A modified bitumen waterproofing layer consists of asphalt derivatives that come in 3′ wide rolls. There is usually a base sheet and a cap (top) sheet that make up the system. The cap sheet comes in different thicknesses. Both the base and cap sheets can be attached to the substrate in different fashions (torched, mechanically fastened or adhered). There are two types of modified bitumen – APP and SBS. The APP type is primarily asphalt. The SBS type is a mixture of rubber and asphalt. The most important difference is that APP is not flexible in normal New England winter temperatures so we do not recommend using this product. The SBS type remains flexible in sub-zero temperatures so is more appropriate for New England. Both the base sheet and the cap sheet are typically twice as thick as single-ply membrane. If a roof is to receive heavy foot traffic or heavy wear then this may be a good choice due to the extra thickness. The downside of the modified roof is that it costs more than single ply, since roll widths are only 3′ wide there are more seams to seal correctly (usually with a torch), and it is more difficult to seal around penetrations with the thicker membrane.