We have installed metal roofs on residential homes for many years now. This has been my main source of income over the last few years. I have learned some important techniques for making your roof system more efficient. 2×4 stripping each two foot sees to be the most profitable.
We have laid the metal straight down on the singles, laid it on tar paper, and laid it on 1×4's, and 2×4's etc. There are some important benefits to 2×4 stripping that you might what to know. For one, the spacing between the metal and the original roofing material acts as an insulation space (like storm windows) which does not allow the heat from the sun to penetrate your attic space. This allows easier cooling during summer months.
On most raised rib panels (not standing seam panels) there is open raised rib space every nine inches or so for the hot air to rise from all the way down at your eve to all the way up to your ridge and out the ridge vent made by the metal ridge cap. A vented roof system.
One important fact here is not to over fill the space created by the 2×4's with insulation so blocking the air flow to from your eve to your ridge vent.
* We actually did our own myth buster test on this theory. We built 2 identical boxes of wood with pitched roofs, completely sealed up from the outside. Both roofs were covered with singles.
On one roof we laid the metal roof straight down on the singles.
The other roof 2×4 stripping was added, then the metal roof was laid on top of the 2×4's.
A digital thermometer was installed in both boxes. On the very first test during a 85 degree sunny day the box without the 2×4 stripping rose to 112 degrees, while the box with the 2×4 stripping maxed out at about 95 degrees. That's about a 17 degree difference in the two attic spaces,
That's plenty of justification for spending the extra dollars to strip your new roof with 2×4's.