Asphalt composition shingles are designed to be installed on roof slopes greater than a 4 in 12 pitch. Therefore the asphalt manufactures base their warranties around the minimum recommended pitch of 4 in 12. For those who don’t understand the roof pitch number here is a way to envision it. For every 12 inches you travel on your ceiling the roof will rise 4 inches in height. Your standard 4 in 12 roof pitch works out to an incline of 18.43 degrees for those of you into geometry.
The asphalt shingle manufactures recommend that you install asphalt shingles on a recommended minimum of 4 in 12 or greater however most manufactures will warrant their shingles down to a 2 in 12 pitch if the recommended installation method is used. Installation on roof slopes below a 2 in 12 pitch is not recommended at all. What is the recommended installation requirements for the installation of a shingle below a 4 in 12 pitch? Well every manufacture will vary just a bit but all require the same underlayment procedure and that’s what we are going to discuss.
What is the correct underlayment for the application method for roofs below the 4 in 12 pitch? You have really two choices. The first recommended choice is installing 2 layers of 15 pound asphalt saturated roofing felt with the lap seams offset from one another and all lap seams embedded in an asphalt cement. The other recommended choice is installing an ice & water membrane throughout the entire roof.
The two plies of 15 pound asphalt saturated roofing felt has been the recommended application before the invention of the ice & water membrane. However the two ply roofing felt procedure is rarely installed correctly. Most roofing contractors ignore the application of the asphalt cement at all lap seams in order to keep the cost down. It would be labor intensive to follow the correct application of the 15 pound felt. Installing the ice & water membrane is more expensive if the 15 pound felt is installed without the lap cement. A contract who bids the 15 pound felt method in his bid is trying to win the bid by being cheaper than the next contractor but in reality he is not looking out for his customer.
Ice & water membrane is a self adhering membrane that is made with a modified asphalt. The modified asphalt keeps it very pliable during the cold months of the year and does not become brittle. Because of the modified asphalt any penetration, such as nails, will be sealed from water being able to bypass the underlayment. Since the membrane is self adhering there is no need to apply any lap cement to the membrane. Ice & water membrane can cost more than two times the amount of just 15 pound felt. However if your roofing contractor installed the 15 pound felt as required by the asphalt shingle manufactures then the labor cost alone would drive the cost up to more than the ice & water membrane. In my opinion the benefits of installing the ice & water membrane out weigh the alternative. As a contractor I want to provide my customers with a cost effective solution without sacrificing the integrity of the roofing system.
When you have to re-roof that home that has a 3 in 12 roof pitch and you want asphalt shingles installed, be sure that the contractor you hire installs the ice & water membrane. If he recommends the two ply’s of 15 pound felt be sure that he will follow the required procedure of the shingle manufacture. If he does follow that procedure then you might be paying more than you should.