Guide
What is dry pack mortar?
Dry pack (also called deck mud, mud bed or floor mud) is sand and Portland cement mixed with only enough water to hold together when you squeeze it. It is packed and screeded by hand into a bed that holds any shape you give it: a slope to a drain, a dead-flat floor, a step.
What it is made of
Washed sand and Portland cement, usually somewhere between four and six parts sand to one part cement, by volume. Many crews use a pre-bagged mud bed mix so every batch is the same. Some add a latex or acrylic admixture to the water for a stronger bond.
The water is the difference. Ordinary mortar or concrete is wet enough to pour. Dry pack is mixed until it looks like damp sand: squeeze a handful and it keeps its shape without dripping. That stiffness is what lets it hold a slope while it cures.
Where it is used
Shower floors: the bed sloped from the walls down to the drain, curbed or curbless, with a point drain or a linear drain.
Floors under tile and stone: a level bed over a slab or a subfloor that is out of level, too uneven for large tile, or that needs to come up to meet another floor.
Pool decks, terraces and balconies: a bed sloped away from the house or the pool so water runs off, ready for pavers, stone or porcelain.
Over in-floor heating: a screed laid over hydronic tubes, so the heat spreads evenly and the floor above stays flat. Electric mats usually go on top of the bed, as their maker says.
How a dry pack bed is built
The base is cleaned and checked first. Over concrete, a cement slurry or bonding agent goes down so the bed grabs the slab. Over wood, the bed usually sits on a cleavage membrane and metal lath instead, so it does not crack when the wood moves.
Screed guides are set to the finished height and slope, often with a laser. The mud is spread, packed hard with a wood float or a tamper, then struck off along the guides with a straight edge and floated to a tight, flat surface.
A floor bed is typically about 1¼ inches at its thinnest (a shower bed is often thinner at the drain, around ¾ inch) and can be built much thicker where a floor needs to come up. It can usually be walked on the next day; the next trade follows the cure times of the products going on top.
Why use dry pack instead of a ready-made base
It fits the room as it is. A mortar bed can follow any drain position, any shower size and any out-of-square wall, and it can make up for a floor that is out of level by an inch or more. It is solid under foot, it does not flex under heavy tile or stone, and the materials are cheap and easy to get.
The trade-off is skill: the slope, the flatness and the compaction all come from the hands of the person packing it. That is why it is usually a separate trade from the tile setter.
Questions
Is dry pack the same as concrete?
No. Concrete has gravel and is wet enough to pour. Dry pack is sand and cement with very little water, packed by hand so it can hold a slope.
Is dry pack waterproof?
No. A mortar bed holds water like a sponge. A shower floor needs a liner or a waterproofing membrane, which is installed by the waterproofer or the tile setter.
How long before tile can go on a dry pack bed?
It can usually be walked on the next day. When the membrane or the tile can go on depends on the products used, so follow their instructions.