What Causes a Sagging Roofline?

A sagging roofline is caused by something under the shingles losing its ability to carry weight, most often rotted roof decking, a bowed or undersized rafter, a cracked truss, or a bearing wall that has shifted. It is a structural symptom, not a cosmetic one. Dodd Roofing and Exteriors inspects sagging rooflines across Boise, Meridian, Nampa and the rest of the Treasure Valley, and the right repair depends entirely on which layer gave way.

Is the deck sagging or the frame?

Stand back from the street and sight down the ridge. If the ridge itself dips in the middle, the framing is involved. If the ridge stays straight but the surface between rafters looks wavy or scalloped, the decking is the problem. Deck sag is the smaller repair. Frame sag has to be corrected before any new roof goes on. Both show up clearly during a roof inspection.

What makes roof decking dip?

Moisture, almost every time. A slow leak at a chimney, valley or pipe boot soaks the sheathing until it loses stiffness. Poor attic ventilation does the same thing more quietly by holding warm humid air against the underside of the deck all winter. Older Treasure Valley homes sometimes have thin or spaced sheathing that was never sized for modern shingle weight. Our guide to attic ventilation requirements in Idaho covers that side of it.

What makes the framing sag?

Age and load. Rafters cut too small for their span will bow after a few decades. Truss webs crack and connector plates back out of the wood. Homes that had a wall removed for an open floor plan sometimes lost a bearing point the roof was counting on. Added weight matters too, including a second layer of shingles over framing sized for one.

How urgent is it?

Treat it as urgent if the dip appeared recently, grew over one season, or shows up alongside cracked ceiling drywall, doors that suddenly stick, or daylight visible in the attic. Those point to active movement. A shallow, long standing wave in old decking is less alarming but still needs a look before winter snow and spring wind load it further.

Can it be fixed without a full replacement?

Often yes. Rotted sheathing is cut out and replaced sheet by sheet. Sistering a rafter or adding a support post is a targeted structural fix. What we will not do is roof over a sag, because new shingles telegraph every dip and the rot keeps spreading underneath. If the roof is already near the end of its life, correcting the structure during a full roof replacement is the cleaner path. Get someone on the roof and in the attic, not just at the curb. Call Dodd Roofing and Exteriors at (208) 295-9421 for a free look in Boise, Meridian, Nampa, Caldwell, Eagle, Star, Kuna, Garden City or Middleton.

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