Does your roofer actually understand ventilation?

Two identical shingle roofs, installed the same week on the same street, can age ten years apart. The difference is under them. One attic has intake at the eaves and exhaust at the ridge and runs a few degrees above the outside air; the other has a couple of gable louvres, an attic at 150°F every afternoon from May to September, and shingles that bake from underneath until the asphalt goes brittle and the granules let go.

Ventilation is also the least glamorous line on a roofing estimate, which is why it is so often missing. This guide is the whole subject in plain numbers, so that when a roofer says "ridge vent" you can ask the next question.

Attic ventilation is a loop. Cooler outside air enters low, through vents in the soffits under the eaves. It warms in the attic, rises, and leaves high, through a ridge vent along the peak or vents near it. The movement is driven by heat and by wind across the ridge; no fan is needed when the two ends are sized to each other.

• Intake is the supply: continuous soffit vents, or individual soffit vents, along the eaves on both sides of the house.
• Exhaust is the outlet: a ridge vent along the peak, or box vents near it.
• Balance is the rule: at least as much intake as exhaust. Exhaust without intake pulls air from wherever it can, including the living space; intake without exhaust does nothing.

The measure is net free area, the actual open area of a vent after its screens and louvres are accounted for, quoted on the product. A ridge vent might provide 18 square inches per linear foot; a soffit vent, 9 square inches each. Sizing the system is arithmetic, and it is done before the roof goes on.

The International Residential Code, which Texas cities adopt, sets the minimum in section R806. The net free ventilating area of an attic must be at least 1/150 of the floor area of the space ventilated. It may be reduced to 1/300 where at least 40% and not more than 50% of the required area is provided by exhaust in the upper portion of the attic, no more than three feet below the ridge, with the balance provided by intake at the eaves. In other words, the code rewards a balanced system with a smaller requirement, because a balanced system works.

Worked through for a 2,000 square foot attic floor:

On the balanced ratio that attic wants roughly 25 to 27 linear feet of ridge vent (at 18 sq in per foot) and about 55 to 65 soffit vents (at 9 sq in each), or the equivalent in continuous soffit vent. The exact count depends on the products, which is why an honest estimate names them and counts them.

The most common ventilation mistake in Texas is adding exhaust to exhaust. A homeowner has gable louvres and a hot attic, so a roofer adds a ridge vent, or a power fan, or a pair of turbines. Now the attic has several outlets and one weak inlet, and air takes the shortest path: the ridge vent pulls from the gable vent a few feet below it instead of from the soffits forty feet away, and the bottom of the attic never moves.

• Ridge vent plus gable vents: the gables become intake for the ridge, the soffits are bypassed, and the air short-circuits across the top of the attic. The fix is usually to close the gables.
• Power fan plus ridge vent: the fan pulls air in through the ridge vent, and in rain can pull water in with it. A power fan wants a closed ridge and generous soffit intake.
• Exhaust with no intake: the exhaust pulls from the house through can lights, attic hatches and wall plates, so you pay to cool air that goes straight out of the roof.
• Bathroom and dryer fans venting into the attic: not a ventilation system but a moisture source. They belong outside.

One exhaust type, matched to intake, wins every time. More holes is not more ventilation.

• Shingle life. Asphalt shingles are rated for years of sun from above, not 150°F from below. Heat dries the asphalt, the mat goes brittle, the granules release early, and a 30-year shingle curls in twelve. Most shingle manufacturers make adequate ventilation a condition of their warranty for exactly this reason.
• The deck. Warm, humid air that cannot leave condenses on the underside of the sheathing on cool nights. Over years it swells, delaminates and rots the deck from the inside, and the nails rust.
• Mold and air quality. A damp attic grows mold on the framing and the insulation, and the house breathes some of it.
• The cooling bill. A 150°F attic radiates heat into every ceiling below it all evening. Homeowners who fix ventilation report a cooler upstairs and a lower bill; we see the attic temperature drop by tens of degrees on the day the system is balanced.
• Insulation. Moisture flattens fibreglass and cellulose, and wet insulation insulates almost nothing.

• Upstairs rooms that stay hot into the night while the air conditioner runs.
• Shingles curling, cupping or losing granules well before their age.
• Rusty nail tips, dark stains or a musty smell in the attic.
• Insulation that looks flattened, damp or discolored.
• Soffits with no vents at all, or soffit vents painted over or blocked by insulation from inside.
• Gable louvres plus a ridge vent, or turbines that spin all summer over a hot attic.
• A bathroom or dryer duct that ends in the attic.

None of these is a diagnosis on its own. Together they are the attic telling you it needs a plan, and the plan is a measurement, not a guess.

The Roof Grade inspection starts in the attic: intake and exhaust counted, the attic temperature and moisture noted, fans venting into the attic flagged, insulation contact with the roof deck checked. On every replacement quote the ventilation is sized to the attic and written as a line item with counts: the soffit intake, the ridge or box exhaust, and anything that has to be closed off so the new system works. On a repair or a maintenance visit we correct the small things (a blocked soffit, a torn screen, a fan into the attic) because they cost little and change a lot.

If your estimate from anyone, including us, says "ridge vent" and nothing about intake, ask where the air comes in. The answer tells you whether the roofer understands ventilation or is just selling the vent.