(click any image to enlarge)
At its simplest, a screed is any mechanism used to produce a completely flat plane. The notched end of the 2 x 4 in the picture above shows us a pre-measured end which “rides” the rails of other wood pieces, permanently embedded along equally pre-measured lines, but, in this case, in two dimensions – height/level and width. By pulling the sand which is poured over the compacted base rock in the picture above, we reach a perfect plane, suitable for laying the brick pavers on top of. Here, they will be fitted into position only needing a final pass with a compactor to compress down into their final resting spots.
Here is an even better look:
Here is a look at a product from PaveTech which I have tried a few times. It works just fine and you can stand up while screeding, a never-to-be-underestimated benefit. I always personally have opted for my own constructed devices, but these are most definitely worth trying. Many people definitely prefer them.