Exterior Stucco, Weather Barriers, and Moisture Management
Exterior work is judged by how it manages water. Stucco is a drained assembly: the barrier and the drainage path matter as much as the mix.
Weather barrier and flashing
Install the weather-resistive barrier shingle-fashion, upper layer over lower, with laps typically 50 mm vertical and 150 mm (6 in.) horizontal. Flashing at sills, heads and roof-wall intersections must shed water onto the outside face of the layer below, never behind it. Weep screeds at the base are commonly at least 100 mm (4 in.) above grade and 50 mm (2 in.) above paving, with the drainage path kept clear.
Lath and three-coat stucco
Galvanized self-furring lath fastened about 150 mm on centre, lapped 13 mm at sides and 25 mm at ends, held off the sheathing so plaster keys behind it and water can drain. Scratch coat about 10 mm, brown coat about 10 mm, finish about 3 mm, for a total near 22 mm (7/8 in.). Cure the scratch before the brown and keep cement plaster damp while curing. Control joints are typically spaced no more than about 5.5 m (18 ft) apart and about 13 m2 (144 sq ft) in area, plus at building joints and substrate changes.
Finishes and conditions
Cement-based or acrylic finishes, coloured or coated with a breathable product. Do not apply in freezing conditions or onto hot, dry, windy surfaces; typically above 4 degrees C with protection after application.
Worked example
A leak appears at the corner of a second-floor window after driving rain.
Reasoning: trace the water path from the outside in. Check whether the head flashing laps over the window flange and whether the barrier above laps over the flashing; check the corner sealant joint; check that the sill drains outward; check that lath and scratch do not bridge the flashing. Water that gets behind the finish must find the drainage plane and exit at a weep screed, so the repair addresses the layer sequence, not just the visible crack.
Traps
Reversing a lap; fastening lath tight to sheathing with no furring; omitting or blocking the weep screed; no control joints; coating with a non-breathable film that traps moisture; applying stucco in wind and heat without damp curing.