Attic Ventilation: The Part of Your Roof You Never See
How attic ventilation works on a Michigan home: intake and exhaust, the 1/150 and 1/300 rules, blocked soffits, and the signs your attic isn't breathing.
The roof repair or replace decision usually comes down to four things: how old the roof is, how much of it is damaged, what shape the decking is in, and how long you plan to stay in the house. A roof under about 15 years old with one leak or one patch of storm damage is usually a repair. A roof near the end of its life, leaking in more than one place, or already carrying two layers is usually a replacement, even if a patch would stop today’s drip.
Start with the roof’s age
Home inspectors’ life-expectancy charts put architectural shingles at around 30 years and 3-tab shingles at around 20. In Michigan the real number is often lower. Freeze-thaw cycles, ice dams, humid summers over a poorly vented attic and a few hard storms all take years off. A lot of 1970s through 2000s subdivision homes in Rochester Hills, Troy and Sterling Heights are on their second or third roof for exactly that reason.
If you don’t know the age, check your closing paperwork or the inspection report from when you bought the house. Re-roofing needs a permit, so your city or township may have a record too.
Age changes the math on any repair. Money spent on a 10-year-old roof can buy another decade. The same repair on a 24-year-old roof may buy a season.
How much of the roof is damaged
Damage in one spot is a repair: a cracked pipe boot, a few shingles torn off by wind, a flashing joint at the chimney. Damage spread across several slopes is a different conversation.
You’ll hear a rule of thumb for this. If roughly 30% or more of the roof is damaged, or a repair would cost more than about a quarter to a third of a new roof, replacing usually makes more sense. Roofers use it, and at least one major shingle manufacturer’s guidance draws the cost line at 25%. It’s a gut check, not a code requirement or an insurance rule, and the percentage is hard to judge without someone on the roof.
Ballpark: National cost guides put a typical roof repair at roughly $400 to $2,000, and a common leak repair at about $350 to $1,500. Replacement costs many times that; our Michigan cost breakdown has the ranges.
Leaks in more than one place
One leak usually has one cause. Two or three leaks in different parts of the house usually mean the roof is wearing out everywhere at once: brittle shingles, dried-out sealant at the flashing, worn pipe boots. You can chase each one, but there will be a next one.
Leaks at the eaves every winter are often ice dams. An older roof without an ice barrier at the eaves is a strong argument for replacing, since today’s code requires one and a new roof is when it goes in.
Layers and decking
Michigan’s residential code limits a roof to two layers. If your house already has two, any replacement is a full tear-off, and even repairs get harder: nails have to go through both layers, and water can travel between them before it shows up inside.
Decking is the other deal-breaker. From the attic, with a flashlight, look for:
- dark stains or damp spots on the underside of the boards
- daylight showing through
- sagging between rafters, which can show from the street as a wavy roofline
- rusty nail tips, a sign of moisture in the attic
Soft or rotted decking can’t be fixed by patching shingles over it. It gets cut out and replaced, which happens most cleanly during a roof replacement.
Matching discontinued shingles
A repair only disappears if the new shingles match. After 15 or 20 years of sun, even the same product in the same color won’t match exactly. Many older roofs were also built with products that aren’t made anymore. Production of 3-tab shingles has dropped sharply as the major manufacturers phased most of them out, and colors get discontinued.
On a back slope nobody sees, a slightly off patch is fine. On the front of the house, decide whether you can live with it before you pay for it.
If you’re selling in the next few years
A buyer’s inspector will note the roof’s age and condition, and an old roof turns into a negotiating point. Some insurers will cover an older roof only at actual cash value, which can come up when the buyer shops for a policy.
A new roof also rarely pays for itself at sale. The 2025 Cost vs. Value report estimated that an asphalt roof replacement recoups about 68% of its cost at resale nationally. If the roof is sound with years left, a documented repair is usually the better move before listing. If it’s at the end, replacing it can make the house easier to sell.
A quick guide
| Situation | Usually |
|---|---|
| Roof under 15 years old with one leak or one damaged area | Repair |
| A few shingles blown off in a windstorm, rest of the roof sound | Repair |
| Cracked pipe boot or loose flashing at a chimney or wall | Repair |
| Hail bruising scattered across several slopes | Replace |
| Leaks in several places at once | Replace |
| Shingles curling, cracking or losing granules across the whole roof | Replace |
| Two layers already on, and more than a small repair needed | Replace |
| Soft, sagging or rotted decking in more than one spot | Replace |
| Damage on roughly 30% or more of the roof | Replace |
| Selling soon, roof sound with years left | Repair and keep the paperwork |
| Selling soon, roof at the end of its life | Replace, or expect to price it in |
If the damage came from a storm, our posts on checking your roof after a storm and hail damage insurance claims in Michigan cover the steps before you decide.
When the answer isn’t obvious, an inspection settles it. A roofer who walks the roof and checks the attic can tell you whether a roof repair will hold or whether you’d be paying to patch a roof that’s done. A good roofer will also tell you when the repair is the right call. J&M Roofing gives free estimates across Oakland and Macomb counties, and you can request one here.
Frequently Asked Questions
Is it worth repairing a 20-year-old roof?
It can be, if the damage is in one spot and the rest of the shingles are flat, sealed and holding their granules. If the roof shows wear across every slope, a repair usually just buys time until the next leak.
Can I replace just one side of my roof?
It can be done a slope at a time, but the new shingles won’t match the older, faded ones, and the old side will need replacing before long. It makes the most sense when one slope took storm damage and the rest of the roof is fairly young.
How do I know if my roof decking is bad?
From the attic, look for dark stains, damp spots, daylight through the boards or sagging between rafters. The full picture only shows once the old shingles are torn off, which is why a good estimate includes a per-sheet price for replacing decking.
Does a new roof increase home value?
It helps a house sell and removes an objection, but it rarely pays for itself. The 2025 Cost vs. Value report estimated that an asphalt roof replacement recoups about 68% of its cost at resale nationally.
