The ideal roof pitch for a home or building that will be subjected to accumulating snow varies depending on the location and purpose of the building in question, the overall building design, the local climate and building codes.
Roof Pitch Explained
Roof pitch is a ratio of the change in vertical height in the roof, a measurement called the rise, over a given horizontal span, called the run. Pitch is expressed as the amount of rise per each 12 inches of run, so a roof that rises 6 inches over every 12 horizontal inches has a pitch of 6:12.
Pitch gives an indication of the steepness of the roof's slope and its height at the roof's peak. A roof with a 12-foot span from eaves to peak and a pitch of 6:12 will have a peak that vertically rises 6-feet above the eaves.
The National Oceanic and Atmospheric Administration's National Weather Service division indicates that water weighs 62.4 pounds per cubic foot, or 5.2 pounds per square foot at a depth of 1 inch. When the agency provides snow load information after major storms, it measures how much snow weighs based on the weight of water, known as water equivalency.
Using these measurements, 2 inches of snow has a water equivalency of about 10.4 pounds per square foot on a flat roof, but this drops down to 8 1/2 pounds per square foot with a steeply pitched roof angle of 35 degrees -- almost equivalent to a 9:12 roof pitch. The roof's pitch helps to redistribute the snow's weight. But these numbers change radically based on whether the snow is light and fluffy or wet and heavy where you live.
Regional Snow Load Differences
Differences in local climate are the major factor in determining the weight of snow on a given roof. Snow that has a high liquid content can be as much as seven times heavier than dry snow. Regional and local variations in altitude, humidity and temperature results in snow that varies dramatically in weight from place to place.
Some locations, such as mountainous regions, may also experience much more snow accumulation over a shorter period of time than others, which is often why mountain homes have steep roof pitches.
In general, snow in the Western states tends to be drier and lighter than snow on the East Coast and surrounding regions, but there are exceptions. Your local building authority can provide you with information on local snow load ranges and how those ranges impact roof designs.
Ideal Roof Pitch
The ideal slope for a roof in especially snowy parts of the country is one that is steep enough to cause the snow to slide off the roof quickly and prevent it from accumulating. Roofs with shallow pitches are likely to retain snow, and the accumulating weight can cause the roof to fail. In general, a roof with a pitch greater than 6:12 will effectively shed snow in most conditions.
The material from which the roof is constructed also plays a part at how effectively the roof sheds snow. Metal roofs tend to be smoother and more slippery than shingle roofs and may shed snow effectively with pitches as shallow as 4:12. Roofs made from rougher materials, such as asphalt shingles, may require a steeper pitch to quickly shed snow. Roof pitches between 4:12 and 6:12 generally retain snow on roofs constructed with relatively rough materials.
When a roof is designed to be strong enough to handle typical local snow loads, it may in fact be preferable to retain snow on the roof. A layer of snow on the roof acts as an insulator and shields the roofing material from damaging exposure to sunlight. Retaining snow on the roof fitted with snow guards also prevents injury to people below when large amounts of snow or ice fall from the roof.
Snow guards prevent large accumulations of snow from falling in dangerous chunks from the roof, and they can also prevent large amounts of snow from sliding from upper roof slopes to lower roofs. Snow guards are especially important on steeply sloped roofs with pitches greater than 7:12.
Proper snow guard spacing depends on the roof pitch. On roofs with pitches between 2:12 and 6:12, install guards in two rows across the bottom edge of the roof, spacing the guards 2 feet apart within each row and staggering the rows so that they are offset horizontally relative to each other. On roofs with pitches greater than 6:12, do not stagger the rows and space the guards 12 inches apart within each row.