New Skin for an Oceanfront Dormer

Dormers are a tricky cladding problem in the best of circumstances. If the dormers in question face the ocean on the shore of an island on the coast of Maine, it gets tougher. In July, JLC’s Coastal Connection went up on the roof with Mark Pollard, lead carpenter for Thompson Johnson Woodworks, on Peaks Island, Maine, to watch Pollard deal with the dormer-flashing challenge.

The dormers shown here are exposed to rough conditions in every season: baking sun in summer, snow and ice in winter, and wind-driven sheets of rain just about year-round. These dormers were part of the Cape roof of a house built in 2001. The homeowners reported water damage to sheathing, windows, and room interiors upstairs caused by the intrusion of wind-blown rain.

When the siding was stripped, Pollard found a collection of improper construction details, including reverse-lapped housewrap and flashing. The owners approved a plan to re-clad the dormers entirely with a rainscreen system. In the next few pages, you can see how Pollard waterproofed the rough windowsills with Grace Ice and Water Shield and Zip System membrane, re-flashed the intersections between the different roof slopes using site-bent vinyl-coated aluminum flashing, and installed new flashing at the dormer-to-roof intersections and at the bottom of the window opening before applying vapor-open self-adhering Henry Blueskin weather-resistive barrier material to the wall. Over the Blueskin, he applied MortairVent rainscreen material to allow free drainage for any wind-driven rain that might penetrate behind the siding.

REVERSE LAPS

As the photos here show, the carpenters who originally built the house had made a few classic water-management mistakes. At the window head, the Typar housewrap was reverse-lapped, so that any moisture penetrating the wood-shingle siding could find its way behind lower layers of housewrap and onto the top of the window—and indeed, the window showed tell-tale water stains and mildew damage.

“Whoever did the roof was the hero,” said Pollard, “because they did such a good job of sealing where the water was getting behind the shingles. It was the Grace Ice and Water Shield on the roof, lapped up onto the dormers, that saved the house.”

Eventually, the homeowners did become aware of the dormer wall leaks, said Pollard: “They knew something was up, because this window wouldn’t close very well. And inside, where they have just a pickling stain on the window trim and the wall beadboard, it was starting to get black at the bottom of the sash. So they knew the rain was penetrating.”

In a few areas, in fact, the water did begin to make its presence known downstairs. “Under the middle dormer,” Pollard said, “they noticed damage in the first-floor ceilings.”

Pollard and his crew stripped away the original cedar shingle siding and the Typar housewrap, but left the Grace Ice and Water Shield in place. The Ice and Water Shield is not breathable, Pollard noted, but the plywood behind it appeared to be sound. And the water-tight, vapor-open material applied to the rest of the dormer, he hoped, would be enough to enable the whole dormer to stay in good condition going forward.

THREE-DIMENSIONAL FLASHING

Flashing for the intersection of a dormer and a roof is tricky. You can’t make the required shapes by bending a single piece of metal, but two-piece metal-flashing buildups typically have leak points. Last year, JLC presented contractor Kyle Diamond’s method for making a one-piece dormer corner flashing out of two pieces of bent copper soldered together (see “Best-Practice Apron Flashing,” Mar/15). Mark Pollard’s method uses vinyl-coated aluminum coil stock instead of copper, and he seals the seams with Zip tape.

For this job, Pollard started by snipping and bending two pieces of aluminum: one that would lie flat on the main roof below the dormer, with a bent leg extending up the dormer face, and a second piece that would lie to the side of the dormer, with a leg up the side wall and a flap bending around the dormer face. A short vertical leg on the side flashing piece serves to direct flowing roof water past the dormer corner.

Holding the two pieces in position, Pollard temporarily stuck them together with a piece of tape across the face. Then he flipped the pair over and taped the seam on the back. In service, the hidden tape joint serves the same function as solder in a soldered copper flashing. Removing the temporary tape from the front, Pollard slid the flashing under the shingles against the dormer cheek and face, blind-nailing it to the roof (where roofing shingles would cover the nails) and to the dormer cheek (where housewrap and siding would cover the nails).

Said Pollard: “I made a bunch of these two or three years ago and I threw them out behind my shed. Just left them. Just to see if I could trust it. And they’re sticking like crazy, so …”

WATERPROOFING THE SILL

The windowsills in the dormers received a double peel-and-stick membrane and metal flashing sequence. First, Pollard applied Grace Ice and Water Shield to the rough framed sill, extending the membrane down onto the shingles that butt up to the dormer face, and using small pieces to patch gaps at the window corners. Then he installed a piece of vinyl-coated aluminum flashing, bent into the windowsill and lapping onto the roofing below the dormer. He fastened the flashing down onto the sill using stainless steel nails.

Pollard applied a bead of silicone sealant to the prepped window-sill before nailing the flashing in place, explaining: “The Ice and Water Shield and the silicone are here to stop the wind off the ocean from blowing heavy rain up under the shingles and into the wall from underneath.”

Next, Pollard (helped by carpenter Tyler Strout) attached wood shingles to create a shallow pitch on the sill, in order to direct water down and out in case the window itself leaks in a driving rainstorm. Then he applied a strip of Henry WB25 window and door flashing membrane (a polyolefin film laminated to rubberized asphalt). As with the Ice and Water Shield below the flashing, Pollard used small pieces of the membrane to patch small seams or holes at the corners.

Henry WB25 requires a proprietary primer, Blueskin Spray Prep, that comes in a rattle can. It’s applied like rubber cement: You spray the surface (in this case, the wood shingles), then let the primer tack up and dry before you peel the backing off the membrane and stick it down. WB25 has a split backing tape to make it easier to position and apply, especially in corners.

HOUSEWRAP, WINDOWS, AND RAINSCREEN

With the windowsill waterproofed, Pollard and Strout went about protecting the walls with Henry Blueskin VP100, a waterproof, vapor-open membrane that comes with a peel-and-stick adhesive backing. In the photo on page 59, Pollard is smoothing the Blueskin onto the front face of the dormer (note the small “bow-tie” patch of WB25 protecting the lower corner of the rough opening).

Compared with mechanically-fastened housewrap, Pollard pointed out, Blueskin can be easier to install. Instead of trying to hold the roll of material with one hand and nail or staple it with the other, the worker can peel off a small piece of the split-release backing and position the sheet by adhering it in a few places before peeling off the remainder of the backing. For a permanent bond, it’s important to hand-roll the sheet with a rubber roller.

Pollard and Strout wrapped the Blueskin VP100 into the window opening, then set the pre-cased window in place (first laying a bead of silicone adhesive behind the side and top casings). Above the window, they applied a typical cap flashing, then taped the flashing to the wall with a 4-inch strip of VP100. Following Henry’s printed instructions, they applied a bead of silicone sealant to the seam where the window-top strip lapped onto the Blueskin on the wall.

Pollard’s last step before installing new wood siding on the wall was to apply a layer of MortairVent rainscreen material, an open-weave synthetic fabric that creates a 3/8-inch drainage space behind the siding.