What types of decking go under a metal roof?

Solid decking

Plywood or OSB sheathing, the same deck used under shingles, is the standard for homes. It supports the underlayment fully, gives clips and screws solid wood to grip, keeps rain noise down, and helps prevent condensation drips. Most standing seam systems and nearly all metal shingle and stone-coated steel products call for it. Thicker panels hold fasteners better and make a flatter roof, which matters because metal shows waves in the deck.

Purlins or battens

Rows of 2x4 or 1x4 lumber run across the rafters or trusses at the spacing the panel maker specifies, often 24 inches. Exposed fastener panels are screwed into the purlins. This is common on barns, carports, porches and over existing shingles. Without a solid deck and underlayment, though, purlin roofs are louder and can drip condensation in humid weather, so they need closures, ventilation and sometimes a membrane.

Older board decking

Many older Georgia homes have 1x board decking with small gaps. It can work under metal if it is sound, but rotted or widely spaced boards should be replaced or overlaid with sheathing first.

Insulated nailbase panels

For vaulted ceilings with no attic, insulated panels with a wood top layer give the roof both insulation and a nailable deck.

Steel roof deck

If you came across Type B, N or form deck, those are corrugated structural steel decks used on commercial buildings under membrane roofs. They are not used under residential metal roofing.

See OSB vs plywood roof decking and metal roofing in Georgia.