What is metal roof decking?
Steel deck on commercial buildings
On warehouses, schools and stores, ribbed steel deck spans between steel joists and carries the roof. Insulation boards and a membrane roof go on top. Its profile and gauge are chosen by an engineer for the span and load, and you rarely find it on houses.
Decking under a residential metal roof
- Solid deck. Plywood or OSB sheathing, the same deck used under shingles, covered with a high temperature underlayment. This is the norm on Georgia homes, and it gives the panels a continuous backup layer against leaks and a quieter roof in heavy rain.
- Purlins. Horizontal wood or steel strips laid across the rafters with panels screwed to them. Common on barns, sheds and some older mountain cabins, but it leaves no continuous layer to catch a leak.
- Over existing shingles. Some metal goes over one layer of shingles, sometimes on purlins. That hides the condition of the deck underneath, so it pays to check the deck first.
What it costs
Steel deck is a commercial bid item priced by gauge and span. For a home, the deck under metal is the same sheathing used under shingles, and normally only damaged sheets are replaced unless the whole deck is unsound. See OSB vs plywood roof decking and metal roofing in Georgia.
What to ask before a metal roof goes on
Ask whether the old roof is coming off, how the deck will be checked, what underlayment goes down, and whether the panel maker's installation guide is being followed. Those answers matter more to how long the roof lasts than the panel brand.