Short answer
Generally, no—you should not tie two standard bathroom exhaust fans into one roof vent. The best practice is one fan per dedicated duct and exterior termination. The safe, code-friendly alternatives are either:
- Use a multi-port inline fan designed to serve multiple bathrooms with a single roof cap, or
- Where allowed by your local code, combine the ducts using a properly sized wye, upsized main duct, and reliable backdraft dampers on each branch (still not ideal).
Why this matters
Combining two fans into one vent can cause:
- Moisture and odor backflow into the other bathroom
- Reduced airflow and higher noise (static pressure increases)
- Condensation in the ductwork and potential mold or ceiling stains
- Code and warranty issues (many manufacturers and AHJs discourage or prohibit it)
If you need one termination for two rooms, choose a multi-port inline fan system (e.g., Panasonic WhisperLine or Fantech multi-port). These are designed for shared exhaust with proper balancing and dampers.
Code and sizing basics
Always check your local mechanical code and the fan manufacturer’s instructions. Many jurisdictions require dedicated terminations. If combining is allowed, size for the total airflow and install backdraft dampers.
Typical bath fan ratings: 80–110 CFM (intermittent)
Minimum duct size: 4 in. (for short runs at ≤80 CFM), 6 in. preferred for quiet/longer runs
Combined duct (two fans): often 6 in. minimum; roof/wall cap must match duct size and CFM
Max equivalent length: per manufacturer (often 50–100 ft). 90° elbow ≈ 5 ft (rigid) to 15 ft (flex)
ASHRAE 62.2: 50 CFM intermittent or 20 CFM continuous per bathroom (common requirement)
Best practice: Separate vents (recommended)
Materials and tools
- Rigid galvanized round duct (26–30 gauge) or UL-181 listed insulated flex duct (R-6 or R-8)
- Roof cap or wall cap with backdraft damper and screen (match duct size; low-resistance design)
- Foil HVAC tape (UL-181) and duct mastic (no cloth “duct tape”)
- Spring-loaded inline backdraft damper (optional but helpful)
- Pipe straps/hangers, screws, sealant compatible with roofing
- Safety gear: eye/ear protection, gloves, dust mask/respirator, roof harness, stable ladder
- Cutting tools: hole saw, aviation snips, reciprocating saw, drill/driver, utility knife
Steps
- Plan the route
- Choose the shortest, straightest run to the exterior. Prefer rigid duct for lower resistance.
- Avoid terminating in the attic or soffit. Roof or wall termination is best.
- Keep terminations away from intake vents and ridge vents to prevent re-entrainment.
- Cut and install the termination
- For a roof cap: remove shingles as needed, cut the hole, slide the cap’s flashing under the upper courses, fasten, and seal per manufacturer instructions.
- For a wall cap: core drill or saw the hole, install sleeve/cap, and seal exterior penetrations.
- Run and seal the duct
- Slope the duct slightly toward the exterior so condensation drains out, not back to the fan.
- Limit elbows; support duct every 4–6 ft. Keep flex duct stretched smooth (no kinks or crushes).
- Seal every joint with mastic and foil tape. Add a spring-loaded inline damper near the fan if the built-in damper is flimsy.
- Insulate in unconditioned spaces
- Use insulated duct or wrap with R-6/R-8 to prevent condensation.
- Test airflow
- With the fan on, use a tissue test or smoke pencil at the grille to confirm strong draw; check exterior cap for good discharge and damper operation.
Time/cost: 3–6 hours DIY per run; $75–$200 in materials per fan, plus roof cap and tools.
Alternative: Multi-port inline fan (one termination)
A purpose-built inline fan with multiple intakes can serve two or more bathrooms and exhaust through one properly sized roof/wall cap.
Why it works
- Engineered for combined flows
- Built-in or add-on backdraft dampers at each branch
- Quieter (fan is remote in the attic)
Steps (overview)
- Select a multi-port inline fan sized for the sum of both bathrooms (e.g., 2×80 CFM = 160 CFM). Choose a 6 in. main exhaust duct and cap.
- Mount the fan in the attic using vibration-isolating straps. Follow clearances and orientation labels.
- Run individual 4–6 in. branches from each bathroom to the fan’s ports; install backdraft dampers at each branch.
- Run a short, smooth 6 in. duct from the fan to the exterior cap; seal and insulate as above.
- Wire controls per manufacturer (independent switches/timers or continuous low-speed with boost).
Time/cost: 1 day DIY; $300–$600 for the fan plus ducting and cap.
If you must combine two standard fans (verify local approval)
- Use a metal wye fitting near the termination, not a tee.
- Install a spring-loaded backdraft damper on each branch, oriented correctly.
- Upsize the common duct and cap to at least 6 in., rated for combined CFM.
- Keep runs short and balanced; identical fan models reduce cross-flow risk.
- Expect some performance loss; this is a compromise.
Safety considerations
- Roof work: use fall protection, stable ladder, and avoid wet/windy days.
- Attic hazards: watch for exposed nails, low headroom, and insulation. Lay boards to distribute weight.
- Electrical: turn off power when working near fan housings; comply with GFCI/AFCI requirements if modifying circuits.
- Air sealing: seal around fan housings and ceiling penetrations to avoid moist air leaks into the attic.
Tips for best results
- Upgrade to 6 in. duct where possible for quieter, stronger exhaust.
- Add a humidity sensor or timer switch to ensure adequate run time after showers (20–30 minutes).
- Choose low-resistance exterior caps; some “louvered” styles restrict flow.
- Periodically clean grilles and check damper operation.
Common mistakes
- Venting into an attic or soffit
- Using uninsulated duct in cold spaces (leads to condensation and staining)
- Long, kinked, or crushed flex runs
- Relying only on flimsy built-in flapper dampers
- Undersized common duct when combining flows
When to call a pro
- Steep or high roofs, or if you’re not comfortable with safe roof work
- Complex routing, structural considerations, or long runs that require friction-loss calculations
- Local code/permit questions or multi-port system design
Bottom line: For two bathrooms, either run two separate vents or install a multi-port inline fan designed for the job. That approach avoids backflow, preserves airflow, and keeps you aligned with good practice and most code interpretations.