What Is a Building Maintenance Unit (BMU)?
Definition: A building maintenance unit (BMU) is a permanently installed, powered facade access system dedicated to the maintenance of a specific building. It typically consists of a roof car or carriage that travels along a roof-mounted track, a jib or boom that reaches out over the roof edge, and a suspended gondola that carries workers up and down the facade. BMUs are also called roof cars, roof rigs, roof carriages, or window washing rigs.
Unlike a temporary swing stage, which must be rigged, inspected, and moved to each new drop location, a BMU stays on the building and is engineered around its exact geometry. The gondola can travel vertically down the facade while the roof car moves horizontally along the track, allowing crews to reach large, tall, or irregular facades without re-rigging. That makes BMUs the preferred choice for buildings that need frequent, recurring access for window cleaning, glass replacement, sealant work, and facade inspection.
BMUs range from economical fixed-jib roof cars on straight facades to telescopic, luffing, and articulated jib systems that reach over setbacks, high parapets, and complex architectural features. In North America, the ASME A120.1 standard requires the hoisting equipment to be located at roof level once vertical travel exceeds 490 feet, which is why BMUs are standard on supertall towers. Through Scaffold Exchange, you can find companies across the U.S. that design, install, inspect, and service building maintenance units in your area.
Key Components of a Building Maintenance Unit
Every BMU is custom-engineered for its building, but most systems share the same core components working together as a permanent powered platform installation.
Roof Car / Carriage
The powered structure that travels along the roof and positions the gondola horizontally at each working location. Houses the hoisting drums on roof-powered systems.
Track or Runway
An L-shaped guide rail, I-beam rail, or load-bearing concrete runway mounted on the roof or parapet that guides the roof car around the building perimeter.
Jib or Boom
Fixed, telescopic, luffing, or articulated arms that extend the gondola out over the roof edge, clearing parapets, setbacks, and facade features.
Gondola (Suspended Platform)
The working platform that carries crews and equipment down the facade, fitted with guardrails and a control box for operating the system from the platform.
Building Face Restraints
Guide rails, guide buttons, or tie-in anchors on the facade that keep the gondola stable against wind and prevent it from swinging away from the building.
Common Applications & Building Uses
Building maintenance units are specified during building design and serve the structure for its entire life. They are common on high-rise commercial, residential, hospitality, and institutional buildings throughout the United States.
Routine exterior window cleaning on high-rise buildings
Curtain wall glass and panel replacement
Sealant, caulking, and gasket maintenance
Periodic facade inspection and condition assessments
Supertall towers where vertical travel exceeds 490 feet
Buildings with setbacks, high parapets, or complex facades
Interior atriums, glass roofs, and large enclosed spaces
Signage, lighting, and facade fixture maintenance
How BMUs Compare to Other Permanent Access Systems
A BMU is the most capable permanent facade access option, but not every building needs one. Here is how it compares to other permanent motorized access systems and which buildings each suits best.
Roof sockets with removable davits
- Lower installed cost than a BMU
- Davits moved between fixed roof sockets
- Used with portable suspended platforms
- Best for low and mid-rise buildings
Track-mounted roof carriages
- Carriage travels a roof track or runway
- Simpler than a full jib-equipped BMU
- Moves the platform between drop locations
- Best for straight facades with open roof space
Permanent powered facade access
- Vertical and horizontal gondola movement
- Custom-engineered for one building
- Jibs reach over setbacks and parapets
- Best for high-rise and complex facades
Overhead trolley track system
- Track fixed to the building above the facade
- Trolleys carry the suspended platform
- Minimal rooftop equipment required
- Best for curved or continuous facade runs
Fixed gantry access structures
- Rail-mounted frame spanning the surface
- Common on atriums and glass roofs
- No suspended rigging required
- Best for interior and overhead glazing access
Find BMU Installers & Service Providers Near You
Use the Scaffold Exchange map to search by location, filter by equipment type, and connect directly with local companies that install, inspect, and service building maintenance units.
OSHA Compliance & Safety Standards
Building maintenance units in the U.S. are governed by OSHA standard 29 CFR 1910.66, Powered Platforms for Building Maintenance, which covers powered platform installations permanently dedicated to the interior or exterior maintenance of a specific structure. Design and installation also follow ASME A120.1, Safety Requirements for Powered Platforms and Traveling Ladders and Gantries for Building Maintenance. Building owners and employers share responsibility for keeping the installation inspected, tested, and safe to use.
- Installation inspected and tested before initial service and after major alterations
- Comprehensive annual inspection plus regular 30-day inspections
- Building owner certifies in writing that the system is inspected and maintained
- Personal fall arrest systems used by workers on the platform
- Operators trained on the specific installation and emergency procedures
- Inspections performed by a competent person per manufacturer intervals
1910.66
Powered Platforms for Building Maintenance — U.S. General Industry Regulations
OSHA Interpretations & Rulings →