Shingles and Roof Slopes Less Than 4/12
I have never felt asphalt composition shingles to be an adequate design solution for any building, much less a post frame one. Some potential post frame building owners look to shingled roofs in an errant idea of added protection against hail (not so, read more here: https://www.hansenpolebuildings.com/2011/09/steel-roofing/).
Asphalt composition shingles are designed and recommended by their manufacturers to be installed on roof slopes 4 in 12 pitch or greater. Therefore asphalt shingle manufacturers base their warranties around this minimum recommended pitch of 4 in 12. For those who don’t understand roof pitch numbers read a more thorough explanation: https://www.hansenpolebuildings.com/2018/09/roof-slope/.
Most manufactures will warrant their shingles down to a 2 in 12 pitch, if following their installation method. Never recommended would be installation on roof slopes below a 2 in 12 pitch.
While every manufacturer will vary just a bit, but all require same underlayment procedure for roofs below 4 in 12 pitch. You have really two choices. First recommended choice would be installing two layers of 15 pound asphalt saturated roofing felt with lap seams offset from one another and all lap seams embedded in asphalt cement. A second choice – install an ice and water membrane throughout entire roof beneath roofing.
Two plies of 15 pound asphalt saturated roofing felt had been recommended application before invention of ice and water membrane. However two ply roofing felt installation procedure are rarely correctly followed. Usually neglected, in order to keep installation time and costs down, would be asphalt cement application of all lap seams. It would be labor intensive to follow a correct application of two layers of 15 pound felt.
Installing ice and water membrane will be more expensive than two layers of 15 pound felt installed without lap cement. In fact, ice and water membrane can cost more than twice of the two layers of 15 pound felt. Made from modified asphalt, ice and water membrane self-adheres, keeps very pliable during cold months and does not become brittle. Because of modified asphalt being used, any penetration, such as nails, will be sealed from water being able to bypass underlayment. Since ice and water membrane self-adheres which eliminates any need to apply lap cement.
In my humble opinion benefits of installing ice and water membrane outweigh alternatives. I want to provide my clients with a cost effective solution without sacrificing roofing system integrity.