The weakest points of every roof—where water is most likely to invade—are joints, seams and holes where vents and pipes pierce the sheathing. That’s why many contractors advocate replacing the entire roof instead of patching in a new section. But, per installed square foot, metal roofing costs more than new maple flooring. Thus, homeowners Mills and Marianne Fleming decided to keep their 112-year-old metal roof and tie in a new metal roof over their addition. They also selected an acrylic coating system that will prolong the life of both roofs, unify them visually and help seal the seam between old and new.

The house's standing seam terne metal roof was extended out over the addition and ended in a gable facing the lane, with the rest of its ridge perpendicular to the building. The addition’s 3/4-inch plywood roof sheathing was covered in roofing felt, an asphalt saturated fabric that keeps the new construction underneath dry until the roof is finished. Rosin-coated craft paper was sandwiched between the two materials to prevent the asphalt in the felt from corroding the steel. The 28-gauge, 1/50 inch thick terne metal—steel coated with a lead/tin alloy to retard rust—was thin enough to cut easily and to bend into place with hand tools. Although it weighs only 12 ounces per square foot, it’s surprisingly strong and durable. Each terne metal panel measured 17 inches wide, with 1/2-inch wings on either side. The wings on adjacent panels were folded together with a metal-bending tool called a hand brake to form the standing seams. Water has nowhere to invade because screws don’t pierce the metal membrane. To anchor the roof to the sheathing and to prevent the occasional hurricane from lifting the panels, S-shaped stainless steel cleats, spaced 12 inches apart along the seams, were nailed into the sheathing, then folded into metal seams.

At the joint between the old roof and the new, the existing roof was peeled back and the new sheathing was patched in. Then layers of roofing felt and rosin paper were inserted and the old roof was trimmed. Finally the new roof was tucked beneath this edge, creating an 8-inch overlap that was sealed with butyl rubber. The layers were secured to the wood sheathing below with screws and 2-inch galvanized washers.

Two chalk lines snapped along the peak of the dormer provided a guide for trimming the excess metal and folding the tops of the panels to form the ridge cap. The ridge cap was created by hand. Eighteen-inch tongs were used to push the metal from each side of the dormer into a standing position. The pieces were folded over twice, much like the seams between panels to form a crisp edge, the seam was flattened with a 16-ounce sheet-metal hammer. The ridge assembly is airtight as well as watertight, so vents were installed every 12 inches along the soffit for attic ventilation. The new roof has a 20-year warranty but with proper upkeep, chances are it will last as long as the 112-year-old main roof.

Top Coat

The old roof was power-washed to give the acrylic coating a solid, clean surface to adhere to. It wasn't necessary to remove the entire 1/4-inch thick crust of more than 15 coats of paint that had been applied during the past 112 years. Rust spots where the terne plating had worn away were scraped clean with a wire brush and treated with a rust inhibitor. The old roof was ready for its acrylic and mesh coating.

A coating of a polyester mesh and acrylic system then sealed the joint between old and new metal and visually blended the two sections. The polyester reinforcing mesh comes in rolls of 4-inches, to bolster the seams, and 16-inches, to cover panels. (The wider fabric was applied along the joint to provide extra strength.) The mesh was laid down while the base coat was wet. Once the base coat was dry, two additional coats of acrylic were rolled over the mesh. Mesh was not necessary on the new roof. Instead, the contractor applied a single coat of the acrylic component. Each coat takes about 15 hours to dry thoroughly, depending on humidity, temperature and color. (Dark colors tend to dry more quickly.) At an installed cost of $3 per square foot and a 10-year life expectancy, the acrylic-polyester treatment could go on three times before it approached the price of a new metal roof.

State of the Art Ventilation
Cooling This Old House

2nd Floor:
Guest Bedroom | Guest Bath | Bedroom
Laundry | Master Bedroom | Master Bath | Attic