Sagging Roofline: Causes and Fixes for Des Moines Homes
Stand across the street from a 1920s house in Highland Park or a 1950s ranch on the south side and sight along the ridge. A lot of them dip in the middle, and a lot of homeowners have been told it's "just an old house." Sometimes that's true. Sometimes it's a roof carrying two layers of shingles on rafters that were undersized the day they went in, and the next 12-inch snowfall is a problem. Here's how to tell the difference.
What a sag tells you by its shape
A dip along the ridge, with the slopes still flat, is usually the ridge board or the rafters spreading at the walls. Common on pre-war houses where the ceiling joists that tie the walls together were cut for an attic stair or a bathroom remodel.
A dish in the middle of one slope is decking or rafters failing between supports. Often water damage under a valley or around a chimney that rotted the deck, or a rafter that was notched for a vent.
A wave across a slope, with a regular pattern, is decking sagging between rafters. Typical on 1960s–70s ranches in Urbandale and the east side sheeted with thin plywood or particleboard, or on any roof where the shingles have been laid over a second layer.
A sag at the eave only is the rafter tails, the fascia, or the overhang rotted from years of gutter overflow and ice dams.
The causes we see most
Undersized rafters. Houses built in Des Moines before the 1950s were often framed with 2x4 or 2x6 rafters on wide spacing, sized by the carpenter's judgment rather than a code table. They carried a cedar shake roof fine. They've since been given two or three generations of asphalt, which is heavier, plus 35 inches of snow a year.
Two layers of shingles. A second layer adds roughly 250 pounds per square. On a 20-square roof that's two and a half tons the framing didn't sign up for. Iowa's code allows two layers, but on old framing that allowance is the problem, not the solution.
Cut or removed framing. Collar ties and ceiling joists removed to open an attic or vault a ceiling. Rafters cut for a skylight or a fan without headers. Every one of those changes the load path.
Wet decking. A leak that ran for years under a valley or beside a chimney turns plywood to cardboard. The deck sags between rafters, then the rafters start to follow.
Snow and ice load. A wet March snow on top of an ice dam is the heaviest thing the roof carries all year. Old framing that's marginal shows it first, and sometimes fails.
Foundation or wall movement. Less common, but on a house where the walls have spread, the roof follows.
Is it urgent?
Go in the attic with a flashlight and look for:
- Rafters that are cracked, split along the grain, or bowed
- Ridge board pulling away from the rafters
- Daylight or water stains at the sag
- Ceiling joists that have been cut and not headed off
- Purlins or braces that have been removed, or are sitting on nothing
- New cracks in the plaster or drywall ceilings below, or doors upstairs that have started to stick
Any of those, plus a sag you can see from the street, is a call to make this week, and before the snow. A gentle dip that hasn't changed in years, with sound framing in the attic, is a note for the next roof replacement.
What the fixes cost
Typical Des Moines ranges, not quotes:
| Fix | What it involves | Cost |
|---|---|---|
| Replace sagging decking | Tear off the shingles over the area, replace plywood, re-roof that section | $90–$150 per sheet plus $500–$1,500 for the roofing over it |
| Sister rafters | Bolt a new rafter alongside each weak one, jack the sag out first where possible | $150–$400 per rafter |
| Restore collar ties or ceiling joists | Reinstall the ties that were cut, sometimes with steel straps | $500–$2,500 |
| Add a purlin and braces | A beam under the rafters mid-span, braced to a bearing wall | $1,500–$5,000 |
| Rebuild rafter tails and fascia | Cut back rot at the eave, scab new tails, new fascia and drip edge | $800–$2,500 per side |
| Full re-framing of a slope | Rare; when rafters are too far gone to sister | $8,000 and up, engineer required |
Most of the framing work happens from the attic and doesn't need a new roof. If the roof is due anyway — and on a house with two layers and a sag, it is — combining the framing work with the replacement saves money because the deck is open.
When to get an engineer
For a purlin, a ridge repair, or anything involving a vaulted ceiling, a structural engineer's letter is worth the few hundred dollars. It tells us what size to build, and it gives you a document for the next buyer's inspector. Des Moines and most suburbs will want a permit for structural work on the roof; our guide on roofing permits covers when that applies.
Two layers and a dip: our advice
If you own a pre-war Des Moines house with two layers of shingles and a ridge that dips, don't add a third layer — the code won't allow it and the rafters won't either. Tear off to the deck, fix the framing from the attic while the roof is open, sheet over the planks, and put one light layer of architectural shingles on top. That roof will outlast the framing question for good. A roof inspection that includes the attic will tell you which situation you're in.
Need roof work in Des Moines?
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