SMACNA provides guidelines on properly designing rectagular gutters.

Rectangular gutter design.
(The following is taken from the recently published sixth edition of SMACNA's Architectural Sheet Metal Manual.)

In designing rectangular gutters, the following considerations apply:

The front edge of the gutter should be lower than the back, so any overflow will spill over the front of the gutter. The elevation difference should be half of the gutter width, with 1-inch minimum.

Some machine-manufactured gutter profiles do not have that 1-inch differential. Hanging gutters at sloped roofs should have the front edge in line with roof slope line.

A minimum ratio of the depth to width should be 3-to-4.

Widths of less than 4 inches should be avoided, because they are hard to maintain.

Provisions for thermal expansion and contraction in gutters: ends of each gutter section will occur no more than 50 feet apart, with at least one downspout in between. For a continuous appearance, adjacent ends are telescoped or enclosed with covers to accommodate expansion.

Downspouts should be located at corners if water will not drain in two directions from the corner. Expansion should normally be away from corners and downspouts. A decision should be made to anchor the gutter to force expansion in given directions or to allow freedom of movement in any direction. The support system must accommodate either case.

Gutters should be properly supported with hangers. Gutters should have sufficient strength in conjunction with supports to remain intact when full of water. In areas where ice or snow slides in the direction of the gutter, means of avoiding damage should be used. The volume of ice created when water freezes is 9 percent greater than the original volume of water; the expansion force has damage potential. Special construction features may be necessary because of wind load.

Rectangular minimum gauges for gutters.

Joints

Joints in gutters must be lapped 1 inch, riveted on 2-inch centers and soldered. In welded gutters, field welds may not be practical on metals thinner than .08-inch aluminum or 14-gauge steel or stainless steel.

With metals that cannot be soldered or welded, a compatible sealant must be applied continuously within a 1-inch lapped joint, and the sealed lap must be riveted on 1-inch centers. All rivet applications must receive sealant after installation. Designers specifying pre-painted gutters must designate the type of joint(s) to be permitted. Lapping joints in the direction of flow is strongly recommended, but gutter-outlet connections may not fit this condition.

The selected gutter material must account for electrolytic corrosion at contact points with other metals and at supports. Special selection is required for corrosive environments such as coastal or polluted industrial atmospheres. If the roof runoff has contaminants or electrolytic liquids, the effects of these must be considered in gutter design. Chemically treated wood may be incompatible with certain metals.

Designers often prefer level gutters for their appearance and alignment with the rest of the roof. When pitched gutters are preferred but potentially unattractive, they may be concealed within a metal enclosure that is supported from the pitched gutter system, its supports, the building or an independent support system. This arrangement is sometimes called a double-gutter system.

When selecting the material and thickness of the gutter, consider:

  • The gutter-support system, including expansion provisions.

  • The inherent strength and rigidity of the gutter material in the selected profile.

  • The weight of the water in the gutter.

  • Wind, snow and ice effects.

  • Maintenance and assumed service life of the system, including corrosion and incidental damage if any is foreseen.

    The omission of other materials, such as zinc-tin alloy, lead-coated copper and zinc-copper alloy is not intended to proscribe their use. Their properties may require less widely known conditions of use.

    Twelve of the most commonly used rectangular gutter styles are shown in Figure 1-2. Other less common styles are also used. The minimum dimensions are provided on one gutter bead to illustrate the appropriate size of the individual breaks. Very small dimensional beads cannot be fabricated.

    The more longitudinal breaks a gutter has, the more rigid it will be. Less rigidity in the gutter can be compensated for by a stronger support system. Therefore, styles like "A" should be of heavier gauge material and more rigidly supported than styles like "G" or "J."

    (For information on ordering the Sheet Metal and Air Conditioning Contractors' National Association's Architectural Sheet Metal Manual, Sixth Edition, write SMACNA, 4201 Lafayette Center Drive, Chantilly, VA 20151-1209; call (703) 803-2989; see www.smacna.org on the Internet.)