Some manufacturers provide pumpable flowing screeds which can achieve very level finishes. Most of these screeds are anhydrite compounds and are based on a calcium sulphate binder.
They are quicker to apply than a traditional sand and cement screed and may be applied to a minimum thickness of 25mm if bonded, 30mm if unbonded, or 35mm if a floating finish is required.
They can also be used in conjunction with underfloor heating systems where a minimum 30mm cover to the pipes is needed. A bonded screed is bonded to the slab or substrate below, and the main way that bonded screeds fail is that the bond between the screed and the substrate fails.
This is more likely to happen if the screed is too thick. An unbonded screed is separated from the slab or substrate below, and the main way that an unbonded screed fails is to lift or curl. This is more likely to happen if the screed is too thin. Bonded screeds should therefore be thin, normally less than 50mm. Unbonded screeds should be thick, normally 70mm or more, and mm or more if curling must be avoided. Correctly specifying the depth and type of screed starts early in the design process.
The issues that dictate the design of the screed include the architecturally specified floor finishes, the construction tolerances and the provision of falls. There may also be structural requirements such as preventing disproportionate collapse and the development of composite action with the concrete slab below. Sometimes, the use of a screed can be avoided.
If a screed is needed it can be either a traditional cement sand screed or more recently-developed proprietary pumpable self-smoothing screeds. These types are explained below, together with a list of related definitions and guidance on screed depths.
There are particular definitions concerned with specifying screeds. These are traditional screeds and are suitable for all applications, provided they are specified correctly. The biggest drawback is the drying time; BS estimates the drying time for a sand cement screed as one day for each millimetre of screed thickness up to 50mm thick.
Further guidance on drying times can be found in the Code. These screeds can be laid as bonded or unbonded. However, they must not be used with reinforcement because the calcium sulfate is corrosive to steel in damp conditions.
These screeds are also generally not suitable for use in damp conditions or where wetting can occur. These screeds are all proprietary products and therefore vary from one supplier to another, the guidance given here is therefore generic and the manufacturer should be consulted before specifying. If they are intended to be used as a wearing structural screed then the manufacturer should be consulted. A levelling screed may be chosen for various reasons.
It might be to provide a smoother, flatter surface than can be achieved economically by the structural slab. If you are looking to lay thin floor coverings, and looking to take out the imperfections of the substrate. Unbonded Specialist calcium sulphate flow screed can be laid at 15 mm. Sand and cement modified rapid strength gain , can be laid at 35 mm.
Traditional screed , one to three, one to four, can be laid at 50 mm Floating Calcium sulphate flow screed can be laid at 35 mm, for domestic, and 40 mm for commercial projects.
Sand and cement modified screed for rapid strength starts at 35 mm. Traditional sand and cement screed is 65 mm for domestic and 75 mm for commercial. Our Accreditations. Quote Me. About Speed Screed Speed Screed Limited was founded on the key principle of providing first-class customer service. Speed Screed Limited. Is it over concrete or flexible substrate? Thanks for your help guys Danny. Sorry for jumping into somebody elses thread but this is the exact topic I need an answer to.
What kind of ratio do you use when adding sand to leveller? Do you work with the same kind of as with a standard cement screed or do you just have a bit of a guesstimate about when the leveller looks stiff enough not to slump?
Because there many brands and types of self levelling compounds there is no definitive answer to your question. I use an old saucepan, fill it, then add half a mug of sand and mix well then try a bit, and add more compound or sand till I get it right and mix a full batch. It will not take much sand to stiffen the mix, use bricklayers sand or washed sand, you want no clay content. Good luck. Thanks for the advice - much appreciated. I just finished the screed for my shower. I mixed the lanko with the specified amount of water and mixed it with drill and screed mixer.
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