Deck Mud Calculator

Bagged deck mud products - data sheet figures

Yields and times come straight from the current manufacturer sheets (CUSTOM TDS-441 dated 1/12/2021, Sakrete Sand Mix rev 10/18, Quikrete No. 1103 rev Dec 2022). In Canada, Quikrete packs 25 kg (0.46 cu ft) and 30 kg (0.54 cu ft) bags instead of 60/80 lb, while FloorMud is the same 50 lb bag in both countries. Don’t average across products. Each column of the tables below is figured per bag from its own sheet.

Bags for common floor areas at a 1 1/4 in bed, 10 percent waste included

Bed volume = area x 1.25 / 12 x 1.10. Each product column divides that volume by its own data sheet yield and rounds up. FloorMud uses the conservative 0.375 cu ft per 50 lb bag.

Site-mixed 5 to 1 deck mud per cu ft of finished bed

Apply your waste allowance to the bed volume first, then read this table. Per cu ft of bed you need 18.8 lb portland (1/5 cu ft) and 1 cu ft damp sand, so one 94 lb sack covers 5 cu ft of bed. Sacks round up. Sand doesn’t shrink when the sack rounds up.

Shower pan pre-slope volumes - centered drain, 3/4 in at the drain, 1/4 in per ft

Average depth = 0.75 in + 2/3 of the rise at the far wall (exact for a straight screed from a level perimeter, see the geometry section). Bag columns include 10 percent waste and round up per product. Deepen the drain figure or steepen the slope and the volumes grow in proportion.

What deck mud is

Deck mud is 5 parts damp sand to 1 part portland cement by volume, for floors. It also goes by floor mud, dry pack mortar or sand mix, and the recipe allows up to 1/10 part lime. Mix it with just enough water to hold a ball when squeezed.

That ratio comes from ANSI A108.1A, the standard for setting ceramic tile in a portland cement mortar bed, as published by the NTCA. Shower receptors are commonly mixed richer, at 4 to 1. CUSTOM FloorMud is a 4 to 1 preblend for that reason.

Deck mud isn’t concrete, and it isn’t brick mortar either. It has no gravel, and the Sakrete data sheet is explicit that Sand Mix is not recommended for laying brick or block.

Think of it as a compacted setting bed. You screed it flat or to slope, pack it hard, then tile it, either fresh the same day under the ANSI A108.1A wet-set method or after it firms up overnight.

The bagged shortcut is any sand-cement topping mix. The Quikrete Sand/Topping Mix data sheet lists thick setting beds for ceramic floor tile among its uses. The Sakrete sheet lists mortar bed under ceramic tile installations too, mixed to a dry pack consistency per ANSI A108.1A/B/C and the TCNA Handbook.

How thick the bed has to be

Over a wood subfloor or any cleavage membrane, the bed is 1 1/4 in minimum and 2 in maximum. It wants welded wire reinforcing mid-bed (TCNA Handbook method F141). The bed floats free of the structure, so it needs the mass and the steel.

Over metal lath fastened to a wood subfloor, 3/4 in is the minimum (TCNA F145, which calls for 2.5 lb per sq yd flat expanded lath).

Bonded directly to clean, cured concrete, 3/4 in is again the minimum, this time under TCNA F112. The CUSTOM FloorMud sheet backs this up by requiring diluted latex admix for bonded areas thinner than 3/4 in. An unbonded bed over a membrane on concrete goes back to 1 1/4 in reinforced (F111).

Stay inside the product windows too. FloorMud runs 1/4-3 in, Sakrete Sand Mix 1/2-2 in, and Quikrete Sand/Topping is formulated for placements under 2 in. Past 2 in on a floating bed, plan a structural fill first. If you’re not sure which method applies to your floor, check your local code or supplier.

Shower pan pre-slope and the geometry behind it

The pre-slope is the mud UNDER the pan liner. It has to fall 1/4 in per ft from the walls to the drain. A liner laid flat tends to trap water against the weep holes, which is why this layer is a code item and not a comfort item.

IPC 417.5.2 requires the liner pitched 1/4 unit vertical in 12 units horizontal and sloped toward the fixture drain. UPC 408.7 adds the detail, with lining materials pitched 1/4 in per ft to weep holes in the subdrain over a smooth, solidly formed subbase.

Now the geometry. With the drain centered and the perimeter screeded level, the run to the farthest wall is half the longer side. The rise at the wall is the slope times that run. The average bed depth is the drain depth plus TWO THIRDS of the rise, not half.

Why two thirds? Calculators that use half the rise under-order on every pan. Along every screed line the surface climbs evenly from drain to wall, but the floor area near the drain is only a small patch. Most of the pan area sits out toward the perimeter, where the bed is thickest. Weighting depth by area puts the average at 2/3 of the rise for any pan shape screeded straight from a level perimeter to the drain.

The calculator assumes a centered drain. An offset drain lengthens the far run and adds mud, so measure the longest drain-to-wall distance and push the waste slider up rather than guessing.

The finished mortar bed that goes ON TOP of the liner is a second, separate lift. Run the shower pan mode again with the thickness at the drain set by your drain barrel height, and bear in mind the finished floor has its own slope rules (IPC 417.5, UPC 408.5).

Bagged or site-mixed

Under about 5 cu ft of bed, bags usually win on hassle. A 36 x 36 in pre-slope is 2 bags of 60 lb sand mix, and a whole 40 sq ft bathroom floor at 1 1/4 in is 10 bags of Quikrete 60 lb.

Yields come straight from the data sheets. Quikrete Sand/Topping gives 0.5 cu ft from the 60 lb bag and 0.66 cu ft from the 80 lb. Sakrete Sand Mix gives 0.45 cu ft and 0.6 cu ft for the same two sizes. CUSTOM FloorMud covers 4.5-5.5 sq ft at 1 in per 50 lb bag, and the calculator plans on the conservative 4.5.

In Canada, Quikrete packs Sand/Topping in 25 kg (0.46 cu ft) and 30 kg (0.54 cu ft) bags instead. FloorMud ships to both countries as the same 50 lb bag.

Past that, the traditional site mix is usually cheaper per cu ft. One 94 lb sack of portland cement (1 cu ft loose, the historical US sack) plus 5 cu ft of damp sand yields about 5 cu ft of compacted bed. The cement paste mostly fills the voids between the sand grains rather than adding to the pile. That yield rule is tile-trade practice, not a lab number, so the calculator carries a waste slider and never trims it below 5 percent.

Don’t use play sand from the same shelf. You want washed coarse sand, typically sold as concrete or all-purpose sand (ASTM C33 fine aggregate), or masonry sand.

Go easy on the water. Mix to the ball test. CUSTOM publishes a true dry pack range (5.75-7.75 pints per 50 lb bag). The water printed on Sakrete and Quikrete bags targets a wetter topping consistency, so for a mortar bed the Sakrete sheet says to add just enough water for a dense bed. A soupy mix tends to slump off the slope, shrink, and pack weak.

Walking on it and setting tile

CUSTOM FloorMud takes foot traffic and tile in 12-24 hours and heavy traffic at 72 hours, per TDS-441. Sakrete Sand Mix opens to foot traffic in 24 hours and vehicular traffic in 72, and it has to be kept from freezing for 48 hours.

The Quikrete Sand/Topping sheet publishes no walk-on time. It calls for a moist cure of 5 days at 70 F and warmer, or 7 days at 50-70 F, plus freeze protection for the first 48 hours.

The traditional alternative needs no wait at all. ANSI A108.1A is the wet-set method, where tile goes onto the fresh, just-screeded bed the same day. For a shower pre-slope there’s no tile on this layer anyway. Let it firm up overnight before the liner goes down, so the membrane can’t dent the surface.

Once the bed is ready, tile quantities, thinset and grout are a different job. The tile calculator on this site handles those.

Common questions

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