Gutter Design

The long term maintenance problems and damage caused by a poor gutter design will usually end up being a very expensive problem for you.

Determining the proper gutter design for your home and the required size and number of the downspouts is best left to a professional gutter installer. A professional gutter installer should be familiar with the industry standards SMACNA Architectural Sheet Metal Manual 6th edition, which details the expected rainfall for each area of the country. With the information in the SMACNA, we can make calculations based on the square footage of your home, the pitch of your roof and the average rainfall in your area.

"Rules of thumb" for gutter size and the number of downspouts needed are often based on the length of the roof edge that is being drained, this gutter design approach is flawed because it fails to consider the roof pitch and the total roof area to be drained.


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Proper Gutter Design And Installation Prevents Problems

1. Calculate your roof's gutter design/watershed area.

Lets begin by determining the area of the roof in the horizontal plane. Since the roof area increases as pitch increases, steep roofs will collect more water than flat roofs. So the next step is to figure a roof's gutter design area, multiply the roof area by the appropriate factor from the pitch table (below).

gutter pitch

The pitch of the roof is the distance a roof rises vertically over a 12 inch horizontal distance. So a roof that rises 8 inches vertically for every 12 inches horizontally has a 8/12 pitch.

2. Find the maximum likely rainfall intensity. Residential gutters are often planned to handle the most intense five-minute burst of rain, measured in inches per hour, that's likely to occur in a ten-year period. Find yours on the map below.

rainfall map

3. Multiply your watershed area by the rainfall intensity in inches per hour. This gives you the maximum area a gutter can handle with a rainfall rate of one inch per hour (this is how gutter capacity is normally rated). For example, if the maximum rainfall intensity in your area is eight inches per hour and the watershed area is 495 square feet, you have 495 square feet x 8 inches = 3,960 square feet at one inch of rainfall per hour.

Sample house An 8/12 pitch shed roof in Washington D. C. is 40 ft. wide and its length is 20 ft. 40 x 20 = 800 so the roof's area is 800 sq. ft. The pitch factor for an 8/12 pitch roof is 1.1; when multiplied by 800 sq. ft., that gives a watershed area of 880 sq. ft.

The theoretical 5,520-sq. ft. watershed drained by a 5-in. K-style gutter, divided by Washington's 6.6-in.-per-hr. rainfall intensity, shows a maximum watershed of 836 sq. ft.

4. Calculate downspout size. Downspouts can handle about 100 square feet of watershed area per square inch. Round the watershed area up to the next even 100 then divide by 100 to determine the square inches of downspouts required.

For example, a 1,550 square foot watershed area needs at least 16 square inches of downspout. A 2 inch by 3 inch downspout has a 6 square inch cross section (2 x 3 = 6) while a 3 inch by 4 inch downspout has a 12 square inch cross section. In this example, you need three 2 by 3 inch downspouts (a total 18 square inches cross section) or two 3 by 4 inch downspouts (24 square inches total cross section) to equal the minimum 16 square inches required.

A gutter splash guard must be installed on all inside corners, or anywhere two roof lines meet to form a valley to prevent the rain water from over shooting your gutter. The valley acts to concentrate a higher volume of water (from both roof sections) in one channel leading down to the intersection of two gutters.

We often see early roof shingle wear, staining, or even roof leaks on lower roof slopes in areas where a downspout from an upper or secondary roof slope drains onto the lower roof slope. You never want a downspout to drain water directly onto a lower level roof, because this will cause water and ice damage to the shingles and roof deck. It must always drain into a lower roof's gutter.

gutter outlet

In gutter design, the following considerations apply.

1. Spacing and size of outlet openings. The gutter can never be any more effective than the downspout outlet and the downspout selected to drain it. Downspout sizes must not exceed the bottom width of the gutter.

2. Slope of the roof. The gutter must be of such a design and location that water from a steep pitched roof will not by its own velocity tend to overrun the front edge.

3. Style of gutters to be used. All gutters are not effective for their full depth and width.

4. No gutter length should exceed 50 feet (15.2 m) without an expansion joint which allows for movement due to temperature changes.

5. Gutter support capability. Gutter hangers should be based on full capacity of the gutter. Ice load capacity also affect the size and strength of the system.

Aluminum gutters are available in two sizes, 5 inch or 6 inch. The most common is the industry standard 5 inch ogee gutter, also known as the K style seemless aluminum gutter profile on residential gutter designs.

The standard gutter coil is .027" thick. The heavier .032" coil is available, however, it is more expensive and for residential purposes, the standard .027" coil is quite adequate.

Aluminum is the standard material for gutters and downspouts, it won’t rust, is weather resistant and comes in a variety of colors. It has pretty much replaced galvanized steel in the industry. If your gutter contractor recommends vinyl gutters and downspouts, get another contractor because a pro wouldn’t use it.

The term seemless gutter is a little bit confusing because they actually do have seems but only at the corners and where expansion joints are required.

An essential part of gutter design is to provide joints in runs over 50 feet to allow for gutter expansion due to temperature change.

Gutter And Downspout Colors

Another important aspect of gutter design is the color of your gutters and downspouts. Your gutters should blend well with the colors of your home. Most people tend to match their gutter colors to the fascia trim. Just remember that lighter colors will show dirt and black stripes known as tiger stripes more so than darker colors.

A way to make the effects of nature less noticable is to choose a darker gutter color that blends in with the roof. It is also quite common for the gutters and downspouts to be different colors, for example, you may want to match the gutters to your roof or fascia and match the downspouts to your exterior trim or brick.

Painting your aluminum gutters and downspouts is another option. Before painting, the gutters must be clean. Make sure to use an oil based metal primer and a compatable oil based paint.

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