In construction terms, a roof with a 3-12 roof pitch has three feet of vertical rise, which is otherwise known as roof rise, for every 12 feet of length, or roof run. A 3-12 roof is equal to 14.04 degrees and is a common roof pitch in southern climates.
Roof Pitch Varieties
In geometry terms, a roof's pitch is it's slope. When it comes to architecture, a roof pitch will range between a low pitch of 9.46 degrees (2-12 roof pitch) and a steep pitch of 45 degrees (12-12 roof pitch). A standard roof pitch is 4-12, or 18.43 degrees. A 5-12, or 22.62-degree roof pitch, is considered steep but still walkable if doing construction work, while anything above a 6-12, or a 22.62-degree angle, is dangerous to traverse.
3-12 Roof Pitch Calculation
You can determine if your roof has a 3-12 pitch with a tape measure and a 12-inch carpenter's level. Position your 12-inch carpenter's level horizontally over the roof so that it is parallel to the ground below and the far end touches the roof. Check that the level bubble is in the middle position, which indicates that the center is level. With your other hand, run the tape measure from the end of the level (next to your hand) to the roof below, keeping the tape measure in a perpendicular position. The tape measurement, in inches, will equal the second number in the pitch. For example, if the measurement is 3 inches, your pitch is 12-3, since the level is 12 inches and the rise equalled 3 inches.
A 3-12 pitch is less suited to inclement weather, such as hail, strong winds and icy conditions. If your roof is 3-12 or lower, consider purchasing an ice and water shield to prevent ice dams. The 3-12 roof pitch is susceptible to winter problems because the slope of the roof is less steep and therefore will accumulate more snow and ice. A 3-12 roof is also more susceptible to wind damage because a high wind will hit a low-pitch roof at a lower angle, which creates a partial vacuum effect.
You will need a ladder to gain safe access to the roof to measure the pitch. Keep in mind that you will not need to walk the entire length of the roof, but you will need to work with at least 12 inches of roof run. A 3-12 roof is safe to traverse, but if your roof is unstable or has shingles, try to measure from inside the home. This works well if you have an attic or second floor with access to the slope of the roof.