How to Repair or Replace a Rotted Rafter

Rafters are the diagonal beams that hold up your roof, and when they rot, your entire roof structure is compromised. Water gets in through a leak, sits in the wood grain, and bacteria starts breaking down the cellulose—within a few years you've got spongey, darkened wood that can't carry load anymore. The stakes are real: a failed rafter system leads to sagging ceilings, structural collapse in snow, and water damage that cascades into walls and insulation. Done well, rafter repair means identifying the rot early, fixing the leak that caused it, and either reinforcing the damaged beam or replacing it entirely. This isn't a quick afternoon fix, but it's manageable if you understand the structure and work methodically.

1. Inspect and Document the Damage. Climb into the attic with a flashlight and probe suspected rafters with a flathead screwdriver or awl. Press the tip into the wood—if it sinks more than a quarter-inch without resistance, the wood is compromised. Mark all affected areas with chalk or tape. Take photos and note the extent: is it surface checking, softness along the bottom edge, or deep decay running the length of the beam?
2. Locate and Fix the Water Source. Trace water stains upward to their origin—a roof leak, chimney penetration, or soffit gap. Without fixing the leak first, any repair you make will fail as new moisture sets in. If the leak is minor and accessible, seal it immediately with roofing cement or patch the shingles. If it's a major leak or structural penetration, you may need a roofer before proceeding.
3. Measure and Order Materials. Measure the rafter's dimensions (typically 2×6, 2×8, or 2×10) and its length. For a sister repair, you'll need a pressure-treated or untreated beam of the same dimension, cut to length. If replacing the entire rafter, note the bird's-mouth cut where it sits on the wall plate and the angle cut at the peak. Call your local building department to confirm if permits are required and whether engineered beams are mandated for your region.
4. Install Temporary Support. If the rafter is severely compromised or you're replacing it entirely, install a temporary support post directly below it to carry the load during work. Use a 4×4 post with a beam cap on top, resting on a solid floor or block in the attic. Crank a screw jack under it until it's snug but not forcing. This prevents ceiling sagging and keeps the roof from shifting while you work.
5. Sister the Rafter or Remove and Replace. For minor rot (surface or small section): bolt a new beam alongside the damaged rafter using galvanized bolts spaced every 2 feet, or use bolted plates at 16-inch centers. Align it carefully so both beams sit flush on the wall plate. For severe rot or full replacement: unbolt any collar ties or bracing, carefully pry out the old rafter, and slide the new one in, ensuring the bird's-mouth cut seats fully on the plate and the peak angle is square. Bolt or nail it into place with 16-penny galvanized nails or structural bolts.
6. Secure Connections and Remove Temporary Support. Once the new or sistered rafter is bolted in place, reattach any collar ties, bracing, or secondary beams. Use galvanized fasteners rated for outdoor exposure. Double-check that the rafter sits evenly on the wall plate and that the peak connection is tight and square. Then slowly release the temporary support post, watching for any ceiling movement. If the ceiling doesn't drop, remove the post entirely.
7. Inspect Attic Ventilation and Seal Gaps. Poor attic ventilation was likely a factor in the rot. Check that soffit and gable vents are clear and unobstructed. If insulation is blocking them, pull it back. Seal any gaps around pipes, chimneys, or other penetrations that could admit moisture. Add baffles if needed to keep insulation away from the soffit vents. Good airflow prevents moisture from pooling in the future.
8. Inspect and Document Completion. Walk the attic with the inspector (if required by permit). Point out the repair, the fixed leak, and the ventilation improvements. Take final photos showing the new or sistered rafter in place, bolt locations, and the sealed penetrations. Keep receipts and photos for your home records and future owners.