Sagging Roof, Soft Spots and Rotted Decking: What Re-Decking Costs

A soft spot you can feel through your shoes, or a dip between rafters you can see from the ground, almost always means rotted decking under the shingles, not a problem with the shingles themselves. We replace decking on 38 percent of the roofs we tear off, priced per sheet in writing before anyone starts. A roof sagging evenly along the ridge is a different problem: the framing itself, which calls for a licensed builder, not just new plywood.

I am Aaron Scott, licensed Michigan residential builder and owner of Roofing Army. A Roofing Army project manager is on site for every job, and checking what is actually under the shingles, not guessing at it, is part of why. Soft spots and sags both get called "the roof is sagging" by homeowners on the phone with us, but they are two different problems with two different price tags, and telling them apart before a crew ever gets on the roof saves everyone a surprise.

Is a Soft Spot the Decking or the Framing?

A localized soft spot, one you can find by walking the roof or pressing a broom handle up into the attic ceiling, is almost always a single sheet of rotted OSB or plywood, or a failed section of old plank board, sitting under one area of shingles. It develops where water has been getting into that one spot for a while, usually from a slow flashing leak, a failed pipe boot, or an ice dam that sat over the same few feet of eave for more than one winter. The fix is straightforward: cut out the rotted sheet, nail in new decking, and replace the shingles over it. That is roofing work, and it is the kind of repair we price by the sheet every week.

A roof that sags in a straight line along the ridge, or where the roofline visibly dips when you stand back and look down it from the street, is a different animal. That pattern means the rafters themselves have crept past what they can safely carry, not just the thin decking nailed across the top of them. Construction trade sources like JLC and Fine Homebuilding are consistent on the fix: once a rafter has permanently bowed, the standard repair is to sister a new rafter alongside the old one, or in severe cases replace it outright, not to relay decking over the sag and call it done. That is carpentry and structural work, which is exactly why having a licensed residential builder look at it, rather than a crew that only does roofing, matters. On a Roofing Army inspection, if what we find looks like a framing problem rather than a decking problem, that gets flagged before a shingle is ordered, not discovered halfway through a tear-off.

How Much Does It Cost to Replace Rotted Roof Decking?

We price decking the same way on every quote: a fixed dollar amount per sheet of new OSB or plywood, called a wood allowance, agreed to in writing before the crew starts, not negotiated on your driveway after the old shingles are already in the dumpster. We cover exactly how that allowance works, and what the rest of a real quote should include, in our guide on what is included in a roof replacement quote, so this post will not repeat that math. What is useful here is seeing the real range in practice, because "some decking replacement" can mean very different jobs.

The Madison Heights job is a light touch: one localized plank repair and a modest run of new OSB on an otherwise sound deck, the kind of thing a slow flashing leak leaves behind. The Rochester Hills job is the other end of the scale, a full deck rebuild where nearly the entire roof needed new sheathing before a shingle could go down. On the quotes we write across Southeast Michigan, decking replacement lands somewhere between those two most of the time: when sheets are needed, the median is 48 sheets, with the middle half of jobs falling between 24 and 80. That is a wide enough range that the per-sheet price, not a flat number, is what actually protects you.

Plank, or board, decking, the 1-inch boards used before plywood and OSB became standard, is a smaller slice of what we see: it gets repaired on about 4 percent of our tear-offs, almost always on homes built before plywood decking was common. We see it regularly in specific pockets of our service area, like the pre-1960s homes in Clawson, the early-1900s blocks around our Old Village office in Plymouth, and the 1920s section of Livonia's Rosedale Gardens. On those roofs, the question is usually not whether to replace a sheet of OSB but whether the existing boards are solid enough to nail new sheathing over, or need to come out entirely.

What Causes Roof Decking to Rot in the First Place?

Almost every rotted deck we find traces back to water sitting against wood longer than it should. The most common paths are a flashing leak around a chimney, skylight, or wall that has been dripping onto the same spot for longer than anyone noticed; an ice dam that backs meltwater up under the shingles every winter, which we cover in detail in our ice dam guide; and poor attic ventilation, which traps warm, moist air against the underside of the deck year-round and rots it from below instead of above, a problem we break down in our guide to ridge vents versus box vents. A bathroom fan venting straight into the attic instead of outside, which we see constantly, does the same thing faster. None of these show up from the ground. They show up once the old shingles come off, which is exactly why a per-sheet price in writing matters more than a single flat number.

Does Michigan Code Require Solid Decking Under Shingles?

Yes. The Michigan Residential Code requires asphalt shingles to be fastened to solidly sheathed decks, which is why gapped skip sheathing or missing boards cannot simply be shingled over during a legitimate tear-off. On the century-home plank decking described above, that usually means adding new structural panel sheathing over sound boards rather than nailing directly into gaps, or replacing sections that have failed outright. If decking damage is extensive enough, it can also intersect with Michigan's 25 percent rule for when a repair legally becomes a full, code-compliant reroof, which we explain in our guide to repairing versus replacing a roof in Michigan rather than repeating that rule here.

One more thing worth knowing: routine rotted decking found during a normal re-roof is generally treated as a maintenance issue, not something homeowners insurance pays for. If the decking damage is discovered during tear-off on a storm-related insurance claim, it is usually added to that claim through a contractor's supplement instead, a process we explain in our guide to what homeowners insurance actually covers on a Michigan roof.

Get an Honest Look at What Is Under Your Shingles

You cannot see rotted decking or a sagging rafter from the driveway, and no honest contractor can price it over the phone. A free inspection is how we tell you which problem you actually have, with a written per-sheet price for any decking and a plain answer if what we find is structural rather than cosmetic. We do not take deposits, so getting that answer never costs you anything up front. We serve Southeast Michigan from offices in Oxford, Ferndale, and Rochester Hills. Learn more on our roof replacement page, see current financing plans if a larger repair catches you off guard, or browse real decking numbers for yourself in our job gallery.

Sources: seo/crm-insights.md (488 replacement quotes, decking and plank decking figures); src/data/cities.ts verified housing-stock notes for Clawson, Plymouth Old Village, and Livonia; JLC Online and Fine Homebuilding general guidance on sistering sagged rafters; Michigan Residential Code solid-sheathing requirement for asphalt shingles. Code and lumber pricing vary by jurisdiction and over time; confirm current editions before relying on a specific figure.