Continuing our series about paying attention to the details in roofing projects, this article is about steep slope vs. low slope roofing and related products and code requirements.
Understanding Roof Pitch
Before we can go into steep vs. low slope, we first need to explain roof pitch. Per Wikipedia:
“In building construction, roof pitch is a numerical measure of the steepness of a roof. Roofs may be functionally flat or ‘pitched’.”
Basically pitch, in the roofing industry, is defined in increments of twelve. For example 1/12, 2/12, 3/12 and so forth. In terms of degrees, a 1/12 pitch = 4.5 degrees, a 2/12 pitch = 9.5 degrees, a 3/12 pitch = 14 degrees, etc. With the pitch in mind, now we can look at the various slopes and products.
Steep Slope vs Low Slope Starts with the Roofing Products
Steep slope roofing products such as laminate shingles or architectural shingles are only designed be water shedding. Water shedding means the shingles are not water tight and only shed water. Due their design, steep slope roofing products require an underlayment to be installed beneath them such as 15 lb. felt, 30 lb. felt, or ice and water shield.
Low slope roofing products, such as modified bitumen or TPO roof systems, are designed to be water tight. This is because the roof pitch is lower and water will have a greater tendency to sit/or remain on top of these products for longer periods of time.
Low Slope Roof Considerations
If your home has lower sloping pitches, such as pitches less than 4/12, then there are things to consider while having your roof replaced:
- Steep slope roofing products are allowed to be installed on roof slopes between 2/12 and 4/12.However, they require two layers of underlayment per 2006 IBC R905.3.3.1. Be sure your roofing estimate has the necessary provisions for installation of these to meet code.Failure to install two layers will not only result in potential roof leaks but is also in violation of the building code in most municipalities as well as can violate many manufacturers’ warranties.
- For slopes between 1/12 and 2/12 at least, roll roofing is required. Roof roofing requires a minimum of 15 lb. underlayment along with seams must are fastened and bonded.
- For slopes 1/12 or less, modified bitumen is required. Modified bitumen mechanically bonds on every sq. inch of the product. There are two basic types: self-adhering and hot application. Hot application requires a torch and is melted onsite to bond to the roof surface. Both products require a 90 lb. granulated base sheet to properly install the system on the roof. In our experience in the Wichita, KS area, for low slope applications, modified bitumen is a far superior product vs. roll roofing. It is also costlier.
These are some insights to what should be considered when “paying attention to the details” with regards to your roofs slope and making sure you get the right roofing products. Education is knowledge and you should always consider more than just price alone when selecting a contractor and roof system. Remember, the roof protects everything underneath it inside the home.