Chapter 17 - Getting the Wort Out (Lautering)
17.2 Getting the Most From the Grainbed
The grainbed can be a few inches to a couple feet deep, but the optimum depth depends on the overall tun geometry as well as the total amount of grain being mashed. A good rule of thumb is: "The depth of the grainbed should be no less than one half the shortest dimension of the floor area, nor greater than twice the longest." In other words, the grainbed aspect ratio can vary between 1:2 and 2:1. If the grainbed gets too shallow, i.e., from lautering too little grain in too large a tun, then an adequate filter bed won't form, the wort will not clear, and you will probably get hazy beer. A minimum useful depth is probably about 4 inches but a depth of between 8-18 inches is preferable. In general, deeper is better, but if it is too deep, then the grainbed is more easily compacted and may not let any wort through, making lautering nearly impossible.