerate the setting process. This type of screed tends to be foot traffickable after an hour or two, and is usually dry within 2 days.  This type of system is very expensive and the quality of the screed can be poorer than usual due to the floor layers lack of working time with the mix.

Polymer modified screed is a standard mix with PVA or SBR added in large amounts to create a gluey screed.  When set theses types of screed are resistant to water and other chemicals, they also allow floor screeds to be thinner than non-polymer modified screeds.