Axiom, thank you for your suggestions again. I mentioned earlier that I am in New Jersey (inland area) and we get a lot of rain plus snow during winter. Wind resistance rating on 3 tab shingle is a concern for me. I don’t really understand your suggestion on shingle nailing but wouldn’t 4 nails per shingle make it even more vulnerable to wind?
I have a feeling that roofing companies don’t like it when the customer start asking them to mix third party products. Probably that’s because most manufacturer warranty require them use products from the same vendor. It is kind of difficult when the roofer doing your job don’t buy into the idea and think you are making a mistake. Only 1 roofer (out of the 4 I talked to) has a similar idea and I can’t use him because the guy wants to redo my entire roof ventilation system as the same time.
You don’t say what area of the country you are from.
Snow & ice will tend to cause a low slope roof to leak sooner/more than just rain.
The simple fact is that most people prefer the look of architectural shingles to that of most low slope roofing.
While it’s not a good idea to install shingles on 3/12’s and lower it can be done.
Use Grace ice & water shield on the lower 1/2 to 2/3’s of the roof and 2 layers of 15# on the remainder.
There are a lot of different products out there that call themselves ice & water shield, only a small elite handful are equal to Grace, it’s the most expensive because it’s the best and necessary for this application.
IMO 2 layers of 15# is better than 1 layer of synthetic or 30# felt because the 2 layers makes it somewhat redundant.
Whether or not this makes a difference I can’t say definitively.
When installing the shingles use hot dipped galvanized nails or better, better would be stainless or copper.
When using a product like Grace the weak link is the nails, the nails rust out and the roof leaks through the nail holes.
Standard electro-galvanised nails rust out pretty quickly and this is the source of a large percentage of leaks I repair.
When laying the shingles use a wider step than usual, 9" - 10" and only use 4 nails per shingle.
If you follow these steps you should get the full lifetime out of your roof on the low slope areas.
All that being said you should really used a low slope material on 2/12’s even though the above procedure will work more often than not.
I prefer torch applied APP modified Bitumen for this, it’s available in different colors and is very durable,
Choose an old roofer for this, I doubt many of the newer ones truly know how to do torch applied mod bit.[/quote]