Today I put down the second layer of deck mud (the first layer, the pre-slope, goes under the liner) onto the shower floor. This layer is constructed of a type of mortar that is called “dry-pack” mortar, or deck mud. There seems to be alot of different opinions on just what this should consist of, compounded by the fact that nobody seems to sell a pre-mixed deck mud product. It also seems that there are alot of different things that will work, and they really just vary in how easy they are to get down. If you use a mix that is too wet, you’ll have a hard time getting it to hold its shape. If you use a mix that’s too strong, once it dries it will be impossible to knock out any lumps. If you use a mix with to big an aggregate, you’ll end up with a lumpy floor, and if you use a mix with too much cement, it will shrink when it dries, and could crack.
Therefore, the best mix is a ratio of 5 parts sand to 1 part cement, mixed to a fairly dry consistency similar to wet sand at the beach. This won’t be a extremely strong product, but as its just going in the floor of your shower, it doesn’t need to be. With the high amount of sand in it, it won’t shrink much as it dries, so you don’t have to worry about it cracking, and with just a dry mix its not too difficult to get the proper slope. For more information, from a tiling expert, check out John Bridge’s page on Deck Mud.