There are two types of modified roof systems to choose from which are SBS & APP. Modifiers are added to the rolls to provide more flexibility and give the rolls more stability. They are most commonly installed with two or three plies which involve the base sheet and intermediate sheet with a granulated cap sheet. SBS rolls can be applied using hot & cold asphalt and can also be torch applied. APP rolls cannot be applied using hot asphalt but can be applied using cold asphalt adhesive and can also be torch applied. Some of the new modified bitumen systems on the market today now involve self adhering sheets. The self adhering rolls come with a removable factory applied plastic that is attached to the underside of the sheet that needs to be removed before setting the roll in place.