Deck Mud Calculator

What Is Deck Mud?

Despite the name, deck mud has nothing to do with outdoor decks. Deck mud — also called floor mud or dry-pack mortar — is a simple sand-and-cement mortar used under tile, most famously to build the sloped mortar bed in a traditional shower pan. If you were searching for outdoor deck materials instead, head over to our decking calculator.

The classic recipe is 5 parts sand to 1 part portland cement with just enough water to make it hold together — a "just damp" mix often compared to sandcastle sand. Squeeze a handful and it should clump firmly without water oozing out. That dry consistency is the whole point: it packs into place, holds a slope without slumping, and cures into a hard, flat, dimensionally stable bed that tile bonds to beautifully. The name "deck mud" comes from old-school tile setters who called any horizontal mortar bed a deck, and "floor mud" is the same material by another name.

Deck mud is used for shower pan pre-slopes and top beds, shower curbs, sloped beds on balconies and steam rooms, and thick "mud beds" that flatten out-of-level floors before tiling. It is not a structural concrete and it is not a bonding mortar — it relies on compaction and mass, which is why the mix stays lean and dry.

How Thick Should a Shower Pan Mortar Bed Be?

Code and tile industry standards (TCNA) call for a pre-slope of at least 3/4 inch at the drain, sloping up toward the walls at a minimum of 1/4 inch per foot of run. In a typical 3 by 4 foot shower, that means the bed starts at 3/4 inch at the drain and rises to roughly 1.25 to 1.5 inches at the walls — which is why this calculator defaults to a 1.25 inch average thickness. Larger showers with longer runs to the drain need thicker perimeter depths to maintain the same slope.

In a traditional two-layer pan, the pre-slope goes under the waterproof pan liner, and a second mud bed of at least 1.5 inches goes on top of the liner to receive the tile. Single-layer systems with surface-applied waterproofing membranes use one sloped bed. Either way, keep any single lift of deck mud between 3/4 inch and about 3 inches — thinner beds crack and crumble, and thicker lifts are hard to compact evenly.

Deck Mud Coverage Chart

A 60-pound bag of pre-mixed sand/topping mix yields roughly 0.5 cubic feet of placed, compacted deck mud — about 4 square feet of bed at 1.5 inches thick. Here is what typical shower pans need, assuming a 1.25 inch average bed thickness plus 10 percent for compaction and waste:

Add roughly 2 cubic feet (4 more bags) if you are building a shower curb from mud, and double the pan quantities if you are doing a traditional two-layer pan with both a pre-slope and a top bed. When in doubt, buy an extra bag — sand/topping mix is cheap, and running out mid-pack ruins the bed.

Mixing Deck Mud

For site-mixed deck mud, measure 5 parts all-purpose sand to 1 part portland cement by volume (a shovel or bucket works fine as the measure) and blend the dry ingredients thoroughly before adding any water. Then add water a little at a time — roughly 1 part water or less — mixing continuously, until the mud just clumps in your fist without dripping. It should look barely damp, not wet like concrete. A richer 4:1 mix is sometimes used for curbs and wall mud where extra strength helps.

Mix in small batches so the mud does not start setting before you can pack it. Pre-mixed sand/topping mix (sand mix) is the same ingredients pre-blended at roughly 3:1 to 5:1 depending on brand — just add water sparingly to the same sandcastle consistency. Pack the mud firmly with a wood float or flat trowel, screed it to your slope lines, and let it cure at least 24 hours before waterproofing or tiling.

Deck Mud vs Thinset vs Self-Leveler

These three materials get confused constantly, but each has one job. Deck mud builds thickness and slope — it is the structural bed, from 3/4 inch up to about 3 inches, under shower pans and thick floor beds. Thinset mortar is the adhesive — a sticky, latex-modified cement applied about 1/8 to 1/4 inch thick that bonds tile to the bed or substrate. It cannot build up thickness or hold a slope. Self-leveling underlayment is a pourable cement that flows dead flat — perfect for flattening a floor before tile, but exactly wrong for a shower pan, because a shower bed must slope to the drain, not level itself. Use our tile calculator to figure the tile and thinset that go on top of your finished mud bed.