You're always better off capping parapet walls with metal rather than stucco'ing over them. But what if your client or the project architect doesn't want that look? I recently built a Spanish Colonial–style house near Austin, Texas, and my clients and the architect on the project wanted a traditional look. The roof over the front door was flat and called for a stucco parapet. The challenge was creating a parapet cap that didn't allow massive amounts of water to drain into, and rot out, the framing. (For a demonstration of the ugly failures that can occur with stucco parapet caps, see my blog posting, "Massive Stucco Failure—Lessons Learned," at jlconline.com.)
Framing
First, my carpenters framed the parapet cap with a slope draining to the inside. Any water falling on a parapet needs to drain onto the watertight roof—you don't want it standing on the parapet cap, and you certainly don't want it to dribble down the stucco face of the exterior wall where it can find its way into the framing. A slope of 1 inch across the width of the parapet works pretty well.
We used plywood for the parapet cap, not OSB. Plywood is a little more forgiving when it gets wet—as the cap inevitably will. The sloped plywood provides a much-needed last line of defense.
Housewrap
Next, my frame carpenters wrapped the house in Tyvek Commercial D building wrap and fastened it using a Stinger cap stapler. Commercial D, a crinkly version of Tyvek Commercial wrap, is my go-to housewrap when I'm installing stucco—or just about any facade. It's jobsite tough and it provides an effective drainage plane behind the stucco. (I posted a video, ""Choosing Housewrap When Using Exterior Rigid Foam," on YouTube a few years ago that demonstrates how well the crinkles move water out of a leaky assembly.)
FLAT ROOF FIRST
Next, I brought in the roofer to do the underlayment for the pitched roofs. My go-to roof underlayment for any roof I build here in Texas is Carlisle WIP 300HT. At this time, the roofers installed the roof membrane on the flat roofs, as well, using Carlisle's white TPO, which is perfect flat-roofing material for our hot climate.
As always, we worked from the bottom to the top when doing the waterproofing. We wanted that flat-roof membrane down before we moved on to the parapet-cap waterproofing.
Cap Flashing
If your installers have covered the parapet with the housewrap, be sure to cut it back before applying any flashing. Next, apply a flexible flashing wherever the parapet cap takes a turn. The straight flashing that covers the parapet cap will need to be cut there, so we want the flexible flashing underneath to ensure a watertight seal.
I typically use Tyvek FlexWrap, which is readily available from local suppliers. Carlisle also makes a version called Elastoform; it's a great product, but it's a special-order item, so I usually go with FlexWrap.
In either case, it's critical to use a single material that can easily conform to different surfaces. The sheet membrane we apply over parapet caps doesn't stretch and must be carefully folded around corners (3, 4). Installing the elastic membrane underneath makes what would otherwise be a tough spot easy to detail.
To ensure a lasting bond between layers of waterproofing, I recommend using 3M High Strength 90 spray adhesive. We use it before adhering the FlexWrap, and to hold down any flaps in the housewrap and the cap flashing. This adhesive particularly helps on custom projects, where the cycle time between trades can get drawn out. You never know how long it will be before the housewrap and flashing materials get covered up, and the adhesive helps hold everything together in the meantime.
For flashing over the top of the parapet, I like Carlisle CCW 705 peel-and-stick membrane. This 40-mil–thick product is very similar to ice/water shield products used on roof edges and valleys to prevent leaks caused by ice damming in cold climates. It consists of a rubberized asphalt adhesive laminated to an HDPE cross-laminated film and has good self-healing properties around nail penetrations. There needs to be a thick, durable product under the stucco cap because sooner or later, water will get through the stucco.
A few other manufacturers make this type of 40-mil product, but you might need to hunt around to find one, as it's not typically stocked at lumberyards. I buy mine from a commercial waterproofing supply house in Austin called All-Tex Supply. You might find a similar product at a roofing supply house. Or you could use WIP 300HT, which is also 40 mil thick. Whatever product you pick, you want to ensure that it wraps over the entire cap in one piece and hangs down at least 4 to 6 inches over the housewrap to provide a good shingle effect.
One reason I like CCW 705 is that I can buy it in various roll sizes; my supply house typically stocks it in 12-, 18-, 24-, 36-, and 48-inch rolls. Sam, my carpenter, runs roughly 6-foot lengths to ensure smooth and mostly wrinkle-free installations. We always overlap the sheets by 6 to 8 inches. Whenever we are concerned about adhesion, we spray overlaps with High Strength 90 adhesive.
STUCCO FINISH
Before we applied the lath and stucco, we installed one layer of 30-pound felt. We did this to provide a bond breaker that would prevent the stucco from bonding to the housewrap. The scratch coat bonds to the felt instead, and any water that manages to seep through the stucco can drain freely down the channels in the housewrap behind the felt.
A parapet cap like this one takes some work and some dollars, but I'll sleep better at night knowing that this detail won't someday end up as a callback (or worse, a lawsuit).
Matt Risinger is owner of Risinger Homes, a custom builder and whole-house remodeling contractor in Austin, Texas.
