What is a screed?
Screed is a layer - or several layers - of screed mortar laid directly onto a substrate with or without bonding onto a separation or insulation layer in order to achieve the following functions (either singular functions or a combination of functions combined):
- To reach a specified height
- To be made suitable to receive a certain floor covering
- To be used as is on its own.
In accordance with DIN 18560 “Screeds in Construction”, screeds are divided according to the type of installation method, i.e. floating screeds, screeds on separation layers, composite screeds and underfloor heating screeds. Properties and requirements that screeds need to have are regulated under DIN EN 13813 “Screed mortars and materials".
Simple ways to check the load carrying capacity of screeds:
- First of all trust your eyes. A thorough visual inspection can often identify quite a lot of defects
- A scratch test with the grid-scratch device enables an assessment of the load carrying capacity
- The wetting potential test allows assessment of the surface´s absorbency
- The wipe-test shows wether there are any separating agents on the surface
- The tapping-test gives clues as to the existence of any cavities in the substrate
Cement screed (CT)
This very common screed type is used in all types of buildings. Cement screeds have little susceptibility to moisture, but in the first few weeks after installation show a distinct shrinkage curing behaviour of around 0.5 mm/m. Because of the binding agent used here cement screeds tend to be of a grey colour. They are usually installed as earth-moist mixtures.
Where speed is of the essence rapid screeds are at hand that cure in just a few hours and which can then quickly be co- vered with tiles. On large-scale building sites floating screeds are occasionally used that offer special qualities.
Calcium sulphate screed (CA)
This widely used screed type is also being used in all kind of buildings. Calcium sulphate screeds, also called anhydrite screeds, remain susceptible to moisture even after they have cured hence they have to be protected accordingly. On the plus side, there is hardly any shrinkage which means they can be installed with very few joints. Because they conduct heat better than cement screeds they tend to be very popular as screeds for underfloor heating. Because of their binder calcium sulphate screeds are usually rather light in colour, i.e. slightly yellowish to brownish.
Calcium sulphate screeds are predominantly installed as floating screeds (CAF) so as to save time and in order to ensure a perfect screed distribution.
Mastic asphalt screeds (AS)
This screed is an anhydrous screed on a bitumen basis. Mastic asphalt screeds are primarily used in commercial and industrial buildings, but are also frequently used to repair old timber boarding.
They do not absorb any moisture and can receive coverings immediately after cooling off. They do however tend to expand significantly under heat impact and for this reason mastic asphalt heating screeds in general should not be tiled over.
What’s more, because of their elastic properties these types of screeds are subject to deformation under point loads.
Magnesia screeds (MA)
This screed is sometimes found in so-called stonewood-screeds (additives made of wood or paper pulp) in old buildings. Nowadays it tends to be installed in industrial buildings using mineral additives.
These so-called industrial magnesite screeds feature particularly hard surfaces that can be loaded immediately.
However, prior to usage magnesia screeds need to receive a protective coating, as they are extremely susceptible to moisture.
In old buildings this protective coat is usually red or green.