Drypack (often called “deck mud”) is a stiff, semi-dry mixture of Portland cement, sand, and a minimal amount of water. There are no rocks or lime in the mixture. Unlike the concrete used for driveways or side walks, drypack has a “damp sand” or “snowball” consistency. This allows it to be compacted, shaped, and sloped by hand.
Key Specifications and Use
- Mix Ratios: The standard ratio is typically 1 part Portland cement to 4 to 5 parts sand.
- Water Content: Only enough water is added to reach a damp consistency—roughly 12% water by volume. A common field test is the “snowball test”: the mix should hold its shape when squeezed but crumble when poked. About the same as damp beach sand.
- Thickness: Thickness requirements vary by use. See below for more.
- Compressive Strength: Standard field-mixed deck mud typically reaches 2,500–3,000 psi.
- Curing: For interior applications, allow 24 hours before installing tile. Exterior or wet-area applications may require 72 hours to 14 days depending on the use of additives. A curing time of 72 hours is required before the application of a liquid applied waterproofing membrane.
- Substitute materials; Using a prebagged sand mix or mortar mix is not prescribed as it has too much Portland cement in the mixture. This reduces the drainage capacity of the mix. Concrete mix is not desired nor appropriate.
Un-bonded applications
An unbonded drypack installation is intentionally separated from the substrate by a bond-breaking layer. This “floating” assembly allows the mortar bed and the subfloor to move independently. This prevents cracks in the subfloor from telegraphing through to the finished tile.
Key Components
- Cleavage Membrane: A “slip sheet” such as 15lb roofing felt, plastic sheeting, or a dedicated uncoupling membrane is placed directly on the substrate to prevent bonding. It also prevents the substrate from wicking the moisture from the drypack mixture.
- Reinforcement: Because it is not adhered to the base, the bed requires internal strength. This is usually provided by 2″ x 2″ galvanized wire mesh (wire lath) placed within the lower half of the mortar bed.
Technical Requirements
- Thickness: Unbonded beds generally must be thicker than bonded ones to accommodate the reinforcement and provide structural integrity. The minimum thickness is 1.5 inches. In shower applications, the thinnest part is usually at the drain and rises from there to achieve the proper slope for drainage.
- Movement: The entire system—mortar, wire, and tile—acts as a rigid “raft” that floats over the substrate.
Un-bonded applications
A bonded drypack installation is a method of creating a flat, sloped, or leveled substrate that is chemically or mechanically attached directly to a structurally sound cementitious substrate.
Unlike a “floating” or “unbonded” bed, a bonded installation functions as a single unit with the subfloor.
Core Components
- Dry Pack Mortar: The same as used in a bonded mud bed.
- Bonding Agent: Drypack cannot bond to a substrate on its own. A slurry coat of water and pure Portland cement or a layer of thinset is applied to the subfloor immediately before the dry pack is packed on top.
- Application: The mortar is tamped down and compacted to remove air voids, creating a dense, high-strength layer that resists shrinkage and cracking. This step takes considerable skill.
Common Uses
- Shower Pans: Creating the precise pitch required for water to flow toward a drain.
- Floor Leveling: Raising or leveling concrete slabs that are out of plumb before installing large-format tile.
Key Technical Standards
- Minimum Thickness: For a bonded bed, the thickness can range from 3/4″ to 3″, though 1-1/2″ to 2″ is common for structural stability on concrete floors.
- Curing: While it feels dry to the touch quickly, it requires 72 hours to cure before a liquid applied waterproofing membrane can be applied.
Alternative method:
An alternative to a creating a drypack shower base is to use a custom made foam shower base from industry specialists such as Built With Foam.