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Stucco Flashing Details -

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Flashing Details

The areas of a stucco-clad building that are most susceptible to leaks are windows and doors, sidewall penetrations, and horizontal surfaces. In each case, proper flashing can prevent leaks, but all too often the flashing is either poorly installed or omitted altogether. Standard flashing details are difficult to come by, so over the years I have developed flashing systems that combine code-required techniques with methods of my own that have endured the test of time.

Flanged Windows

Vinyl or aluminum windows with nailing flanges will drain properly only if the lath and flashing papers are correctly layered as follows (see ).

Step 1:

Install a strip of flashing paper horizontally under the opening.

Step 2:

Install vertical strips of flashing paper at either side, making sure to overlap the bottom strip.

Step 3:

Apply a liberal bead of caulk on top of the flashing paper at the edge of the opening.

Step 4:

Install the window frame while the caulk is still wet. If you've used enough caulk, it should squeeze out from behind the flange. Drive the nails in all the way; never bend them over. A bent-over nail may look okay now, but it will eventually rip or puncture the flashing paper, and could interfere with proper layering of the flashing and water-resistant membranes.

Step 5

: Apply a liberal bead of caulk across the top window flange, then run a strip of flashing paper across the top, pushing the flashing into the caulk. For extra protection, we sometimes install a 26-gauge metal Z-flashing or water table over the top of the flange. This flashing extends about 3/8 inch beyond the window corners to keep water away from the frame, which we have found to be susceptible to leaks. When you install the building paper, roll it out horizontally, starting at the bottom of the wall and working your way up. Pay attention to the overlaps, and carefully weave in the flashing material as you go. I like to see 4-inch horizontal and 12-inch vertical laps. Window frames should have a drip edge and stucco key at the outside edge of the sill; otherwise, water can run back under the sill, behind the stucco, and enter the wall through any holes in the flashing paper. When installing window frames, be careful not to jiggle or shift the frame too much. This could loosen the joints, creating a crack where water can get through. Also make sure any weep holes are big enough to drain freely. If the weep holes are too small or can be easily plugged, hard driving rain water can back up over the inside edge of the window and onto the sill.

Flashing for Clad Doors

Doors are susceptible not only to water that falls on them directly, but also to backsplash from dripping eaves and overhangs. As with windows, each step in the flashing is critical to preventing leaks.

Step 1:

If the door will be installed over a wood floor or subfloor, start by placing a metal pan with soldered corners at the sill, fastened in place with one nail driven through each of the wall fins into the edge of the stud (). The fins on the metal pan should overlap the metal deck-to-wall flashing at the bottom of the wall by a minimum 6 inches. If space is tight, make sure the sheet-metal sub carefully caulks the overlap.

Step 2:

Next, install strips of flashing paper vertically, flush with the sides of the opening. The strips should be long enough to cover the pan flanges at the bottom and to extend under the full width of the flashing paper that will be installed at the header.

Step 3

: Caulk the flashing paper 1/2 inch from the opening, then install the door jamb. Use a big enough bead so that the caulk squeezes out when you nail the door flanges. After the door flanges have been attached, shim and fasten the jambs to the studs using a pair of nails or screws 4 inches from top and bottom and 16 inches on-center in between. This will keep movement in the jamb from causing cracks in the stucco.

Step 4

: Lay a generous bead of caulk over the flange at the door head, then bed a final strip of flashing paper in it. Make sure the paper overlaps both the nailing flange and the side strips of flashing paper.

Plant-Ons

In many stucco installations, windows and doors are trimmed with built-up details called "plant-ons." Made of either foam or wood, these extra layers create additional joints and corners that are prime candidates for leaks. The key to preventing leaks at plant-ons is to use two pieces of paper. One piece runs continuously behind the plant-on to protect the sheathing; a second piece runs from under the window or door frame over the plant-on (Figure 3). Avoid folds or bends that will create a dam for water, and be careful not to tear the flashing paper when covering the ends of the plant-on.

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Figure 3. Flashing paper should be used behind wood or foam plant-ons used to trim openings. The plant-ons should be completely wrapped with flashing paper, which must also be properly woven into each course of lath paper for proper drainage.

Install vertical plant-ons after the bottom plant-on has been fully flashed. Run one piece of flashing paper behind the vertical plant-on, and over the horizontal plant-on. A second strip of flashing paper, used to cover the vertical plant-on, should also overlap the horizontal plant-on. Install horizontal plant-ons at window and door heads last, again making sure the flashing is correctly layered and overlaps the side plant-ons so that water drains to the outside, down and away from the framing. We use a hammer tacker to hold the flashing paper in place until we are ready to install the lath, being careful not to put any holes in the paper. We place staples only at the edges of the flashing paper, where they will be covered by the window or door frame and caulking. Any holes created by the staples at the outside edge will be covered by overlapping building paper. All plant-ons should be installed a minimum of 1 inch from the edge of the window or door frame so as to allow enough space for the flashing paper to easily run out and over the plant-on. If the plant-ons are installed too close to the frame, the flashing paper may create a groove or trough that will allow water to pond.

Wall Penetrations

Vents, utility supply lines, and electrical outlets are all prime candidates for leaks. For all of these penetrations, the key to a good flashing detail is to provide adequate backing around the penetration. For electrical outlets, supply pipes, spigots, and other small penetrations located near studs, use a two-piece sheet-metal collar (Figure 4).

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Figure 4.

To flash wall penetrations, the author uses a two-piece sheet-metal collar (illustration), which is caulked to the fixture, then overlapped with flashing paper. A one-piece collar works well when the fixture falls in the center of a stud bay (photo, left). The integral flanges on attic and exhaust vents provide enough support for the stucco, but should still be caulked and carefully overlapped with flashing paper (photo, right).

Carefully cut the profile of the pipe or outlet out of each half of the collar, and allow for the two halves to overlap by at least 2 inches. Nail the collars only at the top so you can slip flashing paper under the lower halves (you may have to install extra blocking in the wall for nailing). After caulking the sheet metal to the outlet or pipe (all copper pipes should be sleeved in PVC to prevent corrosion), layer flashing paper over the top and sides of the collar, making sure to overlap the lower sheet of paper by 4 inches or more. Where an outlet or pipe falls near the middle of a stud bay, use a one-piece sheet-metal collar to provide solid backing for the stucco. Again, caulk around the cutout and nail only the top of the collar, leaving the bottom flapping so it can overlap building paper slipped up from below. Overlap subsequent layers of paper so that they provide continuous drainage to the outside. The same flashing principles apply to larger penetrations, like exhaust vents and attic vents. Most of these fixtures have integral flanges, so no additional collar is needed. But the paper must be layered carefully and should be caulked to the flange. If the paper rips in the wind, it should be replaced.

Where Fascia Butts a Wall

Another weak spot in the weather barrier is the intersection of a fascia or eaves return and a wall. Because the roof pitch directs so much water to these areas, we take extra precautions against leaks. Whenever possible, we coordinate our flashing work with the framing crew so that we can apply a full 36-inch-wide strip of Vycor over the wall before the subfascia and trim boards are nailed in place (Figure 5). The self-sticking membrane should overlap any lath paper on the bottom, but should be overlapped by paper on the top and sides, so that any water spilling onto the wall will drain downward.

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Figure 5. Flashing where roof trim meets a wall should be done before the framing is complete. The author protects the wall with a full-width sheet of self-adhering membrane, which overlaps lower courses of flashing paper at the bottom, but is itself overlapped by paper at the top and sides. It's also essential to install a metal kick-out flashing on the roof to direct runoff away from the wall.

Equally important is a kick-out flashing at the edge of the eaves. This metal flashing is really the first line of defense, because it catches any water coming off the roof and directs it away from the wall surface (for kick-out detail, see "," JLC 9/98). As with all other flashing details, the building paper should be layered properly so that water drains to the bottom of the wall.

Horizontal Surfaces

Horizontal stucco surfaces, such as balcony railings and pot shelves at recessed windows, are especially prone to leaks because they are fully exposed to the weather. Unless they are carefully detailed, water from repeated soakings or ponding will eventually penetrate the stucco base and finish coats and wet the underlying framing. When the wood swells from this extra moisture, it causes the stucco to crack, which leads to more leaks, more swelling, and more cracks. We always inspect the framing before flashing a horizontal surface to make sure that it slopes 1/4 inch per foot -- more, if possible -- so that water will drain freely. If the framing is level or, worse, sloped back toward the building, we won't touch it till the problem has been corrected.

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Figure 6. Leaks at parapets, pot shelves, and other horizontal surfaces can cause framing lumber to swell and crack the stucco. To promote drainage, the horizontal framing must be adequately sloped and should be fully flashed with a self-adhering membrane. The stepped slab also serves to contain leaks.

On a properly sloped horizontal surface, we apply a layer of Vycor adhered directly to the sheathing and extending down over the edges to overlap any building paper coming up the sides (Figure 6). If the surface is in front of a window, we run the membrane under the bottom nailing flange and onto the sill plate. Similarly, where the horizontal surface meets a wall, we run the membrane up the wall where it will be overlapped by building paper. As with all of our flashing, we're careful not to tear or punch holes in the paper, and we make sure that the paper is layered so that water will drain away from the building. Use as few nails as possible on the horizontal surface, and caulk or patch every rip, tear, or hole.

Ron Webber

owns and operates Prime Plastering in Norco, Calif. He has worked as a plasterer for more than 30 years.

Sources of Supply

Davis Wire Co.

5555 Irwindale Ave.

Irwindale, CA 91706

800/350-7851, ext. 217

Super Kraft Asphalt Sheathing Paper

Dupont

P.O. Box 70805

Wilmington, DE 19880-0705

800/448-9835

Tyvek StuccoWrap, Tyvek Tape

Fortifiber Company

300 Industrial Dr.

Fernley, NV 89408

800/773-4777

Jumbo Tex Weather Resistive Paper, Moistop EZ-Seal Window & Door Flashing

Grace Construction Produc

ts

62 Whittemore Ave.

Cambridge, MA 02140

617/876-1400

Vycor

Hal Industrie

9681 187th Street

Surrey, B.C. Canada V4N 3N3

604/888-0777

30-min. Breather Paper 60-min. Breather Paper

Leatherback Industries

P.O. Box 594

Hollister, CA 95023

800/538-5950

Heavy Duty 60-min. Plaster Kraft Paper

MFM Building Products Corp.

P.O. Box 340

Coshocton, OH 43812

800/882-7663

Future Flash