How to Fix a Sagging Roof Ridge

Roof ridge sag starts small. A visible dip running down the center peak of your roof, maybe an inch or two. It doesn't leak yet. The shingles still shed water. But that sag is the roof telling you the framing underneath has given up. The rafters that meet at the peak are spreading apart under the weight of the roof deck, shingles, and snow load—they're no longer being held tight at the top. Left alone, sag becomes worse. Worse becomes structural failure. The fix depends on what caused it, but the core solution is always the same: you need to tie the rafters back together and support them against the load that's pushing them apart.

1. Read the Ridge's Story. Go into the attic directly above where the sag appears. Look up at the rafters where they meet at the peak. Check for signs of rot, insect damage, or previous repairs. Look at the collar ties—the horizontal beams that connect the rafters on either side of the ridge. If they're missing, cracked, or installed too high up on the rafters, that's your culprit. Mark the problem area with chalk or tape so you know exactly where to work.
2. Map Your Load Path. Measure the distance between the outer walls that support the roof—this is your span. Write it down. Then look at what's currently tying the rafters together. Count existing collar ties and measure their height on the rafter. If collar ties are present but only at every other rafter pair, or if they're installed in the upper third of the roof rather than the lower third, you've found weak points. Sag typically means collar ties are either missing, too few, or installed too high to work effectively.
3. Tie the Rafters Tight. For light sag with intact framing, adding collar ties is the fastest fix. Cut 2x6 or 2x8 lumber to span the distance between the two rafters at collar-tie height—typically 4 to 6 feet down from the peak. Position the tie so it pulls the rafters tight together. Nail it with three 16d nails per side, or bolt it with half-inch lag bolts if the wood is hard or if you're reinforcing an older roof. Install a collar tie on every rafter pair across the entire sagged section, not just at the low point.
4. Rebuild the Damaged Rafter. If the sag is more than 2 inches, rafters show significant rot or damage, or collar ties alone won't work because of the span, install a sister rafter or support beam. A sister is a new 2x6 or 2x8 bolted alongside the damaged rafter to share its load. For very large spans, install a structural ridge beam running the full length of the roof, supported at each end by posts that transfer load down to the foundation. This requires calculating loads and often needs a structural engineer sign-off.
5. Add Vertical Support Posts. If your roof spans from outside wall to outside wall without interior support walls, the attic is essentially a wide-open space. This is a high-risk setup for sag. Install vertical posts from the attic floor (or on a beam at the attic floor) that run up to the ridge or to a horizontal beam installed just below the ridge. Space these posts 4 to 8 feet apart depending on your roof span and load. Brace each post diagonally if the space allows. This creates multiple load paths and shares the weight burden.
6. Hunt for Hidden Rot. Once you've addressed the sag, inspect the rafters and ties for soft wood, discoloration, or signs of prior water entry. Poke suspect areas with an awl—if it sinks easily, that's rot. Small areas can be filled and sealed; extensive rot means that rafter section needs professional replacement. Check the underside of the roof deck as well. If you find evidence of old leaks, address the roof leak before finishing this project, or the new repair will fail quickly.
7. Lock Down the Peak. Once new collar ties or structural elements are in place, nail or bolt blocking between rafters at the peak to prevent racking—the tendency for rafters to shift sideways. Use 2x6 blocks cut to fit snugly between rafters and install them at the peak and at 4-foot intervals down the rafters. This stiffens the entire assembly and prevents the ridge from moving at all. Secure with 16d nails (at least 4 per connection) or lag bolts.
8. Document Your Progress. Take photos of your work before closing up the attic. Mark the installation date and details on the rafters themselves with a permanent marker. Check the ridge from the attic or exterior every six months for the first two years to confirm it's holding. A properly executed ridge repair should show no additional sag; if it does, you'll need to engage a structural engineer to diagnose whether you've encountered a larger load problem or a framing condition you've missed.