In its broadest definition, the term screeding refers to any action of smoothing out a pliable material into a flat layer. The term can be used both as a verb ("to screed") or as a noun referring to the tool used to do the flattening. There are tools manufactured expressly for screeding, but you can make use of any straight board, as well.
The term is most familiar in the building trades, where it refers to the action of flattening out poured concrete into a smooth, flat layer prior to finishing the surface. It can also be used to describe the action of flattening out a layer of flooring mortar or sand or gravel underlayment beneath a surface that will be paved. For example, you might have a wavy sub-floor that needs to be filled in with leveling compound. If the compound is not self-leveling, then you must drag a screed across the top to bring the compound to level.
Screeding in Concrete Work
In the process of pouring concrete, a screed is usually a long length of straight 2 x 4 board or an aluminum bar manufactured for the purpose. Whatever object is used, screeding us done by drawing the tool across the wet surface of the concrete. The screed is generally long enough so that each end can rest on one side of the concrete form. The screed is drawn toward the workers with a sawing, scraping motion that simultaneously smooths the surface and pushes excess concrete into any gaps or hollows. It helps to have two workers, one on each side of the screed, especially for large slabs. Aluminum screeds are often used where precision is needed, as they provide a straighter edge.
For more professional, faster screeding, a motorized screed may be used. The tool has a long handle to eliminate laborious bending and tugging.
Screeding is only the first step in finishing, and it is not intended to produce a smooth final surface. The final finish will be created by "floating" the concrete with one or more smooth-faced tools that are designed to draw up fine aggregate and cement to the surface of the slab. For large slabs such as driveways or garage floors, the surface is floated with a large tool known as a bull float. Smaller wood and metal hand floats are used for sidewalks and other surfaces, but whatever tools are used, they can create a smooth surface only if the concrete has first been screeded to form a solid layer with no voids or gaps.