Downdraft Booth Foundations: Pit or Raised Basement

How a Concrete Pit Works

With a pit, a trench or wider excavation is cut into the slab beneath the booth. Exhaust air passes through floor grates into the pit, through filters, and into the exhaust duct. The booth floor stays at the same level as the surrounding shop, so vehicles and carts roll straight in without ramps. Pits are permanent. They require saw cutting, excavation, new concrete work with proper reinforcement, and attention to what lies beneath the slab, including utilities, soil conditions and groundwater. The work is typically done before the booth is installed and takes time to cure.

• Floor level matches the surrounding shop
• No ramps required for vehicles or carts
• Permanent change to the slab
• Requires excavation and new concrete work

How a Raised Basement Works

A raised basement is a steel structure set on top of the existing slab. The booth floor sits on top of it, creating an exhaust chamber underneath. Air passes through grates into the basement and out to the duct. Because nothing is cut into the slab, installation is faster and the change can be reversed if the booth is moved. The trade-off is height and access. The booth floor is elevated, so ramps are needed at each door, and those ramps take up floor space. The added height also affects clear height requirements for the booth, plenum and make-up air. Grates and their supports must also be rated for the heaviest vehicle or cart that will roll across the booth floor.

Building and Workflow Impacts

Each approach affects the building differently. A pit keeps the overall booth height lower, which matters in buildings with limited clear height, but it involves heavy concrete work, noise and dust during construction. A raised basement avoids slab work but adds height and requires ramp space in front of doors, which can conflict with aisles and other equipment. For drive-through layouts, ramps are needed at both ends. Owners of leased buildings often prefer basements because the slab is left intact. Owners planning a permanent booth location may prefer a pit for its flush floor and clean traffic flow.

• Pit: lower overall height, flush entry, heavy construction
• Basement: faster install, reversible, adds height and ramps
• Leased buildings often favor basements
• Permanent locations often favor pits

Drainage and Cleaning Considerations

Pits sit below floor level, so any liquid spilled in the booth or washed from the floor ends up in them. Some pits are designed with drainage, while others are kept dry and cleaned manually. Groundwater intrusion can be a concern in some locations and should be checked before excavating. Basements sit above the slab and are less exposed to groundwater, but they still collect overspray and need regular cleaning. Both require access for filter changes and cleaning, either through removable grates or access panels, and both should be designed so maintenance staff can reach every section.

Retrofitting a Downdraft Into an Existing Building

Retrofits bring their own questions. Cutting a pit into an existing slab means locating buried utilities, checking slab thickness and reinforcement, and planning around production during the concrete work. Some older slabs or sites with poor soil may not be good candidates. A raised basement avoids those issues but may not fit under existing trusses once the booth and plenum are added. If neither works well, side downdraft or semi-downdraft designs offer much of the downward airflow benefit without floor exhaust. A site survey that checks slab, height and access early makes the decision much clearer.