How to Calculate Stucco
Traditional three-coat stucco is applied as scratch coat, brown coat, and finish coat over metal lath on exterior walls. Estimating stucco requires the wall area, a coat thickness, and the number of bags of each component. Lath, accessories, and corner bead are critical add-ons.
Stucco Coverage Formula
A typical 80 pound bag of stucco mix covers about 25 square feet at 3/8 inch thickness.
Bags=Wall Areasq ftCoverage per Bag
Lath Sheet Formula
Lath Sheets=Wall Areasq ft16
Metal lath comes in 27 in x 96 in sheets (18 sq ft gross, 16 sq ft after overlap). Budget 1 sheet per 16 sq ft of wall, plus 10 percent for cuts around windows and corner bead.
Three Coat vs One Coat Stucco
| System | Thickness | Coverage (80 lb) | Coats |
|---|---|---|---|
| Traditional 3-coat | 7/8 in | ~25 sq ft per coat | Scratch + Brown + Finish |
| One-coat (fiber) | 3/8 to 1/2 in | ~50 sq ft | Base + Finish |
| EIFS synthetic | 1/16 to 1/8 in | ~80 to 120 sq ft | Base + Mesh + Finish |
Three-Coat System Thicknesses
- Scratch coat - 3/8 in, scratched with a notched trowel to create mechanical key
- Brown coat - 3/8 in, leveled flat, cure 7 to 10 days before finish
- Finish coat - 1/8 in, applied in desired texture (sand, dash, smooth, Mediterranean)
Lath and Accessories
- Self-furring metal lath - 2.5 or 3.4 lb diamond mesh, 27 x 96 in sheets
- Weather-resistant barrier - two layers of Grade D paper behind lath
- Corner bead - at all outside corners, 10 ft sticks
- Casing bead - where stucco terminates at windows, doors, trim
- Weep screed - at bottom of walls to drain moisture
- Control joints - every 144 sq ft or panels over 18 ft
Control Joints Are Not Optional
Stucco cracks without properly placed control joints. ASTM C1063 requires control joints every 144 square feet and in any panel with length-to-height ratio over 2.5 to 1. Skipping them to avoid visible lines guarantees random uncontrolled cracking within 1 to 2 years.Curing and Weather
- Moist curing - mist each coat for 48 hours after application
- Cure between coats - scratch to brown: 24 to 48 hours; brown to finish: 7 to 10 days
- Temperature - between 40°F and 90°F, no direct sun on fresh stucco
- Protect from rain - 24 hours minimum after each coat
Stucco vs EIFS
True portland cement stucco is a rigid, breathable, 80 to 100 year material when properly installed. EIFS (Exterior Insulation Finish System) is acrylic over foam insulation - lighter but has moisture problems if not installed as a drainage EIFS. Know which you are specifying.Color Finish Coat vs Paint
Integral color finish coat uses pigment mixed into the stucco, never peels or flakes, and never needs painting. Painted stucco is common but needs refreshing every 7 to 15 years. Integral color costs slightly more up front but is far cheaper over the life of the building.