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Why this matters here (and what “ventilation” actually does)

Vancouver’s long, wet season and frequent wind-driven rain load a roof with moisture for months. Ventilation’s job isn’t to “heat” or “cool” the roof; it’s to flush moist air, promote drying between storms, and help keep the roof deck below the dew point when indoor air tries to leak upward. In our climate, a metal roof assembly that ignores drying paths or air-sealing often ends up with condensation at the deck or under the metal skin —especially overnight after humid days. ( [3] , [4] )

Two big ideas drive success:

1. Air sealing first. If warm, moist indoor air reaches a cold deck, you’ll get condensation no matter how many vents you add. Canada’s long-standing guidance puts air-sealing at the top of the list. ( [5] )
2. Balanced ventilation second. When you choose a vented approach, give moisture a low-to-high path: continuous soffit intake feeding a continuous ridge exhaust—designed to a simple area ratio and kept unobstructed year-round. ( [1] , [6] )

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The two viable roof strategies in Metro Vancouver

1) Vented “cold” attic under a metal roof (most common for houses)

• Ceiling plane is airtight and insulated (the thermal boundary).
• Attic is vented: continuous soffit intake + continuous ridge exhaust.
• Roof deck is cold (outdoor-coupled), but ventilation and air-sealing keep moisture in check.
• Metal roofing sits on a solid deck with underlayment; details at ridges/edges/valleys prevent wind-driven rain from entering while allowing air to flow through designed vent paths. ( [1] , [7] )

2) Compact “unvented” roof (cathedral ceilings, retrofits, some low-slope)

• The roof cavity is insulated to code with a correct vapor control approach; there is no attic venting.
• Metal roofing sits over a continuous, code-compliant underlayment package; many metal systems add a ventilated separation space below panels(more on that below) to drain incidental moisture and speed drying on the outside of the deck.
• This path demands design discipline; get the air/vapor control right. ( [8] )

Both approaches work in Vancouver—provided you follow the details that match your slope, roof shape, and interior humidity.

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What the local standards say (straight from RCABC)

The RCABC Roofing Practices Manual (RPM) —the benchmark standard in B.C.—does three crucial things for architectural sheet metal (standing seam, etc.):

• Requires a ventilation space beneath metal panels when the roof slope is less than 1:3 (4:12), requires it for curved panels, and recommends it even above 1:3. This “above-sheathing ventilation (ASV)” layer helps drain and dry any condensate that forms under cold metal skins. ( [2] )
• Calls for a vapour-permeable separation layer over insulation on insulated ASM systems and publishes ridging details that preserve both weather-tightness and airflow. ( [8] )
• Sets dimensional guardrails for ridge and accessory vents (flange widths, sealed heights, construction), which matter in wind-driven rain. In short: vent components must be built like roofing, not like screens. ( [7] )

Explore RCABC-aligned builds: vented metal roofing systems Vancouver • Compare attachment strategies: fasteners vs clips.

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How much ventilation is “enough” for houses?

For conventional vented attics, Canada’s plain-language standard is net free area (NFA) ≈ 1/300 of the attic floor area, split between intake and exhaust (with more intake than exhaust). Practically, that means continuous soffit panels and a continuous ridge vent sized to the product’s NFA. Industry best practice: ~60% intake / ~40% exhaust or 50/50, with intake never smaller than exhaust. Keep the airway open with baffles at every rafter bay. ( [1] , [6] )

Mini-table — quick targets (vented attic)

Remember: if you can’t air-seal, ventilation won’t save you. Seal first, then ventilate. ( [5] )

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Above-sheathing ventilation (ASV): why metal roofs love it

ASV is a slender air space (entangled mesh or batten/counter-batten) between the metal and the deck/underlayment. It’s required by RCABC below 1:3 slope (and for curves), recommended above that, and it does four jobs at once:

1. Drains and dries any condensate under cold metal skins.
2. Cuts heat gain under summer sun (field studies show ASV can match or beat high-reflectance roofs attached directly to the deck).
3. Decouples vibration/noise paths through the panel.
4. Buffers wind-driven wetting at laps by moving liquid water away from seams. ( [2] , [9] , [10] )

For Vancouver’s mild, heating-leaning climate with long wet spells, that drying function is the headline. ( [2] )

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Metal roof ≠ “ventilation optional”: five condensation triggers I find again and again

1. Leaky ceiling plane(pot lights, open chases, attic hatches). Warm, humid air jets upward, hits a cold deck/skin, and condenses. Fix with airtight covers, gaskets, and sealed penetrations. ( [5] )
2. Choked soffits. Painted-shut or insulation-blocked soffit slots starve the attic; ridge vents can’t exhaust what never arrives. Clear them and add baffles. ( [1] )
3. Unbalanced vents. Too much ridge, too little intake creates negative pressure that pulls interior air through cracks—exactly what you don’t want on wet nights. Balance the system. ( [6] )
4. No ASV on low slopes. Below 4:12, Vancouver’s wind-driven rain plus nighttime radiational cooling make the underside of panels a frequent condensation surface. RCABC requires a ventilated separation space here; skipping it invites trouble. ( [2] )
5. Vents not built for wind-driven rain. Small-flange, low-throat vents let rain blow in and wet the insulation, raising indoor humidity and future condensation risk. Use RCABC-style, baffled components. ( [7] )

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Vancouver’s climate nuance: wind-driven rain + long wet spells

Local research instrumented buildings across Metro Vancouver and confirmed what owners feel every November: wind steers rain sideways and upward at edges and ridges; overhangs materially reduce wetting of surfaces. For roofs, that means: build baffled, well-flashed vent components and keep a clear low-to-high air path so the deck dries between systems. ( [4] )

Environment and Climate Change Canada’s 1991–2020 normals show the region’s persistent cool-wet season—conditions where a slightly under-ventilated attic can creep toward the dew point. Good ventilation plus airtightness buys you margin. ( [3] )

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Design playbook — assemblies that stay dry

A) Steep-slope standing seam (vented attic)

• Ceiling plane: air-seal all penetrations; insulate to target R.
• Vent path: continuous soffit intake (unobstructed) to continuous baffled ridge vent sized to NFA ratio.
• Deck: taped joints or high-quality synthetic underlayment; self-adhered in valleys/eaves.
• Metal: standing seam over the deck; consider ASV above the underlayment where exposure or comfort benefits justify it.
• Ridges & accessories: RCABC dimensional criteria (flanges, throat height), with closures that block WDR. ( [1] , [7] )

B) Low-slope (1:6 to <1:3) or curved metal

• Full-coverage self-adhered underlayment on the deck (per slope rules).
• Required ventilated separation space beneath panels(entangled mesh or batten/counter-batten).
• Mechanically locked seams with sealant in the hem; engineered closures at eaves/ridge.
• Venting: if there’s an attic below, keep the intake and exhaust balanced; if it’s a compact deck, manage moisture by design (air and vapor control), then use the panel-side ventilation to dry the outer layers. ( [11] )

C) Compact cathedral roof (no attic)

• One of two directions:
  • Vent the rafter bays continuously from eave to ridge (maintain a real airway).
  • Design an unvented assembly with robust air/vapour control and continuous insulation where required.
• Above the deck, use a ventilated separation space (ASV) under the metal to evacuate incidental moisture and resist WDR. ( [8] )

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Details that make (or break) the whole plan

• Soffits: Perforated/continuous vented panels are only as good as the clear channel above them; baffles are mandatory where insulation wants to spill into the eaves. ( [1] )
• Ridge vents: Choose externally baffled designs with RCABC-style flange widths and raised throats; size to your NFA. ( [7] )
• Skylights & chimneys: Frame proper crickets and keep vent pathways unblocked around these interruptions; leaky wells are classic moisture sources.
• Bath/kitchen fans: Never dump exhaust into the attic; duct to exterior and insulate the ducts to avoid internal condensation. (A common hidden humidity source.)
• ASV terminations: Keep the cavity continuous to high points; add screened inlets/outlets where the profile calls for them so air can actually move. ( [2] )

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Above-sheathing ventilation vs “cool” colors (which helps more?)

Oak Ridge National Laboratory field work showed ASV can equal or outperform high-reflectance roofs attached directly to the deck. Translation: in our climate, ASV not only helps the deck dry—it also trims summer heat flow without relying solely on color. In practice, many Vancouver projects pair cool-pigment PVDF finishes with ASV for a quiet, dry, comfortable roof year-round. ( [9] , [12] )

Color durability & SRI picks: coatings & colors for Vancouver metal roofs

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Troubleshooting: symptoms and fixes

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People Also Ask — concise answers

Do metal roofs need attic ventilation in Vancouver?
Yes— if you’re using a vented attic strategy. Follow the 1/300 NFA rule with continuous soffit intake and baffled ridge exhaust, and air-seal the ceiling. Compact roofs can be unvented when properly designed. ( [1] )

What is “ventilation beneath panels,” and do I need it?
It’s a slim air space ( ASV) right under the metal. RCABC requires it below 4:12(1:3) and for curved panels, and recommends it above 4:12; it drains and dries condensate and helps in summer. ( [2] )

Will more ridge vents fix condensation?
Not without intake and air-sealing. Intake must meet or exceed exhaust, and the ceiling must be tight so indoor moisture doesn’t flood the attic. ( [6] , [5] )

Is ASV only for hot climates?
No. In our wet marine climate, ASV is a drying layer first and a thermal benefit second—a great pairing with metal roofing. ( [9] )

What if my slope is low?
Use the RCABC low-slope package: full self-adhered underlayment, ventilated separation space beneath panels, mechanical seam with sealant, and proper closures. ( [11] )

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Field checklist I give homeowners (and strata councils)

1. Ceiling airtightness: Photo every light, hatch, and chase; plan gaskets/boxes/foam. (Air leaks beat vents, every time.) ( [5] )
2. Soffit intake: Confirm every bay is actually open with baffles in place. ( [1] )
3. Ridge exhaust: Specify a baffled product with RCABC flanges and raised throats; verify NFA. ( [7] )
4. Vent area math: Show 1/300 NFA on the drawing, split intake ≥ exhaust. ( [1] )
5. Low-slope/curved: Include ASV layer in the spec (entangled mesh or battens). ( [2] )
6. Underlayment: Self-adhered at eaves/valleys; full coverage on low slopes as required.
7. Bath/kitchen fans: Route outdoors, insulated ducts, backdraft dampers.
8. Documentation: Keep product data (NFA, ASV materials) with your O&M files.

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Common myths—debunked quickly

• “Metal roofs don’t need ventilation.” False. Metal is a great cladding, but moisture physics still apply; ventilation (or a code-correct unvented design) prevents condensation and keeps decks dry. ( [1] )
• “More exhaust is always better.” No— balance matters. Starving intake can pull indoor air into the attic and make condensation worse. ( [6] )
• “Condensation means the ridge vent leaks.” Usually false. It often means the ceiling leaks air; fix air-sealing and intake first. ( [5] )
• “ASV is only for heat.” Wrong. In Vancouver, ASV is a condensation and drying tool—and RCABC requires/recommends it by slope. ( [2] )

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A note on noise & comfort (bonus upside)

ASV has a side benefit: it decouples the metal from the deck. That can slightly reduce rain impact noise and lower deck heat flux in summer—small but noticeable comfort improvements that ride along with your moisture control. ORNL’s field work documents these effects. ( [9] )

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The bottom line

Metal roofs last and look superb in Metro Vancouver when the assembly respects our climate: airtight ceiling, correctly sized and balanced intake + ridge exhaust, and—on low slopes or curves—an RCABC-compliant ventilated separation space beneath the panels. Add careful flashing at ridges and vents (real flanges, raised throats, external baffles), and your deck dries predictably between systems. Ignore air-sealing, starve intake, or skip ASV on low slopes, and the November fog will find your roof’s cold surfaces. Build the physics into the spec, and ventilation becomes boring—and that’s exactly what you want from a Vancouver roof. ( [1] , [2] )

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Written by Owner (Vancouver).