A.Carl Hagstrom responds: The evenly spaced
ridges of the roofing profile present a real challenge. When
visualizing a flashing strategy for the down-slope face of the
chimney, picture the plane of the roof at the top of the
ridges, not at the lower, flat portion of the roofing profile.
At the sides of the chimney, extend the flashing out past the
nearest formed ridge, fold it over the ridge, and then fasten
through the flashing at the high point of the ridge with
neoprene gasketed roof screws.
This goes against the roofing manufacturers’
recommendations to place screws in the flats of the roofing,
but I’ve used this approach with success.
Fill the areas where the base flashing spans the flat
portion of the roofing profile with the same closure strips
that are used to seal off the metal ridge caps. These are
adhesive-backed foam strips that conform to the profile of the
metal roofing (see illustration, below).
When the chimney penetrates the roof below the ridge, use a
cricket on the up-slope side to divert water around the
chimney. The roofing, the cricket valley, and the chimney base
flashing all come together at the chimney corners and must
overlap in the proper order. First, fasten the lower sheet of
roofing in place alongside the chimney, then install the base
flashings at the bottom and sides of the chimney. As the side
base flashing approaches the valley flashing, it will have to
be shaped to lie in the lower, "trough" section of the roofing
so that the second sheet of roofing will lie flat. This results
in an oddly shaped piece of base flashing, which I form using
painted aluminum coil stock and a fuss-’n’-fit
approach. It’s not the most elegant piece of flashing,
but it’s leak-free and is suitable for the types of
buildings where I install metal roofs.
Next, install the cricket valley flashing so that it
overlaps the side base flashing. Finally, lay the upper sheet
of roofing over the valley flashing.
The roofing manufacturer can provide steel flat sheets that
will match the color of the roof. However, these will most
likely be tempered steel (like the roofing), and can be
difficult to form, even when using a brake. On occasion,
I’ve used painted aluminum coil stock for the flashing
material. It’s much easier to form and can be painted to
match the roofing steel.
Carl Hagstrom is an assistant editor at the Journal
of Light Construction.