Stucco Calculator

Traditional three-coat is a 7/8-inch system over metal lath: a 3/8-inch scratch coat, a 3/8-inch brown coat, and a 1/8-inch finish. One-coat systems (really two coats) use a fiberglass-reinforced 3/8-inch base over rigid foam and lath, then the same finish. That is why this calculator multiplies base-coat demand by 2.33 for three-coat — 3/4 inch of scratch plus brown, which is twice a single 3/8-inch coat, plus about 15 percent because the scratch coat also has to fill and key the metal lath — and by 1.0 for one-coat. Neither figure includes the 1/8-inch finish coat. Three-coat is thicker, more impact-resistant, and what most codes assume by default. One-coat goes up faster and adds insulation, but it must be an evaluated proprietary assembly installed exactly per its evaluation report.

Over wood framing, the IRC and ASTM C1063 call for two separate layers of weather-resistive barrier — typically Grade D building paper or house wrap — then self-furring expanded metal lath fastened into the studs, plus casing bead, corner aid, and a weep screed set at least 4 inches above grade or 2 inches above paved surfaces. The furring dimples hold the lath about 1/4 inch off the paper so the scratch coat can key behind it. Over concrete or CMU, stucco can bond directly with a bonding agent and no lath. Flashing at every penetration matters more than the mix itself.

Cement plaster shrinks as it cures and moves with temperature, so ASTM C1063 limits an unbroken panel to 144 square feet with no dimension over 18 feet, and requires a joint wherever the length-to-width ratio exceeds 2.5 to 1 or the framing changes — at floor lines, for instance. Skip the joints and that movement shows up as random cracking, usually running diagonally out of window corners. Accessory joints are installed with the lath, before any base coat goes on, and cannot be retrofitted later, so lay out your panels while the lath is going up.

For three-coat work, give the scratch coat at least 48 hours before the brown coat, and let the brown coat cure 7 days before the finish. Some specs allow shorter intervals in warm weather, but 48 hours and 7 days are the common defaults. Moist-cure each base coat by fogging it once or twice a day for the first 48 hours, more often in hot, dry, or windy conditions. Stucco gains strength by hydration, not by drying out — rushing the schedule or letting a coat flash-dry is the usual cause of crazing and weak bond.

No. The bag count and cost here cover base coat only — scratch and brown for three-coat, or the single base for one-coat. Finish coat is a separate product sold by color and texture; an 80 lb bag covers roughly 40 to 60 square feet at 1/8 inch depending on how heavy the texture is, so budget about 5 to 8 extra bags for the 300 square feet in the example. Paper, lath, weep screed, corner aid, control joints, scaffolding, and labor are all extra. Installed three-coat stucco commonly runs $9 to $15 per square foot. Treat every number here as a planning estimate: the assembly itself is governed by your local building code and the evaluation report for the system you buy, and a licensed plastering contractor or your building inspector has the final say.