Flashing a chimney on a TPO roof involves creating a secure, watertight barrier where the chimney meets the roofing membrane. You'll use two types of flashing: counter flashing and step flashing. Step flashing is installed first, layering pieces of metal at each shingle or membrane step along the chimney base so water flows down and away without seeping behind. Counter flashing is then installed over the step flashing, bent into the chimney masonry joints to cover the upper edge of the step flashing, creating a tight seal. On a TPO roof, the membrane is typically heat-welded around base flashings, so you’ll need a compatible metal flashing material—usually coated aluminum or stainless steel—to avoid corrosion and ensure good adhesion. This system directs water properly and prevents leaks caused by chimney expansion or settling.
To check if your chimney flashing is functioning well, look for signs of water stains or mold in your attic or ceiling near the chimney, or loose and deteriorated flashing materials around its base. A simple DIY check involves inspecting the flashing seams for gaps or cracks and gently running water around the chimney during a hose test to see if leaks form inside. Avoid walking on the chimney or on the flashing since it can be fragile. Calling a professional roofer is a good idea if you're unsure how to weld the TPO membrane or if the chimney brickwork needs repointing before flashing can be installed correctly. When speaking with a contractor, ask how they will integrate the metal flashing with the TPO membrane seam and if their method complies with manufacturer warranties. Also, inquire about their experience handling thermal expansion differences between masonry and flexible roofing to avoid future leaks.