A soft or spongy spot underfoot means the boards under the shingles have rotted or were too thin to start with. A sagging ridge, or a roof that bows between rafters, points deeper, to the framing.
Both need a contractor to look soon. If the roof sags, creaks or cracks under snow, get everyone out of the house first.
What causes soft spots on a roof?
Rotted decking, most of the time. The decking is the plywood, OSB or old boards the shingles are nailed to, and once water gets into it the wood goes soft.
The water comes from two places. A leak at a vent, chimney or worn patch soaks it from above. Damp house air soaks it from below, when it rises into the attic and condenses on the cold boards, then melts and drips with every thaw.
Look in the attic. Black plywood, rusty nails, wet insulation and frost on the nail tips on a cold morning all point to an attic problem, and the cure is in sealing and venting the attic.
Some decks were weak from day one. InspectAPedia, a building inspection reference site, cites 1/4 inch plywood and 3/8 inch sheathing on some 1960s homes, along with fiberboard and other panels that come apart with age.
Is the sag in the deck or the framing?
Check from the attic with a flashlight. A dip between two rafters is the decking. A dip that follows a rafter, or a ridge line that droops in the middle, is the framing.
The worst sign is outside. A sagging ridge with the tops of the walls bowing outward means more serious movement, InspectAPedia says.
| Where you see it | Likely cause | Usual fix |
|---|---|---|
| Soft spot underfoot, no visible dip | Rot in one or two sheets of decking | New sheets during a repair or reroof |
| Dips between rafters across the roof | Decking too thin, or failing panels | New decking over the whole roof |
| Wavy lines along the joints of the deck | Decking that took on moisture, or no gap between panels | A contractor checks the deck and attic |
| Rafter or ridge sagging | Undersized or rotted framing, missing ties | Sistered rafters, new ties or reframing |
| Ridge sagging and walls bowing out | Framing pushing the walls apart | A structural engineer’s design |
The wavy-line pattern comes from ARMA, the asphalt roofing makers’ trade group, which ties shingle buckling along deck joints to wet decking, board decks and poor attic ventilation.
Why does a roof ridge sag?
The rafters push out as well as down. Ceiling joists or rafter ties that run across the house hold the bottoms together, and without them the walls spread and the ridge drops, according to home inspector training material.
InspectAPedia lists the usual causes: rafters smaller than today’s standard, such as 2x4s or 2x6s, insect or rot damage, and ties nailed high near the ridge instead of lower down, a common 1960s habit. Snow, crews and shingle bundles stacked on the roof during a reroof add weight on top.
Old houses are where to look. About 35% of Sioux Falls homes were built before 1980, and in Canton the median home dates from 1960.
Can a second layer of shingles make a roof sag?
It adds weight, though we found no maker’s figure for how much a square of shingles weighs. The published numbers disagree too widely to use.
The code sets the limit. The model code bans a new layer over a roof that is water-soaked or deteriorated, or that already has two or more layers. Confirm how the city applies it with Sioux Falls Building Services at 605-367-8670.
An overlay also hides the deck. Nobody sees the boards, so a soft sheet stays under the new shingles. One insurer’s guide says a roof laid over old shingles may need replacing after about 20 years.
When is a sagging roof an emergency?
When it changes under snow. FEMA’s snow load guide lists the warning signs: sagging plywood or roof framing, popping, cracking or creaking, cracked or split wood, doors or windows that stop closing, new cracks in walls and severe leaks. If you see them, get out and call a structural engineer.
Snow is heavier than it looks. FEMA puts a foot of fresh snow at 3 pounds per square foot when light and dry, and 21 when wet. A foot of ice weighs about 57.
Rot makes it worse. FEMA says wood that splits or rots carries less load, and tells owners to have a contractor or engineer check the framing and a dry attic before the snow season.
The city’s design snow load and how to clear a roof safely are in snow load and ice dams in Sioux Falls.
A soft spot with no sag is less urgent. Stay off it, and get it seen before winter.
What does replacing rotted decking involve?
It happens when the old shingles come off. The crew finds the soft sheets, cuts them out and nails new ones to the rafters before the underlayment goes down.
Nobody knows how many sheets until the tear-off, so the quote should give a price per sheet. We found no code section that says when a sheet must be replaced. The shingle maker’s deck rules and the contractor decide.
The thickness rules and national prices are in what goes under the shingles.
What does fixing sagging rafters involve?
New wood beside the old, new ties, or new framing. InspectAPedia describes sistering a new rafter alongside a weak one and adding ties across the attic, and says rafters that are badly sagged or rotted should not be jacked back up because they can break.
FEMA says structural problems need a professional, and that a homeowner fix can cause more damage. Ask the contractor whether an engineer should design the repair.
The permit rule for structural roof work is unconfirmed. Call Sioux Falls Building Services at 605-367-8670 before work starts.
Insurance pays for some of it. Standard policies cover damage from the weight of snow or ice, and exclude damage from a lack of maintenance, which is how a slow rot reads to an adjuster. Read how a roof claim works in South Dakota.
Questions about soft spots and sagging roofs
Is a soft spot on my roof dangerous?
It means the decking under the shingles is weak or rotted, so stay off it. It needs repair before winter, and becomes urgent if the roof starts to sag.
Why does my roof feel spongy when I walk on it?
The decking has gone soft from a leak above or damp attic air condensing on the boards below. Some older homes also have decking that was too thin from the start.
Can a sagging roof collapse?
Yes, under enough load. FEMA lists sagging framing, cracking or creaking noises and doors that stop closing as warning signs, and says to evacuate and call a structural engineer.
How much does a foot of snow weigh on a roof?
FEMA puts a foot of fresh snow at 3 to 21 pounds per square foot, from light and dry to wet and heavy. A foot of ice weighs about 57.
Can new shingles go over a sagging roof?
Not over a deck that is water-soaked or deteriorated, or a roof that already has two layers, under the model code. Fix the deck or framing first.
Get a quote
Tell us about the job, with where the roof feels soft or sags, and a local roofing contractor will call, text or email to set up a visit. The quote is free.