Twin wall flue system design, measuring the roof penetration height

The roof penetration height is the height from the hearth which the stove sits on to where the flue penetrates the roof outside. The point to measure on the roof is where the imaginary centre-line of the flue pipe would touch the outside (weatherward) surface of the roofing material.

If the stove is directly below the ridge of your roof then please let us know in the notes section of your application. Whilst it is possible to get a custom-made ridge flashing it is much easier to use a couple of bends to create an offset inside the roof space so as to miss the ridge. If you really do want the flue pipe coming on at the ridge of the roof then please say so in your notes.