It’s not uncommon for roofing contractors to prefer installing asphalt composition shingles on as many roof pitches as they can. Other types of roofing, like modified bitumen and TPO, take extra training that many roofers haven’t had. This results in some contractors using shingles on some roofs that they shouldn’t, particularly when the pitch is too shallow for shingles to work effectively.
What pitch is too shallow for shingles? Generally speaking, 4/12 and below. But with special considerations you can still install shingles down to 2/12 pitched roofs. If below a 2/12 pitch, you certainly want to go with TPO or torchdown.
If you are unsure what pitch your roof is, you can measure it with a ladder and tape measure. The formula is y/x, also known as rise over run. Put the hook of the tape measure on any part of the roof deck, then measure 12 inches horizontally. This is the run. Then from that point measure straight down. The number of inches when measuring straight down is the rise. A 2/12 pitch roof will measure 12 inches horizontally and 2 inches down. An 8/12 pitch roof will measure 12 inches horizontally and 8 inches down.
The two main considerations when shingling a roof between 4/12 and 2/12 pitch are:
- How long is the roof deck in the downward direction.
- What kind of underlayment will be under the shingles.
The length of the roof deck is how far water must travel downward. The steeper the slope, the faster rainwater travels down the roof as well as the less accumulation of possible snow. Slowly traveling rainwater and accumulated snow can seep moisture beneath the shingles. If asphalt composition shingles were seamless, this wouldn’t be a problem. But they are designed for steeper pitches that shed water well enough that the shingles can be layered on top of each other and don’t have to be fully adhered. Shingles work spectacularly on steep enough slopes, but because of the design, any water that doesn’t shed can find a way inside the tiny openings between shingles.
If the roof deck is short enough in the length that rainwater travels down it, shingles can be a viable solution.
On typical roof slopes, roofers will use either felt paper or synthetic underlayment. Quality of these vary greatly, and the best is GAF Tiger Paw Synthetic Underlayment. On a pitch between 2/12 and 4/12, a common practice is to double layer underlayment like these. But that can still lead to problems. Best practice is to instead use ice and water shield, the best of which is GAF WeatherWatch.
Ice and water shield is more protective against any water that seeped through gaps in shingles. 2/12 pitches with ice and water shield instead of multiple layers of felt perform better.
When you have a relatively short roof deck, like on a dormer or a shed, and you make sure to use ice and water shield underlayment, then it is okay to install asphalt composition shingles down to a slope of 2/12.
If you live in the south Puget Sound of Washington State and need a new roof, please call us at 253.445.8950 or fill out our quick estimate form. All estimates are free and comprehensive.