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Hang the Board The Durock we use is the same cement-based product tile installers use on floors and walls under ceramic tile. Typically, we score and snap the panels using standard drywall or utility knives with a heavy-duty blade or carbide scoring tip; for crisp edges, we cut it with a circular saw equipped with either a dry diamond or carbide blade. (We’ve tried using standard masonry "mesh" type blades, but they don’t last.) Sawing creates a lot of dust, so dust masks are standard equipment for the crew. We use 4x8 sheets for the first two courses, which we can reach from the ground, and 32-inch-wide sheets for everything higher (Figure 8).

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Figure 8. With the drainage plane and scaffolding in place (top left), the Durock cement board panels are applied in a staggered pattern (top right). Spaces between panels provide a keyway for the stucco. A vinyl starter bead maintains a clean reveal along the exposed bottom edge of the cement board (left).

It generally takes two men to handle a 4x8 sheet, plus an additional man per level of scaffolding to pass it up. The smaller panels can be managed by one man. Durock installation begins with a starter track, a vinyl accessory with weep holes that allow water to escape from the bottom. Starter track is optional at the base flashing, but we use it to ensure a clean line at the bottom of the Durock. We use a water level to make a level line at the foundation and check the concrete or block for stray mortar and any other obstructions. Then we nail the track using 11/4-inch hot-dipped galvanized roofing nails. We seat the first course into the starter track perpendicular to the framing, taking care not to distort the track or block the drain holes. We use nails to space sheets both horizontally and vertically, creating a 1/16-inch mechanical key like with plaster systems. We use hot-dipped galvanized roofing nails to attach the cementboard and all the accessory channels. We tried the special Durock screws, but found them difficult to use. We follow the recommended fastening pattern and stagger the butt joints as we go. As with drywall, we avoid seams at door and window jambs; instead, we run sheets past the top and bottom of the openings by 6 to 12 inches. To accommodate expansion and contraction, the Durock layer is divided into sections with 3/4-inch-wide control joints that are filled with foam backer rod and caulking. Many homeowners and building designers object to the obvious spaces, which are visible from the street, but they are necessary evils of this system. The effect can be minimized, however, with color-matched sealants. At floor framing lines and at the intersection of dissimilar materials, we use either a vinyl casing bead or L-bead. At the heads of windows and doors, we use a vinyl drip flashing. The edges of openings receive either a 45-degree bead or a casing bead. Any non-reactive metal would work for these flashings, but we typically stick to the vinyl flashings that come with the system to avoid any galvanic reactions with the Portland cement in the base coat, like you would get with aluminum or galvanized flashings. On this house, as is true with many houses using a synthetic stucco system, most openings called for built-up foam detailing (Figure 9).

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Figure 9. Hand-carved foam details are common in synthetic stucco systems (left). For extra durability, the foam trim is either bedded in mesh tape or capped with vinyl beads, as shown here (right). To strengthen these foam details, the system calls for either applying trim bead along the edges or "back-wrapping" the foam using an exterior-grade fiber mesh. For typical rectangular foam details, the mesh is cut wide enough

so that it extends 2 inches under the trim, wraps completely around the three exposed faces, and then extends 2 inches onto the face of the Durock. It’s left loose on the face of the foam buildup until the base coat is applied, at which time it will be embedded in the base coat.