Design Considerations of Built-in Gutters
Built-in gutters serve a function that is special in architectural design because they are more hidden than gutters that are exposed. Architects specifying built-in gutters should bear in mind that any leaks that develop will cause water to go directly into the building. They need to be designed carefully.
In designing built-in gutters, think about the following:
— Materials needs to be corrosion resistant. Copper or stainless steel is favored. Galvanized steel may be used in a galvanized steel roof system, though stainless steel is favored. Prefinished galvalume products or aluminum must not be used. Field painting of stock that is prefinished is impractical, and adherence and color matching are common difficulties.
— All joints must be lapped, riveted, and soldered. Joints that riveted and are sealed with caulk will need maintenance and possess a shorter service life. Failing caulk joints letting water into the building can sometimes cause damage for years before becoming detected. When this occurs, the scope of repairs will extend far beyond fixing the leaking gutter. Copper and stainless steel gutters, with soldered joints, are the only practical metal liner for an eavestrough built into the roof.
— Built in gutters must be adequately supported. Continuous support is favored. Damage possibility is greater with intermittent support. If you use blocks of various sizes to create slope below the built-in gutter, fasten or glue a strip of plywood over the blocks cut to the with of the gutter. This will provide better support than the blocks alone.
— Underlayment should be used beneath the copper or stainless gutter and over a waterproof membrane. The waterproof membrane should be a high temp or HT product specifically designed for use under metal. The slip sheet underlayment can be 30 pound roofing felt or rosin paper. The underlayment serves to minimize condensation buildup on the underside of the metal gutter lining, and it should lap in the same direction as the seams.
— The highest point on the front of the gutter must be a minimum 1 inch lower than the back edge of the gutter. This allows water to spill over the front edge in case it rains heavily or the gutter or downspout become clogged. Designers who would like the front edge more in relation to the back edge for appearance’s sake should use scuppers to prevent overflow caused by excessive rain or clogged downspouts.
— Expansion control is essential. Please see our other articles regarding proper utilization.
— With flat roof systems, always cleat the rear edge of the gutter using the very least apron flashing width. Use a 6 inch minimal apron flashing for cement tile, slate, ceramic tile, and wood shingle roofs. Use a 10 inch minimum width for terra cotta tile. It’s strongly recommended that there be a waterproof membrane used that lies under the roof surface material and laps within the cleat. In areas where ice dams occur, minimum 24 inches should be extended by the waterproof membrane behind the wall that is inside.
— Outlets should only be put into the bottoms of gutters. Clearance must be permitted at the gutter outlets to allow for expansion and contraction.
— Whenever possible, slope built-in gutters to empty points.