Roof Terminology Part 2: Pitch

Here’s some information that will help you understand the “pitch” and “shape” of your roof.

In 8th grade science, students learn the two rules of water are:

  1. Water always moves downhill
  2. Water always takes the path of least resistance

Water on a roof needs to be told where it is going.  It needs clearly defined paths to travel or else it will find its way into your attic space and will eventually drip its way onto your insulation and into your living space.  Understanding pitch is the most important part of directing that water away from the inside of your home.

What is the pitch of your roof? Roof pitch is referred to in terms of the inches of vertical rise in relationship to the inches of horizontal run. For convenience sake, run is always 12. So, a 6:12 pitch roof will have 6″ of vertical rise for every 12″ of horizontal run. A 12:12 pitch roof will have 12″ of rise for every 12″ of run, effectively making it be at a 45-degree angle.

Most metal roofs are designed for roof pitches of no less than 3:12. There are a few vertical seam panels that can be used down to a 2:12 pitch or maybe 1-1/2:12. However, for most roofs less than 2:12 pitch, a special kind of standing seam called a “mechanically seamed” metal roof must be used. This type of product is installed on the roof as side-by-side panels, which are then seamed using an electric machine or hand crimpers to create a folded watertight lock on top of the seams.

In addition to pitch, we need to also look at the “shape” of your roof.  In some cases the geometry of your roof may mean that a particular roof area will carry a large amount of water. This happens most often when a higher roof slope drains onto a lower roof slope. In these cases, special care must be taken, perhaps through a larger “pan flashing,” to spread that water over a wider area and make certain that an individual roof panel is not overwhelmed with so much water that the interlock between the panels is “flooded out,” allowing water to force its way under the panels.

Should you ever install a product at lower than its manufacturer-specified minimum pitch? Absolutely positively 100% “no”.

Contractors will sometimes, through the use of special underlayments or other materials, try to place a watertight roof beneath the metal panels in hope of making the panels work at lower pitches than those for which they are intended. This will eventually fail. Do not let anyone do this to your roof. There are many good alternatives for low-pitched roofs, including metal. Do not allow your contractor to try and make the “wrong” product work. Some manufacturers may offer products appropriate for lower pitch sections of your roof in colors matching the product you may choose for the steeper pitch portions of your roof.