How do you put a metal roof on a lean-to?
Short answer. Give the lean-to enough slope for the panel you choose, usually at least 3 in 12 for exposed fastener panels without lap sealant, set purlins across the rafters about 24 inches apart, and run one-piece panels from the high side to the eave, screwed through into the purlins. Where the lean-to meets a wall, a headwall flashing has to tuck behind the siding to keep water out.
Plan the structure
- Slope: the IRC sets 3 in 12 as the minimum for lapped metal panels without lap sealant. Lower slopes need sealant at the laps or a different panel, so check the manufacturer's rating.
- Purlins: 2x4s across the rafters at the spacing the panel maker specifies, commonly 24 inches. For a lean-to over living space or a finished porch, use solid decking and underlayment instead.
- Overhang: let panels extend past the eave purlin a couple of inches so water clears the fascia or drops into a gutter.
Installing the panels
- Install eave trim and closure strips if the panel profile calls for them.
- Start at the end away from the prevailing wind so side laps face away from wind-driven rain.
- Square the first panel to the eave. Every panel after it follows that line.
- Drive screws with rubber washers into the purlins in the pattern the manufacturer shows, snug enough to compress the washer slightly, not crush it.
- Cut panels with snips or a nibbler, not an abrasive blade, and sweep up metal shavings so they do not rust stain the finish.
The wall connection
The leak point is where the lean-to roof meets the house. A headwall or transition flashing should run up the wall behind the siding and house wrap, then lap over the top of the panels with closures underneath. Caulk alone at this joint will fail.
Permits
An attached lean-to is an addition to the house, and most Georgia building departments require a permit. Detached sheds may be exempt below a size threshold that varies by county.
See metal roofing in Georgia, roof flashing explained and roofing permits in Georgia.