Just a thought....if you are putting a new concrete base in an extension why screed? If it reasonably level why not just use levelling compound?
Leveling compound is darn expensive compare to a cement screed, leveling compound can be used from around 2mm up to 50mm or so, sometimes deeper if sharp sand is added to the mix. And a leveling compound is not designed to be a finish floor, it's needs tiles, timber,etc, where as a cement screed is a finished floor. Whereas a floor screed can be bonded (with a primer) or unbonded (laid on a membrane), a bonded screed is usually around 25mm, & a unbonded can be around 70mm,sometimes up to a 100mm. You can add polypropylene fibres to a cement screed, this will give a stronger screed,better durability, & reduce the shrinkage. And if you have ufh, then a pumped screed is a better option,it flows into every knook unlike a cement screed, & less chance of damaging pipework when laying.
You may have solved a problem for me there Kiab. Have a garage project coming up for a friend and need to level floor up. It's only a 75mm threshold between garage floors and hallway (highest point) and the floor only slopes about 25/35mm over approx 5m. Intending on putting celotex and tongue and groove chipboard down to create a floating floor but need to level it up. Too shallow for screed and don't really want to use firring strips so how well does mixing sharp sand with self levelling work? Any tips?
A old trick, to use Cementone-Cempolatex with sharp sand,for filling holes to a depth of 100mm, allow to dry, then pour your levelling compond as usual. Never used the method to do a whole floor though. https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwiEhf-47tHTAhUlIcAKHfh6DzQQFggvMAE&url=http://blog.jewson.co.uk/wp-content/uploads/2011/08/Cementone-Cempolatex-One-Pack-TDS00179.pdf&usg=AFQjCNHg7iK3wK5_uoV0Ycl0UvvfAkG5Cw&cad=rja You could use a bonded fibre cement screed good for 15mm to 40mm max, the fibre being Adfil or similar. https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&ved=0ahUKEwi-1u2a8NHTAhXpL8AKHde4CEoQFghaMAs&url=http://www.cemcraft.com/index.php?p=product_guides&download_id=9&usg=AFQjCNHcpl7yCp1yEwMEZe8yAClBWExkEw&cad=rja You can also get fibre added leveling compound. https://www.pureadhesion.co.uk/ultr...eveller.html?gclid=CPGv0rLx0dMCFaW17QodV60FWw
Cheers Kiab. I've had a quick Google too and seems to be about 1:1 with sand and compound. Does this sound about right?
Perfect. Could use Grano 6mm to Dust instead of sharp sand, haven't tried, but it's used in granolithic & monolithic screeds .