Modified bitumen is a type of roofing material commonly used for flat or low-slope roofs, consisting of asphalt combined with modifiers like polymers to improve durability and flexibility. It can be installed as a base or cap sheet and is often layered with underlayments for added protection. Underlayment is the layer of material installed beneath the main roofing surface to serve as a moisture barrier and protect the roof deck from water intrusion. While modified bitumen itself acts as a strong waterproof barrier, the choice of underlayment depends on roof design, climate, and compatibility with the bitumen. Synthetic underlayments, which are lightweight and resistant to tearing, are often preferred beneath modified bitumen because they provide excellent water resistance and help maintain warranty coverage. Felt paper, while traditional, may be less durable under modified bitumen and might compromise the roof’s lifespan or warranty if not properly installed.
To check if your current underlayment is effective, a simple DIY inspection involves looking for signs of moisture or deterioration in the attic or beneath the roof deck, especially after heavy rain. Pay attention to soft spots, mold, or water stains, which could indicate a failing underlayment. Unless you have roofing experience and the proper safety gear, it’s best to avoid removing layers yourself. When hiring a contractor, ask specifically if the underlayment they recommend or install is compatible with modified bitumen and whether it meets local building codes and manufacturer warranties. Also, inquire about their experience with modified bitumen roofs and if they follow the manufacturer’s installation guidelines closely. These questions ensure you get a durable installation that protects your roof investment over time.