Vent Hoods for Built-In Grills: Sizing, CFM, and When You Need One

Short answer: If your built-in grill sits under a covered patio, pergola with solid roof, or any structure with overhead enclosure, you need a vent hood. NFPA 96 (the National Fire Protection Association code for commercial cooking equipment) is the baseline — minimum 1,200 CFM exhaust for a 30-36" grill, 1,500-2,000+ CFM for 42"+. Open-sky installs don't need a hood. Anything overhead does.

This is one of the most-skipped steps in outdoor kitchen builds, and it causes two failure modes. Skip the hood entirely under a covered structure: grease vapor accumulates on rafters, ceiling fans, and adjacent siding — you'll see brown staining within a season. Under-spec the CFM: grease vapor recirculates into the patio and the cooking smoke triggers smoke alarms in adjacent indoor spaces. Get it right and the build is invisible (which is what you want).

When you need a hood vs when you don't

No hood required: Open-sky install with no overhead structure within 5 ft. The smoke disperses up into ambient air. Most freestanding cart grills and "island" builds without a pergola roof fall here.

Hood required:

Covered patio with solid roof

Pergola with solid (not slatted) roof panels

Under a second-story balcony or overhang

Inside a screened-in lanai

In any 3-walled outdoor structure (greenhouse-style or gazebo with sides)

Edge case: Slatted pergola (gaps between slats, not solid). NFPA 96 doesn't strictly require a hood here, but in practice the slats trap enough grease vapor that a hood is still worth installing for a high-use grill. Optional but recommended.

CFM math — the actual calculation

Two formulas in play. Use the larger of the two for sizing.

Formula 1 (grill BTU based): Total grill BTU ÷ 100 = required CFM. So a 90,000 BTU Bull Brahma needs 900 CFM minimum. Round up to the next standard size (typically 1,200 CFM).

Formula 2 (cooking-surface based): Cooking surface width in inches × 100 = required CFM. So a 36" grill needs 3,600... wait — that's the indoor cooktop formula. Outdoor adjusts down to width × 40 for typical residential. 36" × 40 = 1,440 CFM.

In practice, use ~1,200 CFM for 30-36" grills, ~1,500-2,000 CFM for 38-42", and 2,000-3,500 CFM for 46"+. Going bigger than the minimum is fine and often the right call — the hood runs quieter at 75% load than at 100%.

NFPA 96 — the clearance requirements

This is the fire code that governs commercial cooking equipment and adopted in residential outdoor-kitchen practice. Key requirements:

Mounting height: The hood must be 36-42" above the cooking surface. Higher than 42" and the capture efficiency drops below 80%. Lower than 36" and you risk thermal damage to the hood filter housings.

Overhang: Hood must extend 6" beyond the cooking surface on each side. A 36" grill needs a minimum 48" wide hood (36 + 6 + 6).

Filter type: Stainless steel baffle filters (NOT aluminum mesh or activated charcoal). Baffle filters trap grease and slow flame spread — critical for outdoor cooking where grease ignition risk is higher.

Ducting: Smooth-walled, fully welded stainless or galvanized duct, sloped 1/4" per foot back toward the hood for grease drainage. Flexible duct (the silver-tape stuff from the home improvement store) is a code violation and a fire hazard — don't do it.

Termination: Duct must terminate 10 ft from any property line, 3 ft from any operable window, and away from any HVAC fresh-air intake. Common mistake: terminating directly above a second-story bedroom window.

Hood styles for outdoor use

Island hood (suspended): Hangs from the patio ceiling. Most common for outdoor installs because the grill is rarely against a wall. Requires structural support in the ceiling — a 1,500 CFM hood + ducting weighs 80-150 lbs.

Wall-mount hood: Bolts to a structural wall behind the grill. Common in three-walled outdoor structures (lanais, sunrooms). Simpler install than island.

Under-counter hood: Pull-out hood that retracts into the countertop when not in use. Mostly indoor use; not appropriate for outdoor grills.

Downdraft hood: Vent rises out of the countertop behind the grill. Generally avoid for outdoor use — capture efficiency drops dramatically in any wind, and most don't carry NFPA 96 certification.

Makeup air — the often-overlooked piece

Code in some jurisdictions (CMC, IRC) requires makeup air for hoods above 400 CFM. A 1,500 CFM hood pulls 1,500 cubic feet of air out of the patio every minute — that air has to come from somewhere. In a screened lanai or partially-enclosed structure, the hood will create negative pressure that pulls air back through the back-pressure path (which can mean reversing the flow of indoor exhaust fans, backdrafting a fireplace, etc.).

Solutions: open the structure on at least one side, install a passive makeup air vent (a louvered intake on an opposite wall), or use a powered makeup-air system (MUA) that injects fresh air into the patio at a rate matching the hood exhaust. For most residential outdoor builds, just making sure the patio has at least one fully-open side handles this.

Brand recommendations

Vent-A-Hood: Outdoor-rated hoods specifically engineered for residential outdoor kitchens. 304 stainless, baffle filters, NFPA 96 compliant. $2-5K for the hood, $800-1,500 for ducting and install accessories.

Best by Broan: Lower price point ($1-2K), good for backyard builds where the grill is moderate-use.

Zephyr: Higher-end residential, some outdoor-rated models. $3-7K for outdoor-spec units.

Fire Magic: Outdoor-kitchen specialty brand, integrates well with built-in grill cutouts. $2-4K.

Avoid using indoor-rated hoods outdoors. The corrosion resistance isn't sufficient — you'll see rust within 2-3 seasons even on stainless models not specifically rated for outdoor exposure.

Realistic install budget

Hood: $1,500-5,000

Ducting (rigid stainless, 8-15 ft run): $600-1,500

Roof penetration + cap: $300-800

Electrical (dedicated 20A circuit if not present): $400-1,200

Labor (HVAC contractor + electrician): $800-2,500

Total: $3,600-11,000

This is the line item most homeowners under-budget. If you're planning a $20K outdoor kitchen with a covered roof, expect to spend $4-8K on the vent hood system. It's not negotiable from a code/safety perspective and not negotiable from a property-protection perspective.

Related reads

[Built-in grill install guide](/journal/built-in-grill-install-guide) — cutout, gas, drainage

[Outdoor kitchen design guide](/journal/outdoor-kitchen-design-guide) — full layout planning

[Outdoor kitchen mistakes to avoid](/journal/outdoor-kitchen-mistakes-to-avoid)

FAQ

Can I just use a kitchen exhaust fan from Home Depot? No. Indoor hoods aren't rated for outdoor exposure, the CFM is usually too low (400-600 is common for indoor), and they typically use aluminum mesh filters that don't meet NFPA 96. The cost difference between a code-compliant outdoor hood and an indoor unit is real but the indoor unit will fail (rust, motor burnout) within 2-3 years anyway.

What if my patio is open but has a ceiling fan directly overhead? Hood needed. The fan is the smoking gun — it will distribute grease vapor across the patio and accumulate brown staining on the fan blades and motor housing within a season. Either remove the fan or install a hood.

How loud is a 1,500 CFM hood? 60-72 dB at high speed, 45-55 dB at medium. Premium hoods (Vent-A-Hood Magic Lung, Zephyr Tempest series) use blower designs that significantly reduce noise. Mid-tier hoods at full speed are louder than most people expect.

Does the hood need to be on a dedicated circuit? A 1,200+ CFM hood with lights draws 10-15A; a dedicated 20A circuit is required by code in most jurisdictions. Don't share with the grill rotisserie or any other outdoor outlet.

Can I add a hood retroactively to a built-in grill that wasn't designed with one? Yes, but requires roof penetration (or a wall through-penetration if wall-mounted) and ducting routed through the existing structure. Plan for $3-6K retroactive vs $2-4K if planned at original build.