Roof Ventilation in a Wet Climate: What Washington Homeowners Should Check

What is roof ventilation supposed to do?

A vented attic works by moving outside air continuously through the space between the insulation and the roof deck. Air enters low, at the soffits or eaves, and leaves high, at or near the ridge. The pressure difference driving that flow comes from warm air rising and from wind across the roof, so it costs nothing to run and has no moving parts. What that airflow removes is moisture. A household generates a substantial amount of water vapour every day through cooking, showering, laundry, and simply breathing. Some of it migrates upward through ceiling penetrations, light fittings, hatches and gaps around plumbing, into the attic. In a ventilated attic that vapour is carried out. In a sealed or blocked attic it stays. The secondary job is temperature. Ventilation keeps the deck closer to outside temperature, which reduces condensation in winter and reduces heat load in summer. In this region the moisture function is the one that decides whether the sheathing survives.

Why does intake and exhaust balance matter so much?

Ventilation is a circuit, and a circuit needs both ends open. If there is exhaust at the ridge but the soffit intake is blocked, the exhaust vents still try to pull air, and they pull it from the easiest available source. That source is usually the conditioned house below, drawn up through ceiling gaps, which means the ventilation system is actively importing warm, humid indoor air into the attic and depositing its moisture on the cold underside of the deck. The general rule of thumb used in the trade is to provide at least as much intake area as exhaust area, and often slightly more intake. Getting the ratio approximately right matters more than getting it exact, because a badly unbalanced system can be worse than a modest one that is balanced. Mixing exhaust types is the other common fault. A ridge vent combined with powered fans or high gable vents on the same attic can short-circuit, so the ridge vent becomes an intake for the fan instead of drawing air along the full length of the roof. The result is a section of attic with no air movement at all.

How does trapped moisture rot a roof deck from below?

Warm, moist air rising into an unventilated attic meets the roof sheathing, which in a Washington winter is cold because outside is cold. Water vapour condenses on that cold surface. It appears first as frost or beads on the nail points protruding through the deck, then as damp patches on the sheathing, then as dark staining and mould growth on the underside of the plywood or board. The damage runs from the inside out, which is why it is so often missed. The roof covering above may look entirely serviceable while the deck underneath is softening. Homeowners frequently discover it only during a tear-off, when the sheathing turns out to need replacing across a whole section that never leaked from above. Other signs show up earlier if you look for them. Insulation that is damp or compressed, a musty smell in the attic, rusty nail tips, mould on the rafters near the eaves, or paint peeling from an upstairs ceiling all point at attic moisture rather than at the roof covering. In a climate where the wet season runs roughly October into June, the attic has months to accumulate what it cannot expel.

What can a homeowner check without going on the roof?

Start from inside the attic on a cold, dry day with a torch. Look at the underside of the sheathing for staining, frost, or dark mould patches, and at the nail points for rust. Look at the eaves where the roof meets the outside wall: you should be able to see daylight or feel air movement at the soffit line. If insulation has been pushed into that gap, the intake is blocked, and that is the most common fault we find. Check where the bathroom and kitchen fans terminate. A duct that stops short and discharges into the attic is pumping the wettest air in the house directly at the roof deck, and it must be extended to the outside. Check the attic hatch too, because an uninsulated or unsealed hatch is a large opening for indoor air. From the ground outside, look for continuous soffit venting along the eaves and for a ridge vent or other exhaust near the top of each roof plane. A house with a ridge vent and no visible soffit venting is a house worth investigating, whatever the roof covering is.

Does the roof covering change the ventilation requirement?

No. The attic below has the same moisture load whatever is on top of it, and it needs the same balanced intake and exhaust. This is one of the reasons ventilation is worth understanding whether or not you ever change your roof. Correcting blocked soffits, rerouting a bathroom fan, or sealing an attic hatch are useful things to do under any covering. What a roof replacement gives you is access. A tear-off is the point at which the deck is visible, the soffit condition can be seen, and exhaust venting can be installed or corrected properly rather than retrofitted through a finished roof. It is far easier to fix an intake problem when the roof is open. This is why Interlock assesses attic ventilation at every Washington estimate, including houses that end up not buying anything from us. A new roof installed over an under-ventilated attic inherits the same condensation problem, and the sheathing keeps deteriorating underneath a covering that is doing its job perfectly well.

Where do ventilation problems show up most in western Washington?

They show up wherever the attic cannot dry, which in practice means the same properties that grow the most moss: shaded lots, north-facing slopes, and houses surrounded by conifer canopy from Everett through Seattle and Tacoma to Olympia. Outdoor air in this region is humid for much of the year, so ventilation has to work with a smaller moisture gradient than it would in a drier climate, and it needs the airway fully open to do it. Older housing stock adds its own pattern. Homes built before continuous soffit venting became standard practice often have only a handful of small vents, and later insulation upgrades frequently blocked those. Retrofitted recessed lights and bathroom fans added over the years each create another path for indoor air to enter the attic. Interlock has completed 3,223 Washington installations since 1997, including 394 in Seattle since 1998, 180 in Tacoma since 1997, 80 in Everett, 67 in Olympia, 50 in Bellingham and 92 in Vancouver. Decking replaced on those jobs is consistently concentrated on the slope that gets the least sun and the least airflow. Washington licence #INTERMR75406.

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