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Exterior stucco is portland cement plaster applied in at least three coats over metal or wire lath, or two coats over masonry or concrete (IRC R703.7.2). IRC Table R702.1(1) requires 7/8 in of finished thickness over wire lath — typically a 3/8 in scratch coat, a 3/8 in brown coat and a 1/8 in finish coat — and 1/2 in over masonry. QUIKRETE's data sheets give an 80 lb bag of Scratch and Brown Base Coat Stucco 20–24 sq ft at 3/8 in, 15–18 at 1/2 in and 10–12 at 3/4 in, and an 80 lb bag of Finish Coat Stucco about 70 sq ft at 1/8 in. The Stucco Calculator finds the wall area with gables and openings, adds waste and uses the low end of each range, then adds the lath in square yards, lath fasteners at 7 in on centre along each stud (R703.7.1) and weep screed along the foundation plate line (R703.7.2.1). A 40 × 30 ft house with 9 ft walls, two 6/12 gables, 10 windows and 2 doors has 1,295 sq ft of wall; with 10% waste, three-coat stucco takes about 143 bags of base coat, 21 bags of finish coat, 159 sq yd of lath and 140 ft of weep screed.
IRC R703.7.2 requires at least three coats of cement plaster over metal or wire lath and two over masonry or concrete; IRC Table R702.1(1) sets 7/8 in minimum exterior thickness over wire lath and 1/2 in over masonry. Weep screed goes at or below the foundation plate line (R703.7.2.1); lath fasteners at most 7 in apart along framing (R703.7.1).
| Coat | Thickness | Coverage per bag |
|---|---|---|
| Scratch or brown | 3/8 in | 20–24 sq ft |
| Base coat | 1/2 in | 15–18 sq ft |
| Scratch + brown | 3/4 in | 10–12 sq ft |
| Finish coat | 1/8 in | about 70 sq ft |
For three-coat stucco over lath, QUIKRETE's base coat data sheet gives 10 to 12 sq ft per 80 lb bag at the 3/4 in of the scratch and brown coats together, and its finish coat about 70 sq ft per bag at 1/8 in. A 1,295 sq ft house plus 10% waste needs about 143 bags of base coat and 21 of finish coat, using the low end of the range.
The IRC (Table R702.1(1)) requires 7/8 in of cement plaster over wire or metal lath on exterior walls — typically a 3/8 in scratch coat, a 3/8 in brown coat and a 1/8 in finish coat. Over masonry or concrete the minimum is 1/2 in in two coats.
At least three over metal or wire lath, and at least two over masonry, concrete or gypsum backing (IRC R703.7.2), applied to ASTM C926.
Yes, on exterior stud walls. IRC R703.7.2.1 requires a corrosion-resistant weep screed with a 3 1/2 in flange at or below the foundation plate line, at least 4 in above the ground or 2 in above paving, with the lath ending on its flange.
IRC R703.7.1 fastens lath with 1 1/2 in, 11-gage nails with a 7/16 in head or 7/8 in, 16-gage staples no more than 7 in apart along each stud or furring strip. On 16 in studs that is about 1.3 fasteners per square foot.
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