Shingles on a “Low Slope”

September 28, 2022

Why you should be very, very worried about composition shingles on a low slope roof

First, what’s a low slope roof? What’s a roof slope!?

Roof slope is typically denoted as a ratio, like 4/12. This is “Rise over run”, but in super easy language: for every 12 inches sideways, 4 inches up. Sometimes roof slope is measured in degrees (if you’re a super nerd) but in the trades it’s referred to as a ratio, like 10/12.
Roof Pitch Diagram
What’s low slope then? This range between 2/12 and 4/12 that’s a bit of a gray area in the manuals. First, it’s legal to put laminated, architectural asphalt composition shingles on that shallow of a roof. It’s often done on double-wide trailer park roofs and badly framed back patio covers.

Some manufacturers allow it with a code-minimum double coursing of felt paper, some require ice & water shield peel n stick rubberized membrane, and some manufacturers outright void the warranty altogether. Why the wide range? They all know it’s going to leak. Some shingle manufacturers are trying to be transparent. Some manufacturers are used to getting sued and have expert legal teams, so they just wiggle out.

Wsrca Bulletin

What you should know is that installing a laminated architectural shingle on a low slope roof will:

  1. Fail early
  2. Seep water past the shingles whenever it’s wet for a prolonged period (hello Seattle!)
  3. NOT be covered by the manufacturer or your installer when it does
  4. Very likely have severe mold issues in the attic cavity

Sounds like a pretty bad deal, right? So why does it get recommended so often? Simple.

Most roofers don’t even know.

How can they not know?

Glad you asked!

One (or more) of the following:

  1. They never read the warranty (it’s your problem, in the end)
  2. Never read the installation instructions
  3. Haven’t been in business long enough to get the callbacks
  4. Will also wiggle out of responsibility, and blame the shingle maker

Low Slope Architectural PuddleLook at this example where you can see actual mini puddles at each course on the laminated architectural shingle head-lap. You think that’s going to work for decades? Think again!

It’s not just us.

This WSRCA Technical Bulletin, released in 2009 with reference to the NRCA’s 2005 publication along the same lines, says clearly they do not recommend installing an architectural shingle on a low slope roof. Regardless of what the manufacturer will “allow.” It’s just going to fail.

At RoofSmart, we’re dedicated to educating ourselves so we can give our customers better homes. Sometimes we won’t tell you what you want to hear, but we’ll always give you the best information on the topic and try to steer you right. To us, that’s what RoofSmart means! Our Meticulous Roofing Nerds are standing by if you ever need our help in the Seattle or Greater Puget Sound area.

A Metal Roof May Bet a Better Option

Standing Seam metal roofing can be installed down to 1/12, or less! If you don’t want to keep replacing your roof, metal might be worth considering. It’s often double the price up front, but instead of replacing your roof every 10-15 years: you’re all done!

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