Never go below 4" Rise Per Foot for roofing Shingles. Have we all not seen enough damage from poor specifications? No matter what manufacturer specifications say, just say "No."
Bob Wewer - Damage from low slope shingle installation
It seems that the old timers had a lot more on the ball than the modern construction gurus of today. It has been for a long time in this era now that low sloped roofs have been cladded with asphalt shingles. In the designation of "low slope," here I am referring to the subject like an old timer from centuries prior. It used to be that in the old days (like the 1800's to early 1900's and before asphalt shingles) when primary natural cladding was used on steep slopes. These steep slopes were no lower than 8" rise per foot. This dividing line caused the use of metal roofing on lower porch roofs, etc.
There were no low sloped shingle applications. There was no "ice and water" underlayment either. There was only the primary water-shedding or waterproofing material and the construction methods were superior to today in regard to precipitation protection. The rooftops lasted longer and functioned better.
Even with the current array of lower sloped roofs and specifications by manufacturers that have blurred the lines between steep and low slopes, we can still do the job right and without nonsensical methodology. The link above (at the top of the page) leads to a page that demonstrates the problem within our industry of acceptance of faulty specifications. An absolute line has to be drawn at four inches rise per foot as the limit for roofing shingle use. This is far lower than what the old timers would have accepted and we should realize that there is a definite line that should not be crossed when specifying a product that relies solely upon lap and slope to shed water.
by Bob Wewer