Built-in gutters require special design considerations
(Editor's note: The following is taken from the Sheet Metal and Air Conditioning Contractors National Association's "Architectural Sheet Metal Manual," fifth edition.)
Built-in gutters serve a special purpose in architectural design in that they are less visible than exposed gutters. Architects specifying t-in gutters should be aware that leaks which may develop will cause water to go directly into the building. They should design cautiously.
In designing built-in gutter, the following considerations apply:
1. Materials should be corrosion resistant. Copper or stainless steel is preferred. Stainless steel is preferred in a galvanized steel roof system, but galvanized steel can be used. Aluminum or pre-finished metal should not be used. Field painting of pre-finished stock is not practical; adherence and color matching are common problems. 2. All joints must be riveted and soldered. Joints that are sealed and riveted should be expected to require maintenance and have reduced service life. Longitudinal seams are not used, except in custom radius conditions.
3. Built-in gutters must be adequately supported. Continuous support is preferred. Damage potential is greater with intermittent support.
4. Underlying for gutters shall consist of a smooth unsaturated rosin-sized building paper weighing 5 lbs. per 100 sq. ft. (0.244 kg/m2) used as a slip sheet over a single ply of 30 lbs. (1.46 kg/m2) asphalt saturated roofing felt. Felt underlying serves to minimize condensation build up to the underside of metal gutter lining and it should lap in the same direction as the seams. When using terne metal, omit the felt underlying; one ply of rosin paper is sufficient.
5. The highest point on the front of the gutter must be a minimum of one inch (25 mm) lower than the back edge of the gutter. This allows water spill over the front edge in case the gutter or downspout becomes clogged, or should excess rainfall occur. When the designer must have the front edge higher than the back edge for aesthetic appearance, scuppers should be used to prevent overflow from excess rainfall or clogged downspouts.
6. Expansion control is critical. Provide horizontal clearance from supports, gutter ends and corners. Prevent or control movement at downspout locations. Tests and experience show that with different shapes, metals of different thickness are needed to avoid buckling as expansion occurs.
7. Width across bottom of the gutter should be at least four inches. (254 mm).
8. With non-metal roofing, continuously cleat the back edge using a four-inch (102 mm) minimum apron flashing width. Four-inch (102 mm) width is used for asphalt and fiberglass shingles. Six-inch (152 mm) minimum apron flashing is used for slate, cement tile, ceramic tile and wood shingle roofs. Ten-inch (254 mm) minimum width is standard for terra cotta tile. Note: Consider requiring a weatherproofing membrane that laps the cleat and lies under the roof material. In ice dam areas, the waterproofing sheet should extend at least 12 in. (305 mm) behind the inside wall lines. It should be 36 in. (910 mm) behind the wall line with a roof pitch less than 5 in. (127 mm) in 12 in. (305 mm).
With metal roofing, the sheet is also recommended. See eave and gutter details for the type of metal roof. The metal roof system may need special features to lock to the gutter in a waterproof manner when ice dams are present with ponding behind them. The metal edge may have provision for thermal expansion. Exposed and concealed fasteners should be evaluated for water tightness. Use only one-piece combination gutter-flashing in ice and heavy snow areas.
9. It is recommended that angle A be 908 and angle B be 1208. If this is not practical, then other angles may be used. In icing areas, the use of two 908 angles should be avoided.
10. Outlets should only be placed in the bottoms of gutters. Clearance must be allowed around gutter outlets to allow for movement.
11. When the built-in gutter is behind a parapet wall, overflow scuppers can be used in the wall.
12. Where the built-in gutter terminates at a fascia, a continuous drip edge is desirable. 13. Where practical, slope built-in gutters towards drain points.
(For information on ordering SMACNA's Architectural Sheet Metal Manual, write SMACNA at P.O. Box 221230, Chantilly, VA 20153-1230 or call (703) 803-2980; fax (703) 803-3732; www.smacna.org.)