Do I need ridge vent, soffit vent, or both?

TL;DR — Use both: continuous soffit intake + continuous ridge exhaust.
Size by net-free area (NFA) (typ. 1:300 of attic floor area), target 40–60% intake at soffits and 40–60% exhaust at ridge. Don’t mix too many exhaust types and never rely on ridge vent without soffit intake.

Why both soffit + ridge?#

A passive system needs low intake and high exhaust to create stack effect and wind-assisted flow. With only ridge vent you depressurize the house and pull conditioned air from below; with only soffit you trap heat and moisture.

Common short-circuits

Box/power fans + ridge together (stronger fan pulls from ridge instead of soffits).
Hip roofs with tiny ridges (exhaust area too small for the attic size).
Painted-over or perforated vinyl soffits with solid sheathing behind them.

Quick sizing (NFA)#

Measure attic floor area (length × width in ft²).
Start with 1:300: area ÷ 300 = total NFA (in²).
Split 50/50 (or 60/40 intake/exhaust when you have long eaves).
Convert product ratings:
- If your ridge vent is 18 in²/ft, and ridge length is 40 ft → 18 × 40 = 720 in² exhaust.
- If soffit strip vent is 10 in²/ft with two eaves totaling 60 ft → 10 × 60 = 600 in² intake. Add baffles or more intake to reach balance.

Pro tip: NFA is net area of free openings, not the slot cut. Always use the manufacturer’s NFA per foot.

Where ridge + soffit makes sense#

Steep-slope (≥ 3:12) roofs with continuous eave overhangs.
Attics that allow a clear air path from soffit to ridge (no fire blocks).
You can still use ridge on hip roofs, but you may need hip vents or additional high vents to meet the exhaust NFA.

When it fails (and fixes)#

No soffit intake → add intake (continuous strip or individual vents) and install baffles above insulation.
Slots bridged by underlayment at ridge → re-cut/clear slot; verify the vent’s slot width requirement.
Mixed exhaust (box + power + ridge) → pick a single strategy and rebalance NFA.
Ice-dam/humid zones → add full-height baffles at eaves and air-seal penetrations; ventilation does not replace air-sealing/insulation.

Code & warranty notes#

Many U.S. jurisdictions accept 1:300 (reduced from 1:150 if specific air/vapor conditions are met). Check your local adoption.
Some shingle manufacturers require intake/exhaust balance for warranty.
In HVL/TAS regions (high wind), use wind-tested vents and corrosion-resistant fasteners.

Cost & labor (ballpark)#

Cutting ridge slot + installing baffles: $12–$25 per linear foot depending on access.
Typical retrofit time: half-day to two days.