The Application of Stucco
Phase One: The Preparations For Stucco (continued)
Lath For Stucco
The Components of Stucco Lath
Flashing / Drip Cap
Lath For Stucco
Lath is a term describing the system of components that secure mortar to the supporting structure by means of a mechanical bond. The primary functions of stucco lath are to provide perimeter alignment for the stucco membrane, and an interlock between the supporting structure and the stucco mortar. Stucco lath must be able to receive and support fresh stucco mortar, and provide reinforcement for the first mortar application. Ideally, lath and stucco mortar combine to form an integral unit. This unity depends on the type and effectiveness of the bond between the two systems.
A mechanical bond is obtained by a process involving characteristics of the mortar as it changes from a plastic to a hardened state. Upon application, the plastic stucco mortar completely encases and clings to the lath. As it hydrates the mortar slowly hardens around the lath, eventually resulting in the formation of permanent keys. The internal support provided by the embedded lath serves to increase the strength of the resulting stucco membrane.
Except for soffits, stucco is usually applied over a prepared surface that provides a solid backing for the stucco application. The solid backing is usually made of wood sheathing or masonry. In some instances, namely low budget construction, stucco is applied over a frame constructed structure without a solid backing. This is not recommended. The increased strength that sheathing provides to a frame constructed structure serves to protect the stucco application by minimizing stress transferred to the stucco membrane.
Obsolete forms of stucco lath
Wood lath was used almost universally through the early twentieth century. Made of sawed wood slats, it was time consuming to install; expanded and contracted depending on moisture content; and deteriorated after time.
Wood lath has been replaced by lath components made of metal. Their advantages over wood lath include: non-combustibility, ease of application, stability, durability, uniformity of units, availability, and generally improved results.
Early wire laths, such as woven chicken wire, did not have the structural integrity necessary for stucco, and were time consuming to install properly.
The Components of Stucco Lath
There are a wide array of different stucco lath components used to provide both structural and esthetic qualities to the stucco application. These stucco lath components can all be basically grouped into the following four categories: building paper, stucco bead, sources of interlock, and fasteners.
Building paper provides separation from the supporting structure, weather resistance, and a backing surface having suction properties conducive for stucco mortar application.
Stucco bead includes such components as stucco stop, flashing, expansion bead, and corner bead. They are used to define the perimeters and profile, as well as to increase the structural integrity of the stucco membrane. Stucco beads are usually formed from 0.45 mm (~1/64 inch) thick galvanized steel with perforated or expanded flanges. All stucco bead must be corrosion resistant.
The main sources of interlock for conventional stucco applications are provided by wire lath and expanded metal lath.
Fasteners attach the other stucco lath components to the supporting structure.
Qualities of a stucco lath application
The quality of a stucco lath application is determined by both its structural and esthetic properties. The structural quality of a stucco lath application is determined by the extent that the stucco membrane is protected from stress, provided with internal support, and connected to the supporting structure. The esthetic quality of a stucco lath application is determined by the suitability of the perimeter edges and angles formed by the stucco bead.
Unless otherwise specified, all perimeter edges of the stucco membrane should: be straight, be either horizontal or vertical, and intersect at right angles. All stucco bead should be undamaged and installed with the longest practical length. Combining smaller scraps of stucco bead does not provide the esthetics produced by one continuous piece. Furthermore, each form of stucco bead is produced by several manufacturers, and comes in a variety of different styles. To provide esthetic continuity, all stucco bead installed on a project should be of the same style.
After application the wire lath and metal lath must be entirely behind the surface plane of the eventual stucco membrane.
The stucco lath components are discussed in the approximate order of their installation.
Except for the building paper, which may be installed with staples, all stucco lath components are fastened to the supporting structure with either large headed nails or screws.
Screws are normally used for securing components to metal studs, while nails are used for almost every other installation.
All fasteners must be corrosion resistant.
Nails must be long enough to penetrate 25 mm (1 inch) into framing members or, where sheathing is used for attachment, to the full depth of the sheathing.
38 mm (1½ inch) long roofing nails provide excellent results.
18 mm (¾ inch) base 13 mm (½ inch) base
Stucco stop has many uses. It provides a straight edge for most perimeters of the stucco membrane; it separates the stucco membrane from adjacent materials; and it provides a guide of the stucco membrane's profile.
Stucco stop is manufactured with base widths of either 13 mm (½ inch) or 18 mm (¾ inch), usually in 3 m (10 ft.) lengths, with either a wide or a narrow flange.
Whenever possible, always use the wide flanged stucco stop. Installation is easier and faster. Furthermore, many times narrow flange is not able to provide the necessary overlap required.
The lathing usually begins with the installation of the stucco stop. The usual areas of installation are where the sheathing meets the foundation, over roof flashing, and around vents.
The installation of stucco stop
The installation of stucco stop over long distances requires the use of a chalk line. A level should be used for shorter lengths or vertical applications.
To ease the installation, stucco stop is usually manufactured with pre-punched nail holes. However, to get a better grip on the stop, helpful when securing either end of a piece, nail directly through the metal flange.
When installing a system of adjacent stucco stops, do not install each stop permanently by immediately hammering the fasteners flush. Instead, pre-install each stop by just tacking them into place. After all of the adjacent stops are tacked in place, the alignment and suitability of the system can be judged. Any corrections to the system can be easily performed by removing the required fasteners, adjusting the stop, and re-tacking them into place. Once the entire installation is deemed correct, the fasteners can be hammered flush. Extra fasteners are then added for additional security.
Drip caps are used to protect components from water penetration. Formed by installing the stucco stop so they overlap other components, the drip caps make it difficult for water to penetrate into certain areas by forcing the water to run down the face of the stucco membrane until the water is below the component requiring protection.
The minimum drip cap overlap should be at least 6 mm (¼ inch), however, 13 mm (½ inch) is superior. The largest overlap is determined by the flange width of the stucco stop. Drip caps larger than 13 mm (½ inch) become increasingly prone to damage.
Diagram 5 Various drip caps formed by the installation of stucco stop
Joining two lengths of stucco stop
The proper method to join two separate pieces of stucco stop is shown in diagram 6. One of the pieces is cut and bent to form a lip. This lip is then slid into the other stucco stop, providing a secure attachment and assuring accurate alignment. Additional fasteners at all connections further increase the security of the alignment.
Diagram 6 Attaching stucco stop
Diagram 7 Cutting and fitting stucco stop for projecting corners
Diagram 8 Cutting and fitting stucco stop for reentrant corners
Diagram 9 Cutting and forming stucco stop
Installation of upside down stucco stop
Whenever the top perimeter of the stucco membrane requires a stucco stop to be installed upside down (such as under soffits, vents, and flashing) extra fasteners may be required. When installing stucco stop upside down, fasteners using the pre-punched holes (which are now on the bottom), do not provide adequate support. Therefore, to increase support, additional fasteners should be installed through the metal flange.
Installing stucco stop over foundations
Installed over the wood sheathing of the supporting structure, building paper performs many useful functions. It provides a water and vapor barrier between the supporting structure and the stucco membrane, and supplies membrane separation from the supporting structure, helping to reduce stress buildup.
Of the different types of building papers available, the most popular form is tar paper. Other forms, usually known as house wrap, provide excellent results but are much more expensive.
Although they all must be water proof, building papers which absorb moisture, such as tar paper, reduce mortar sags by providing suction properties conducive to mortar application. Building papers that absorb very little moisture provide poor suction properties. Consequently, the initial mortar application may sag tremendously.
Building paper over sheathed walls must have no holes; should be water repellent; and conform to the current regulated requirements of the projects locale. See Regulated Requirements, page 128.
The installation of building paper
Except for soffits, building paper must cover the entire wood sheathed backing surface.
The installation of the building paper usually begins after the stucco stop installation, before control joint installation, and simultaneously with flashing installation. See diagram 13.
The building paper is installed by rolling it out horizontally against the sheathed surface and fastening it with nails or staples. The building paper must sit flat against the sheathing, with minimal wrinkles. See diagram 12.
All joins of separate pieces of building paper must overlap a minimum of 75 mm (6 inches), and all lower pieces must be overlapped by higher pieces, forming a shingle effect. Installing the building paper from bottom to top facilitates the overlapping of a lower piece by a higher piece.
Soffits and building paper
Since any moisture that accumulates in the soffit must be encouraged to drain, soffits should not be sealed with building paper.
Diagram 12 The installation of building paper
Flashing / Drip Cap
Manufactured in a variety of sizes and styles, flashings are used to protect components and restrict moisture entry into the wall assembly by channeling and directing the moisture away from components, junctures, or openings, in the membrane of the exterior finish. The required style and size are determined by the component, juncture, or opening, that the flashing must protect.
Flashings are manufactured with or without a returned edge. Those that possess returned lips are recommended, as they are safer, less prone to damage, and easier to install.
Flashing must be installed:
- at all horizontal junctures between separate exterior finishes, except when the upper finish overlaps the lower.
- over all components, such as doors, windows, vents, etc., that are exposed to moisture.
In addition, if a component, juncture, or opening, must be flashed, then to provide esthetic continuity it is usually best to flash all similar components, junctures, or openings.
For round or arched components that require flashing, use a vinyl flashing and cut the flange to allow for easier bending.
Stucco stop over flashing
The decision whether to put a stucco stop on top of a flashing is mainly determined by the choice of stucco finish. Although not structurally required, the installation of a stucco stop on top of a flashing does provide a guide to assist in mortar application. This guide is sometimes beneficial, especially for float finishes. However, do not install a stucco stop on top of a flashing if its installation serves to hinder the application of the mortar. If the wall or flashing are not aligned correctly it is usually easier for the mortar applicator to mask this defect without a stucco stop.
The installation of flashing
The flashing should overlap the edges of the component by at least 13 mm (½ inch) and should sit with a slight downward forward slant, allowing water to run off and providing an expansion slot in case of settlement. The upper flange of the flashing must be over lapped by building paper, and should extend under the building paper at least 5 cm (2½ inches).
A control joint is a pre-formed break in the stucco membrane, used for both structural and esthetic purposes.
Structurally, a control joint is used to control and relieve stresses in the stucco membrane, thereby reducing the possibilities of cracking or buckling. These stresses are found at joins of dissimilar materials or where differential deflection is anticipated. Stress is caused by expansion and contraction of a structure, and is usually due to settlement or varying climatic conditions. Control joints must be installed wherever structurally required.
The stucco lath component used to form a structural control joint is called an expansion bead. Expansion bead is usually manufactured in 13 mm and 19 mm (½ and ¾ inch) profiles. The required profile depends on the chosen thickness of the stucco membrane.
When installed horizontally for structural purposes, the expansion bead must be positioned as shown in diagram 16. The top flange has round holes and the bottom flange has oblong holes. The oblong holes allow the bottom flange to move according to stress forces acting on the stucco membrane
Control joint applied for esthetics can be installed anywhere, however all structural applications require the flanges of the expansion bead to span the structural joint.
The installation of expansion bead
Installation technique of expansion bead over joists
1. Cut off a small piece of expansion bead to be used as a guide.
2. Place this guide as indicated in diagram 16, near one end of the joist.
3. Tack a nail just above the upper flange, so that the nail protrudes from the surface.
4. Repeat this at the other end of the joist.
5. The building paper should now be installed.
6. Chalk a line between the two protruding nails and over top of the building paper.
7. Install the expansion bead using this chalk line to guide the upper flange of the expansion bead.
Control joint installation over floor joists
Diagram 16 Wall section cut out showing the correct position to install expansion bead.
The top flange is applied to the horizontal 2x4 of the wall, located just above the flooring. The bottom flange is applied to the joist. The top flange should be nailed flush every 15 cm (6 inches), while the bottom flange should be nailed non-flush every 60 cm (2 ft.). This application allows the bottom flange to ride on the joist.
Control joint between wood and masonry materials
Attach the rigid flange to the wood and let the floating flange sit flush on the masonry surface.
Expansion bead at projecting corners
At all projecting 90º corners the expansion bead should be cut at 45º, and applied as shown in diagram 18. Expansion bead for projecting corners with other angles should be cut at the appropriate angle, and applied in a similar fashion. A hacksaw and a miter box with the appropriate angle should be used to cut the expansion bead.
Control joint formed from back to back stucco stop
Sometimes control joints are formed by two back to back stucco stops, usually with a separation of 13 mm (½ inch).
The separation gap must be caulked or treated to control and prevent water penetration.
Creating independent membrane panels
The individual panels created by dividing the stucco membrane with structural control joints must be independent. Therefore, the sources of interlock used for each panel must also be independent and must not continue under the control joints, nor span the structural joint.
Sources of Interlock
The two principle sources of interlock are wire lath and metal lath.
Wire lath provides the main source of interlock between stucco mortar and the solid backing of the supporting structure. It is made of 16 gauge crisscrossing galvanized wires welded together at each overlap, creating square shaped openings approximately 50.8 x 50.8 mm (2 x 2 inches) in size. These joints are usually very strong, yet the welds do fail on occasion.
Usually purchased in 4.6 m² (50 yard²) rolls with widths of either 122 or 137 cm (48 or 54 inches), wire lath incorporates a self furring design employing ribs to space the lath from the backing surface approximately 6 mm (¼ inch). This self furring design permits stucco mortar to surround and eventually embed the wire lath, providing a strong mechanical bond. To take advantage of this design, the wire lath must be applied so the self furring ribs face the backing surface.
Paper backed welded wire lath (not shown)
A variation of wire lath, incorporating a combination of galvanized wire fabric, sheet stiffeners and slot perforated paper fabricated between face and back wire, paper backed welded wire lath is usually used when the framing of a wall does not include a solid sheathed backing.
The installation of wire lath
The most effective wire lath application method must provide checks and balances to compensate for the variety of potential problems that may arise in the life of the stucco membrane.
The combination of building paper and wire lath is usually applied over a solid backing made of wood sheathing attached to wood studs. Wire lath is applied over the building paper and is attached to the backing with nails. Although the majority of the nails should penetrate the studs behind the sheathing, the importance of the nails piercing a stud is directly related to the thickness and strength of the sheathed backing. The stronger the sheathing, the less important that all of the nails hit studs. Conversely, the weaker the sheathing the more imperative it is to hit the studs.
The maximum distance between nails used to support wire lath should be 40 cm (16 inches) horizontally and 15 cm (6 inches) vertically. The distance between studs used in the construction of exterior walls is usually 40 cm (16 inches). This translates to approximately 8 squares of wire lath. The basic patterned formed by nails supporting wire lath is a rectangle 8 squares wide by 4 squares high. Of course, this is only when the wire is applied horizontally or vertically. When applying wire lath on any other angle it is difficult to harmonize the basic nailing pattern with the placement of studs. Therefore, to provide extra support when applying wire lath on an angle, nail in distances of 4 squares, forming boxes 4 squares by 4 squares.
The application of stucco mortar invariably puts stress on the lath system. Any slack in the wire lath system prior to the mortar application would contribute to weakening the integrity of the eventual stucco membrane. Therefore, a proper wire lath application must be taut, with minimum slack.
The basic method to produce a taut wire lath application is to initially anchor the lath in one direction, and then tighten the lath by stretching it in the opposite direction. All nails that attempt to stretch the lath must have at least one anchor nail to counteract the stress. Any attempt to stretch the lath without initially anchoring it in the opposite direction would not result in stretching the lath, but would instead pull the lath away from the previous nails.
Most of the nails contributing to the wire lath application are installed against the intersection of the horizontal rows and vertical columns, but all nails must be installed so they rub against the wires of the lath. The lath is stretched by installing the nails on a slight angle. As the nails are hammered into the solid backing, they rub and push the wire in the direction that the nail is pointing.
To gain the maximum benefit of each nail, the nails location relative to the nearest horizontal row and vertical column must correspond to the direction the nail points. For example:
If the nail points up then the nail must be located directly under a row.
If the nail points down then the nail must be located directly above a row.
If the nail points to the left then the nail must be located directly to the right of a column.
If the nail points to the right then the nail must be located directly to the left of a column.
The majority of the stress acting on the wire lath is due to the weight of the mortar. To compensate, most of the fasteners should be directly below the horizontal wires and point up.
The installation begins by cutting a length of wire lath from the roll. Larger lengths become more manageable by initially tacking them to the backing surface before proceeding to install them. After placing and tacking the piece of wire lath to the supporting structure in the approximate location of its installation, the first step in attaching the wire lath is to create a box.
The basic nailing pattern for installing wire lath
i) creating a box.
The box provides a starting point in wire lath installation. The wire lath is now anchored in all directions and can be stretched in all directions away from the box.
The basic nailing pattern (continued)
ii) creating a control column
Providing the initial horizontal anchor for the wire lath, a control column should be aligned with the corresponding column of the original box, as determined by the direction of the nails. The nails of a control column should either all face left or all face right. (In our example the nails are all facing left.) The nails that are to be applied above the box created in step one should all face up. The nails below the box should all face down.
Although all of the nails above the box must face up, the sequence that the nails are installed in is critical. A nail facing up should not be installed below a previously installed nail on the same column. Vice versa, a nail facing down should not be installed above a previously installed nail on the same column. Therefore, in the following example, nail #1 must be installed before nail #2, etc. It should be noted that since there are anchor nails above it and no nails below it, nail #5 can be installed at any time during this example. Nails in a control column need not be slanted either left or right, since doing so without any counter force would likely warp the wire lath.
The basic nailing pattern (continued)
iii) stretching the wire
Since the nails in the example control column face left, the only way to stretch the wire is to push it to the right. Notice that the second column is also aligned with the nails in the original box.
Nails #4 and #6 provide extra vertical support to the wire lath.
Notice that all of the nails installed on this page of the example face to the right.
The basic nailing pattern (continued)
iv) stretching the wire in the other direction.
The wire is now anchored in both horizontal directions. Therefore, the wire can now be stretched in either direction. (In our example, it will be stretched to the left.)
The idea of the basic nailing pattern is to continually push the secured edges of the wire lath away from the central anchor points. Therefore all nails on the left of the previously nailed vertical columns must be on the right of their particular column and pushing the wire to the left (away from the anchored column).
Again, notice that all of the nails in the above example face to the left.
The basic nailing pattern (continued)
Each successive horizontal row or vertical column of nails not only contributes to the tautness of the wire lath, but also slightly relieves the tension on the welds of the previous row or column. This contributes to balancing and controlling the stress produced in the wire lath system.
For added security in case of weld breakage, there should be at least two nails pulling the wire in every direction. Therefore, the control column should not be near the edge, but instead near the middle of the piece of wire lath.
A row or column inadvertently cut so as to disrupt the tension lines of the rows or columns, can create havoc on the structural integrity of the wire lath application. Therefore, all exterior vents, electrical outlets , windows, doors, and any other encumbrance, must be installed before the lath application. If the wire lath must be cut, then before cutting, add new anchor nails to counter the lost stretching nails.
Upon completion of the installation, the wire lath should be taught in every direction. You can test this by plucking the wire and letting it snap back against the wall. If it doesn't snap back then the wire lath has too much slack in the system and must be repaired before mortar application.
Since it must be entirely enclosed by the stucco mortar, the wire lath must be installed entirely behind the surface plane of the stucco membrane.
Wire lath corner reinforcement
The direction of the stress produced by differential deflection of two intersecting panels depends on whether the corner created from the intersection is projecting or reentrant. For projecting corners the stress usually acts to pull the stucco system towards the supporting structure. However, for reentrant corners the stress usually acts to pull the stucco system away from the supporting structure.
Wire lath applications over projecting corners should at least continue the distance required to attach a vertical column to the first stud around the corner.
Whenever a projecting corner is to be stuccoed without a corner bead, extra support for the mortar can be achieved by double wrapping the corner with two separate overlapping pieces of wire lath. Even though, if properly applied, one layer of wire lath and a corner bead have proven to provide ample support for projecting corners, if your local building codes requires that all projecting corners be 'double wrapped' with two pieces of wire lath, then it is best to comply.
Wire lath corner reinforcement (continued)
There are two schools of thought when it comes to lathing reentrant corners. The basic difference between the schools is to what extent should the stucco system resist differential movement between the two intersecting panels of the supporting structure that form the reentrant corner.
The unrestrained method accepts the fact that stucco can not resist excessive structural movement, and the stucco system should instead find ways to minimize the potential damage.
The unrestrained method uses no continuous wire lath reinforcement around reentrant corners. Consequently, in case of differential movement between the panels, the result tends to be a crack forming at the apex of the join between the panels. A crack that would probably form anyway.
Stucco relies on the integrity of the supporting structure for support, and at no times should stucco be designed to relieve the structure of this duty.
Diagram 22: unrestrained method
The restrained method relies on the belief that stucco can and should indeed resist structural movement, even if the potential result is greater damage to the stucco membrane.
The restrained method attempts to connect the intersecting panels together as rigidly as possible using the restraint afforded by metal reinforcement and interlocking bond between the different coats of stucco on each panel. This is achieved though continuous wire lath reinforcement around the reentrant corner, and may include the additional reinforcement provided by cornerite.
There are major potential problems that may arise from using this method. If there is excess differential movement between the panels, the resulting stress will tend to pull the lath away from the supporting structure. Consequently, instead of just causing a minor crack in the corner apex, this method may cause uncontrollable cracking and buckling of the stucco membrane.
Diagram 23: restrained method
Wire lath nibs
The nibs of wire lath, as shown in diagram 24, become an important consideration in quality lath application. Bending the nibs before positioning and applying the wire lath serves to protect trim and other components that may be damaged by the sharp edges. After applying the wire lath, the nibs can then be positioned near the trim.
When cutting a length of wire lath off of a roll, always cut the row wire close to the column. This gives you the longest length of nib. The shorter the nib the harder it is to bend. Longer nibs provide the most control. Cutting the roll in this fashion also makes sealing the roll that much easier.
The stress relief column, as its name infers, helps to alleviate stress build up and provides a back up in the event of a weld failure in the last column.
For most columns of a wire lath application, the basic nailing pattern provides security against random weld breakage. However, the last column not only has the most stress acting on it, but also has no back up column in case of weld breakage.
To adequately protect the last column, a stress relief column should be created about 2 to 3 columns prior to the last column.
Minimum overlaps of wire lath
The overlap of the joins between the different pieces of wire lath helps to reinforce and strengthen the stucco membrane.
The minimum overlap in most stucco manuals is 5 cm (2 inches), however this author recommends that all wire lath overlaps be at least 10 cm (4 inches).
Vertical overlaps should be stag-gered and should not occur in line with any structural member used to framed openings, such as doors or windows.
Metal lath is formed by punching, slitting, and expanding, plain or galvanized steel coils or sheets. There are variations of metal lath, all providing excellent results for particular applications. Since it is more expensive than wire lath, metal lath is usually only used on soffits, on walls supported by metal studs, or for small patches. There are two general types of metal lath: those with ribs and those without. Metal lath without ribs is called diamond mesh.
Diagram 26.1 Rib lath (High rib)
Diagram 26.2 Rib lath (Low rib)
Manufactured from copper alloy steel and then painted, the distinguishing feature of rib lath is the evenly spaced strips of solid metal (ribs), that provide stiffness to the sheet. Rib lath is manufactured with various sizes of ribs. The ribs of flat rib lath have a depth of not more than 3 mm (1/8 inch). Flat rib lath, like diamond mesh metal lath, provides all purpose mortar support, but its added rigidity due to the ribs permits wider spacing of supports. Rib laths incorporating deeper ribs are also available. The deeper ribs facilitate self furring and the additional rigidity permits wider spacing between supports than with flat rib lath. Rib lath is the form of metal lath that is usually used on soffits.
Diamond mesh is fabricated by cutting and expanding either copper alloy steel which is then dipped into protective paint, or galvanized sheet steel. Manufactured in different sizes, large sheets of diamond mesh are used for the overlap of masonry surfaces, while small narrow strips, called stripite, are used for patching.
Narrow strips of diamond mesh, commonly called stripite, are used for patching and small irregular areas. Stripite is manufactured in rolls, usually 10 cm to 20 cm (4 to 8 inches) wide.
A bent strip of diamond mesh formed in a right angle to fit and span reentrant corners, cornerite provides reinforcement and continuity between two intersecting planes of stucco.
The installation of metal lath
Metal lath is initially cut to size, fitted onto the supporting structure, and then attached to the supporting structure with fasteners. After application the metal lath must be entirely behind the surface plane of the eventual stucco membrane.
When securing rib lath, the majority of fasteners should reside beside the ribs. Further-more, when overlapping two pieces of rib lath, any ribs on the first piece must be in line with their counterparts on the second. See diagram 26.3.
Wire corner bead Expanded metal corner bead
Corner bead provides a straight guide edge as well as reinforcement for projected corners. There are two main types of corner bead: wire corner bead and expanded metal corner bead. Both types are manufactured in a number of standard lengths, and each type incorporates flanges of varying widths that facilitate installation. As a general rule, corner bead with the wider flanges are usually easier to install. Wire corner bead is formed by galvanized xx gauge wires that are welded together. This type of corner bead is usually used for dry dash stucco finishes. The more frequently used expanded metal corner bead is usually fabricated either from pure zinc alloy, or 0.45 mm thick galvanized steel.
Flexible corner bead used for curved corners is called arch corner bead.
Even though the installation of corner bead is mandatory for all float finishes, and is highly recommended for most other stucco finishes, wet dash finishes offer the esthetic choice of rounded corners if no corner beads are installed.
All corner beads must be rigid, straight, and true. The use of a plumb bob or a string is recommended to attain a straight edge. The use of one longer corner bead in place of two shorter ones facilitates easier installation.
The corner beads projecting edge should provide a guide that is proud of the sheathing by the same distance as the chosen profile of the stucco membrane. See diagram 28.
The installation of corner bead
The first step of its installation is measuring and cutting the corner bead to length. The corner bead although not forced to fit, must span the entire length of the projected corner less approximately ¼ inch. This gap, small enough to be unnoticed by a stucco mortar applicator, is an insurance in case the structure settles before mortar application.
Since they are produced in maximum lengths of 3 m (10 ft.), any projected corner longer than 3 m (10 ft.) re-quires at least two corner beads. The installation of corner beads to long corners is made easier through the use of a plumb line.
Using pre installed benchmarks such as stucco stop, control joints, or wood trim, tack in the corner bead, with a few nails. Sight down both planes of the corner bead and make sure that the projecting edge is straight.
Move the projecting edge of the corner bead by either lightly tap with a hammer, or pushing the corner bead using nails angled in the direction that the bead must move. When the projected edge of concern is the correct distance from the backing in both planes, tack it in place. Continue to straighten the corner bead until it is straight in both planes, and held firmly in place.
The trick to installing straight corner bead, without a plumb line, is to continually move different segments of the corner beads projected edge to that segments correct straightened position. Begin by using nails to divide the bead into to two large sections, and then continually dividing these sections into smaller sections, until the distance between nails is approximately 15 cm (6 inches).
The corner bead must be held firm and rigid enough to resist movement or damage caused by the application of the stucco mortar.