Flashing a chimney on a modified bitumen roof involves carefully integrating both counter flashing and step flashing to create a watertight seal where the chimney meets the roof. Modified bitumen is a durable, rubberized asphalt material usually applied in sheets, requiring precise installation around roof penetrations like chimneys to prevent leaks. Step flashing is metal flashing shaped to fit over each course of roofing material, directing water away from the chimney, while counter flashing overlaps and covers the step flashing, sealing the vertical joint on the chimney itself. The process starts by cleaning the chimney surface and cutting the bitumen roofing to fit snugly around the chimney base. Step flashing is then woven in between the bitumen layers, and counter flashing is installed into grooves or reglets cut into the chimney masonry to overlap the step flashing, ensuring proper water runoff. Simple DIY checks include looking for visible gaps or rusted metal near the chimney, water stains inside the attic, or peeling in the modified bitumen close to the chimney, which might indicate a flashing problem.
If you’re checking your chimney flashing yourself, it’s safest to visually inspect from the ground or gutters with binoculars, or carefully use a sturdy ladder to look for loose or damaged flashing and cracked mortar joints. Avoid walking on the bitumen roof itself, as it can be slippery and might get damaged if not walked on correctly. If you suspect flashing issues or aren’t comfortable spotting potential leaks, it’s best to call a professional roofer experienced with modified bitumen systems and chimney details. When speaking with a contractor, ask how they ensure the step and counter flashing are integrated properly with the modified bitumen membrane, especially how they seal overlaps to maintain the roof warranty. Also, inquire about the materials they use for flashing—will they use corrosion-resistant metals like aluminum or copper, and how do they secure the counter flashing into the chimney masonry without causing damage? These questions help ensure the repair or installation is long-lasting and code-compliant.