*PLUMBERSSTOCK EXPERT* responded:
Hi there! Thank you for your interest in our Metal-Fab 12MF 12" B-Vent Standard Flashing. According to Metal-Fab, the "plate" dimension of the flashing is 24" x 24" and the opening in the plate is 16.63" in diameter. The opening at the top of the flashing cone is approximately 1/8" to 1/4" larger than the outside dimension of the vent pipe. The outside dimension of the 12MT is 13 inches so the opening at the top of the cone will be slightly larger than 13 inches. For reference, please check pages 13 and 20 of this Brochure. We hope this helps. If you want to order this product, just click "Add to Cart" and put your postal code and you're ready to go. If we can be of any further help, you may call our customer support team at 435-868-4020 between 8:00 AM and 7:00 PM MT or e-mail them @ email@example.com. Have a great day!