1x4 is fine on shallow pitched roofs, anything above a 6/12 with steel I prefer the 2x’s. No steel manufacturer will warranty their product if applied directly on asphalt shingles…. At least not here, it is clearly stated in all of their literature. Do what you want, don’t try and get an insurance to help out.

I ain't buying it. Taylor metal product, a major player does.

Taylor Metal Products StreamLineTM Standing Seam
roofing products can be installed for either new or
re-roofing applications.
We recommend installing the StreamLine™
Standing Seam over a rigid continuous substrate
such as plywood sheathing or decking. We
recommend that the plywood be 15/32" or thicker
or 22 ga metal deck. For best results the substrate
should be smooth, flat and free of debris.
Cover the entire roof area with 30 lb. ASTM rated
felt paper. Apply the felt by rolling it out horizontally
across the roof starting at the eave. Allow a 3"
overlap for each course.
Lap end joints 6". Maintain the rule of keeping uphill
courses lapped on top of downhill courses of felt.
Tears and cuts should be replaced with new felt or
repaired with roofing lap cement.
To prevent bonding between the copper and roofing
felt, a layer of smooth building paper or a rosin
sized slip sheet should be laid over the felt before
installing the copper roofing.
Ventilation
Proper ventilation is necessary for full roof life.
Check local codes for venting requirements.
To provide for ridge and/or hip ventilation, remove
(for retrofit) or leave out (for new construction) 2" of
sheathing on both sides of the ridge center. Cover
opening with flyscreen and secure the flyscreen
with staples. Apply felt paper up to the edge of
opening as explained in “Underlayment" section.
Taylor Metal Products recommends allowing
ventilation between the outer roof deck and the
insulation. Lack of ventilation may trap moisture.
The rib of the panel is not a source of ventilation for
the area beneath the roofing panels.
Re-Roofing
The StreamLineTM Standing Seam can be installed
with felt over most existing asphalt, composition,
fiberglass shingles or rolled roofing. Tile, gravel,
wood shingles/shake,metal or any other type of
roofing material should be removed to the bare
sheathing. Inspect the substrate for damage or
rot and replace sheathing as necessary. Apply the
underlayment as described above.
Consider the following when installing the metal
roofing over existing roofing materials:
• Building Codes: Local building codes will typically limit
the number of layers of roofing allowed. Check with
your agency.
• Solid Fastening: Check the condition of the substrate.
Damaged or rotted plywood or decking will not provide
for secure fastening. Repair or replace damaged or
rotted substrates.
• Appearance: Irregular substrates may affect the
overall appearance of the metal roofing product. Panel
deformation may occur, however, product integrity will
not be affected.
• Roof surface: Any warped or loose shingles must be
nailed down along with any protruding nail heads.
Remove all moss and other debris, including existing
starter strips. Cut off all overhanging shingles and
remove hip and ridge caps.
• Ventilation: Trapped moisture can cause premature
failure of the metal roofing product as well as
substrate, insulation etc. Provide adequate ventilation
and appropriate moisture protection.
Insulation
Check with your design professional or insulation
consultant for applications or design details. Also
check local building codes to ensure compliance.
Touch-Up Paint
Most of the time touch-up paint is supplied in spray
cans. Spray cans are useful for painting large areas
such as downspouts, pipe flashings, and other pre-
existing areas.
Scratches and scuffs in the finish should be
touched-up but not sprayed. The paint should
be well mixed and sprayed into a small container,
then applied to scratches with a very fine brush
or toothpick,just filling in the scratch. If the area
is sprayed over, the differences in the chemical
makeup will likely cause the touch-up paint to fade
differently than the baked-on finish and cause a
blotchy appearance over time.
Paint paint pens are offered in most Kynar® colors
and work best for covering minor scratches, please
inquire for color options. Paint pens not available
for ArmorTech™ panels.

Wow!!! Some interesting details on this application. I have some research to do. Seems it would be a cleaner and better finished product with tear off. I will consider all options and ma go back to shingles. Thanks

Will project management for beer....
(And a drive around)

You could but 16" & 24" centers don't usually hold out for snow guard spacing.

At least one manufacturer I use (SnoBlox/SnoJax) offers a glue that I have used in the past with no call-backs so far.

The glue reaches 1,500-2,000 psi tensile strength after a 28 day curing time. That's about 2/3 the strength of a mechanically fastened snow guard.

What is the species and grade stamp
of these 1x4s?
Over here Hem Fir are the best I know.

Like Lug said, matching the layout of
metal vs rafters vs snow guards is
a major brain cell tax.

If the tear off is too much work, I'd say good bye.

And, if the homeowner insists on "running the job",
an experienced contractor will tell him good bye.

DECRA

Eastern States Lynx is popular here.
And they do answer the phone.

I had my house stripped to the 1" x 6 " sheathing before installing metal. I did not want the two layers of asphalt shingles being a heat sink under the new metal roof.