Why is my lean-to roof leaking?
Short answer. Most lean-to roofs leak at the joint where they meet the house wall, which needs apron flashing tucked up behind the siding or set into the brick, and many are built with too little slope for the roofing on them. Other common causes are exposed screws with worn washers on metal panels, short end laps, and shingles installed below their minimum pitch.
The wall joint
A lean-to slopes away from a taller wall, so all the water running down that wall lands at the top of the lean-to roof. That joint needs a continuous apron flashing that runs up the wall behind the siding or house wrap, with the siding lapping over it, or counterflashing cut into the mortar on a brick wall. A metal strip screwed to the face of the siding with caulk along its top is the most common failure.
Too little slope
- Shingles: not allowed below 2 in 12, and between 2 in 12 and 4 in 12 they need a double layer of underlayment. Many porches and sheds are shingled flatter than that.
- Exposed-fastener metal panels: the code calls for at least 3 in 12 when lapped seams have no sealant, and lower slopes only with sealed laps.
- Rolled roofing: works at lower slopes than shingles but has a short life in Georgia sun.
- Near-flat roofs: a low-slope membrane is usually the right material.
Other causes
- Rubber washers on screws that have dried and cracked, or screws driven crooked or too tight.
- Panel end laps too short, letting water wick uphill on a shallow roof.
- Polycarbonate panels with failed seals or cracked sheets.
- A gutter that overflows back toward the wall, or no gutter at all.
- Sagging framing that lets water pond.
Fixing it for good
Check the slope first with a level and a tape measure. If the material suits the slope, reflash the wall joint properly and replace worn fasteners. If it does not, the lasting fix is a roofing material rated for that pitch. See roof flashing explained and metal roofing in Georgia.