Why Is My Roof Sagging or Wavy? Causes and When to Worry

A roof that looks wavy usually has a problem in the layer under the shingles: roof decking that has swelled, warped or softened from moisture, or panels that were installed without proper spacing. A roof that sags, with a dip along the ridge or a low spot in the middle of a slope, points to the framing or long-term water damage and should be looked at soon. Neither is a shingle problem, which is why new shingles alone never fix it.

Wavy vs. sagging: two different problems

It helps to separate what you are seeing. A wavy roof has a rippled surface, often with faint lines running across the slope or a pattern that looks like a quilt under the shingles. The roof plane is basically flat, but the surface is uneven. A sagging roof has an actual dip: the ridge line bows down in the middle instead of running straight, or a section of the slope is visibly lower than the rest. Wavy is usually about the deck. Sagging is usually about the structure holding the deck up.

Common causes of a wavy roof

Decking that has swelled from moisture

Most homes built in Livingston County since the 1990s have oriented strand board, or OSB, under the shingles. OSB is good decking when it stays dry. When it gets damp over and over, from attic condensation or slow leaks, the edges swell and do not shrink back. Each swollen seam shows through the shingles as a ridge, which creates the quilted look. Plywood handles moisture better but can delaminate over time.

Panels installed without expansion gaps

Wood decking panels are supposed to be installed with a small gap between them, typically about 1/8 inch, so they can expand in humid weather. Panels butted tight together have nowhere to go when they swell, so they buckle upward along the seams. This shows up as straight lines across the roof, often in a regular pattern.

A layover on top of old shingles

When new shingles are installed over an old layer, every curl and cup in the old shingles telegraphs through. The roof looks lumpy rather than quilted. We explain why we almost always recommend a tear-off in our post on shingling over an old roof.

Attic moisture

In a Michigan winter, warm, humid air that leaks into a poorly ventilated attic condenses on the cold underside of the deck. Spring after spring, that moisture softens the sheathing from below. A wavy roof combined with frost or staining in the attic is a strong sign the ventilation needs correcting along with the deck.

Common causes of a sagging roof

• Long-term leaks that have rotted the decking or the tops of rafters, often around chimneys, valleys and the lower edge of the roof where ice dams form
• Rafters that were undersized or spaced too far apart for the span, which is more common on older homes and additions
• Multiple layers of shingles adding weight the framing was not designed to carry
• Removed or cut collar ties, braces or trusses, sometimes during an earlier remodel or attic conversion
• Heavy, wet snow loads over many winters on a roof that was already marginal
• Settling of the walls or foundation that pulls the roof framing out of line

When a sagging roof is urgent

Most roof waviness develops over years and is not an emergency. Some signs are. Call a professional promptly, and keep people out of the rooms below, if you notice any of the following:

• A sag that has appeared suddenly or gotten noticeably worse after a storm or heavy snow
• Cracks appearing in interior walls or ceilings near the top of the walls
• Doors or windows on the upper floor that suddenly stick
• Creaking or popping sounds from the attic under snow load
• Daylight visible through the roof deck from the attic, or a ceiling that is bowing down

What you can check yourself

From the street, look along the ridge line from the end of the house. It should be straight. Then look across each slope in low morning or evening light, which makes waves and dips easier to see. From the attic, if you can get there safely, look at the underside of the deck for dark staining, soft-looking or delaminated panels, and frost or rusty nail tips in winter. Look at the rafters for cracks, bowing or signs of rot. Do not walk on a roof you suspect is soft.

How it gets fixed

A wavy roof caused by swollen or soft decking is fixed during a roof replacement: the old shingles come off, damaged sheathing is cut out and replaced with properly gapped new panels, and the ventilation is corrected so the new deck stays dry. We photograph any bad decking and show it to you before the new roof goes on. Our post on what is under your shingles explains what we look for.

A true structural sag may need the framing repaired first, which can involve sistering new lumber to damaged rafters, adding supports or restoring removed braces. Depending on the extent, that work may call for a structural engineer or a framing contractor and a building permit. We will tell you plainly when what we are seeing is beyond the roof and needs that kind of evaluation, so the roof goes back on a structure that can carry it.

Sagging and wavy roof questions

Is a wavy roof dangerous?

A slightly wavy roof from swollen deck seams is usually not dangerous, but it means moisture has been at work and the decking should be checked. A sag, especially one that is getting worse, needs attention sooner.

Can new shingles fix a wavy roof?

No. Shingles follow the surface underneath them. Unless the damaged decking is replaced, a new roof will show the same waves.

Is it normal for an old roof to sag a little?

Some older homes have a slight, long-settled dip in the ridge that has not changed in decades. Any sag should still be inspected to confirm it is stable and not caused by rot.

Does a sagging roof mean the whole roof needs replacing?

Not always. Sometimes the problem is localized to one area of decking or one damaged rafter. An inspection from the roof and the attic shows how far it goes.

Get an honest answer on what is under your shingles

We inspect from the roof and from the attic, photograph what we find and give you a written answer on whether the issue is the deck, the framing or the ventilation. The inspection is free. On Top Roofing is an Owens Corning Platinum Preferred Contractor serving Howell, Brighton, Hartland and Greater Southeast Michigan, and our inspectors are HAAG Certified. Call (810) 588-3933 for your free estimate.