The amount of layers that you can put on your roof will vary depending on your location because it is determined by your areas building codes. Many building codes allow for two layers of shingles to be on a roof that has a pitch of less than or equal to 4/12. Roofs with a steeper pitch than 4/12 can sometimes be allowed to have three layers of shingles. Roofs that have more layers on them are generally less safe because if the structure of your building can't handle the weight of your roof it is very unsafe for the building, and anyone who occupies it. Therefore be very careful with multiple layers of roofing.