Bonded screed to increase sub-floor height

The need to raise the height of a sub-floor prior to the installation of a bonded/adhered floor covering is a relatively common occurrence. There are many options available from using plywood, thicker or layered beds of adhesive, or even specialist underlays. However, when a floor covering is to be truly bonded as to give that ultra firm feel and sound as well as to fulfill the technical requirements of your flooring manufacturer, then a bonded screed system is one of the best and often only options to consider.

cleaned-concrete-sub-floorWhen a sub-floor requires raising from 15 mm up to 40 mm (approximate values subject to screed manufacturer spec), utilizing a bonded screed system can be one of start-of-bonded-screedthe best and cheapest options. Conventional smoothing compounds, even at 15 mm depths, can work out extremely expensive as well as take an age to dry. Even the so called ‘deep base’ compounds can put a huge dent in any budget.
bonded-screed-2Such smoothing compounds consist of highly developed formulas consisting of refined sands, polymers, and cements etc. As such, they come at a price. Not as much of a problem when the layer being applied is thin (Typically between 3 and 6 mm), but once the depth starts to touch the 15 mm mark, you’d be shocked at just how much that can add to the overall cost! Bulking aggregates (stone chippings) can be added to smoothing compounds at these depths, but you’d be surprised how tricky this can be in a practical sense due to the immediate rapid setting qualities of such compounds.

bonded-screed-3Screed mixes are bulked out with four to five parts sharp sand (concrete sand) to one part screed cement (I use that term descriptively). To give a rough idea of the difference in cost, a bonded screed can work out around one third the cost of a conventional smoothing compound. This is solely due to the relatively cheap cost of sharp sand at between £2 and £3 per 20 kg bag. You can expect to pay between £15 and £25 for a 25 kg bag of half decent smoothing compound. Therefore, after you’ve added the cost of sceeding cement, at the above ratio, it’s pretty clear we’re by far talking about the cheapest and most effective method when raising sub-floors to such depths.

bonded-screed-4As installers, we are often caught up in time restrains/deadlines. Therefore, we have to look at a system that will harden and dry rapidly. Without the want for this article to sound like some kind of sales pitch, we more than not look towards ‘Ardex’ to provide such screeding systems (when I say systems, in layman’s terms I’m actually saying bags of screeding cement).

bonded-screed-finishedIn the pictures throughout this article, we used ‘Ardex A38’ and sand to create the screed paste, and Ardex E100 additive to create a bonding slurry. The Ardex E100 is mixed with neat Ardex A38 (At the recommended ratio) and applied to the pre-prepared concrete. At the same time the Ardex A38 screed is prepared (Again, at the recommended ratio – see Ardex for all application and mix guidelines). Whilst the slurry is still wet, the Ardex screed is applied.

‘Ardex A38 is typically (Under normal circumstances) walkable within 3 hours and is ready to receive ceramic and natural stone coverings within 4 hours irrespective of screed thickness. Resilient coverings such as wood, vinyl etc can be installed within 48 hours (However, moisture tests should always be carried out prior). After just 1 day, the compressive and tensile bending strength of Ardex A38 exceeds the acceptable minimum attained by an ordinary cement screed after 28 days!’….Source ~ Ardex UK.

Ardex A38 is truly an exceptional product and certainly one to consider for both large surface depths and fast track substrate requirements. This article has in no way been sponsored by any third party. It has been produced from first hand experience.

© Copyright 2016 Wes, All rights Reserved. Written For: Fitmywoodfloor

I am a Pergo trained professional installer of 20 years. I've been up close and personal with lots of floors and have the knobbly knees to show for it...Should you have any questions or comments please feel free to add them below. Thanks for taking the time to call by and I hope the information you've found has given you some insight!........................................................................................................................................................................“When you click on links to various merchants on this site and make a purchase, this can result in the site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network,,,,,,, affiliate window network.” This statement is to comply with current internet regulations regarding transparency to consumers.

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