How do you repair water stains from a roof leak?
Short answer. First make sure the roof leak is actually fixed and the area is completely dry, then seal the stain with a stain-blocking primer, oil or shellac based, before repainting, because ordinary paint lets the brown ring bleed back through. If the drywall is soft, sagging or crumbling, or the stain keeps growing, cut out and replace that section instead.
Step 1: confirm the source is fixed
Painting over a stain from an active leak only hides it. Have the roof repaired, then watch the spot through a couple of good rains. Mark the stain's edge lightly with a pencil, and if it grows, water is still getting in.
Step 2: check what is above the stain
- Insulation: wet fiberglass or cellulose loses its value and holds moisture against the drywall, so replace any that got soaked.
- Decking and rafters: dark, soft or delaminating wood above the stain means the leak ran long enough to need wood repair on the roof side.
- Mold: a musty smell or dark spotting on the attic side of the drywall needs cleanup before any cosmetic repair.
Step 3: paint, patch or replace
- Paint it: the drywall is firm and flat, just discolored.
- Patch it: a small soft area, cut back to sound drywall and patched.
- Replace it: the drywall sags between joists, the tape has let go, or the paper face has bubbled.
Step 4: prime and paint the right way
- Let the area dry fully; a moisture meter takes out the guesswork.
- Roll a stain-blocking primer over the stain and a few inches beyond it.
- Repaint the whole ceiling plane, or at least wall to wall, because touch-ups on flat ceiling paint usually show.
Old stains on the roof decking
Dark stains on attic decking are worth noting during any inspection. If the wood is dry and firm, they may be old history; if it is soft, those sheets should be replaced when the roof is worked on. See common roof leak causes and OSB vs plywood roof decking.