A roof is cooled and dried from below
It is easy to think of a roof as a surface that keeps weather out, and to assume everything that determines its lifespan happens on top of it. A good deal happens underneath. The space below the sheathing needs air moving through it continuously, and when that air stops moving the roof above suffers in two distinct ways — heat in summer and moisture in winter.
How it is supposed to work
The system is simple in principle. Air enters low, at intake vents in the soffits under the eaves. It travels up the underside of the roof deck, warming as it goes. It exits high, through a ridge vent along the peak or through other exhaust vents near the top. Warm air rising drives the flow, and wind across the ridge assists it. The two halves have to be in balance: exhaust without adequate intake cannot pull air from the soffits, so it pulls it from inside the house instead, which is worse than no ventilation at all.
What poor ventilation does in summer
An attic with no airflow becomes an oven. Heat that has passed through the roof covering accumulates against the underside of the deck with nowhere to go, and the sheathing and the shingles above it sit at a considerably higher temperature for longer than they would otherwise. Asphalt shingles age faster when they are hot. The volatile components in the asphalt drive off, the material becomes brittle, and the shingle reaches the end of its usable life earlier than its rating suggested. The rooms below are also uncomfortable, and the air conditioning runs harder.
What poor ventilation does in winter
In winter the problem inverts. Warm, humid air from the house makes its way into the attic and meets the cold underside of the sheathing. Without airflow to carry that moisture away, it condenses. The consequences accumulate quietly.
- Water condensing on the sheathing and dripping onto insulation below
- Insulation becoming damp and losing most of its effectiveness
- Sheathing degrading over repeated wet-dry cycles
- Fasteners rusting where they pass through the deck
- Mould developing on the underside of the roof and on stored items
Ice at the eaves
There is a third consequence that is specific to cold weather and very visible. When heat escapes into an attic, it warms the roof deck enough to melt snow lying on the upper part of the roof. That meltwater runs down the slope until it reaches the overhang at the eave, which sits outside the heated envelope and stays cold. There it refreezes, building a ridge of ice that dams the water behind it. The dammed water then backs up under the shingles — which are designed to shed water running down, not to hold it back — and gets into the deck. The cure is a cold attic, achieved through ventilation and air sealing, with ice and water membrane at the eaves as the second line of defence. Adding heat cables treats the symptom.
The five ways it usually goes wrong
In practice, ventilation problems come from a short list of recurring causes, and most of them were created by well-intentioned work.
- Insulation pushed into the eaves during an upgrade, sealing the soffit intake completely — by far the most common
- Soffit vents painted over during exterior decoration, or decorative panels that were never perforated
- A ridge vent fitted without the deck being cut open beneath it, so it vents nothing
- Two different exhaust types on one roof, which short-circuit and draw from each other rather than from the soffits
- Bathroom and kitchen fans discharging into the attic instead of outside
When to sort it out
The natural moment is during a roof replacement, because the covering is off, the ridge is accessible and the eaves can be seen properly. Correcting intake, cutting the ridge properly, fitting baffles at the eaves to hold insulation clear and re-routing a fan duct are all far cheaper as part of that work than as a separate exercise afterwards. It is also the point at which a competent roofer should raise it without being asked.
Across Atlantic County
Ventilation matters everywhere, and the symptom you notice depends on where you are. In the inland municipalities, where winters are genuinely colder, the presenting problem is usually winter condensation and ice at the eaves. On the barrier islands and along the shore corridor, where winters are milder, it more often shows up as summer heat and shingles that have aged faster than expected. The underlying fault is the same.
