Rooftop RECD Installation: Structural Support and Lifting Access

On a rooftop, the hardest part of fitting a retrofit emission control device is rarely the device. It is the two questions that come before it. Can the roof carry it? And how will it get up there?

Rooftop RECDs appear in two common situations in Indian buildings. In the first, the generator itself sits on the terrace, often in an acoustic canopy, because the building had no basement space or because the design wanted the exhaust discharged high above the street. In the second, the generator is in a basement or ground-level room, but the exhaust riser ends on the terrace and the device is placed at the top of the riser to save room space below. Either way, the device ends up on a structure that was designed for something else: waterproofing, water tanks, lift machine rooms, people walking, perhaps a few solar panels.

This guide treats the rooftop as a structural and logistics problem first, and an emission control problem second. It traces the load from the device to the columns, compares the ways to lift equipment onto a roof, and covers the rooftop-specific issues of wind, water, vibration and lightning.

Part One: Can the Roof Carry It?

Think in load paths, not in kilograms

A structural engineer does not ask only “how heavy is it?” but “where does the weight go?” Every kilogram of the device travels from its feet, through a frame or plinth, into the roof slab or beams, down the columns and into the foundations. If any link in that chain is weak, the installation is unsafe even when the total weight looks small.

The load path for a rooftop RECD has five links:

1. The device feet or saddles, which concentrate the weight onto small areas
2. The support frame or plinth, which spreads that weight
3. The connection to the roof, through anchors, base plates or a concrete plinth
4. The slab and beams, which carry the load to the columns
5. The columns and foundations, which take it to the ground

The aim is to get as much of the load as possible directly onto beams and columns, rather than onto the middle of a slab span.

The loads to account for

Load type	What causes it	Why it matters on a roof
Dead load	Device weight, frame, insulation, pipe	Permanent; must be within the roof’s capacity
Contents load	Ash and soot accumulated in the filter	Small, but adds to dead load over time
Live load during service	Technicians, tools, removed filter modules placed on the roof	Often forgotten; a filter module set down on the slab is a point load
Wind load	Wind on the device, frame and stack	Much higher on rooftops than at ground level
Thermal load	Expansion of hot pipe and housing	Pushes and pulls on supports and on the generator connection
Vibration	Engine vibration transmitted through the exhaust	Fatigue in welds, anchors and slab
Seismic load	Earthquake shaking	Depends on seismic zone; heavier on tall buildings

India’s code for imposed loads on buildings is IS 875 (Part 2), and roof design loads in older buildings were often based on it. Many flat terraces were designed only for maintenance foot traffic and waterproofing layers, not for heavy equipment. That is why a structural check is the first step, not a formality.

What the structural engineer needs from you

Give the engineer a package that lets them check the roof quickly:

• Device weight empty and in service, from the RECD vendor
• Footprint and positions of the feet or saddles
• Proposed frame layout and the beam grid beneath it
• Building drawings showing slab thickness, beam sizes and column positions, if available
• Existing rooftop equipment and its weights: water tanks, cooling towers, the generator itself
• Building age, any past additions to the terrace and visible cracking or leakage

If drawings are missing, the engineer may need to scan the slab to find beams and reinforcement, or take cores. Budget time for that.

Common ways to support the device

Support method	How it works	Best suited to
Steel frame spanning between columns	Frame legs land on or next to column heads, bypassing weak slab spans	Heavier devices; older or lightly designed roofs
Steel frame on beams	Base plates fixed over main beams	Medium devices on reasonably strong roofs
Concrete plinth on the slab	Cast plinth spreads load over a larger area	Lighter devices on well-built slabs
Integrated with the generator base frame	Device carried by the canopy or skid structure	Rooftop sets where the canopy maker approves the extra load
Raised platform shared with other equipment	Common steel platform for generator, device and stack	New rooftop DG installations planned from scratch

Our guide to how to prevent generator vibration issues covers the isolation side of support design.

Part Two: How Will It Get Up There?

Once the roof can take the device, the next challenge is lifting it. The right method depends on the building’s height, the site around it, the weight and size of the device, and how often it will need to come down again for service.

The main lifting options

Mobile crane from the street or compound. The most common method for medium and large devices. A mobile crane lifts the device over the building edge onto the roof. It needs space to set up outriggers, ground that can take outrigger loads, a clear swing path free of overhead cables, and often traffic or municipal permission. Crane capacity drops sharply with radius, so a crane that can lift the device close in may not reach the far side of a deep roof.

Tower crane on an active construction site. If the building or a neighbouring phase is still under construction, the site’s tower crane may be available. This is often the cheapest and simplest route, but it depends on construction schedules.

Rooftop davit, gantry or derrick. A small lifting structure fixed temporarily or permanently on the roof edge, with a winch, raises loads up the face of the building. Suitable for lighter modules. A permanent davit is useful for future service lifts.

Building goods lift. Small modules may travel in a service or goods lift to the top floor, then be moved up a final stair or through a roof hatch. This only works for compact, light items and is limited by the lift’s car size, door size and rated load.

Manual carry in pieces. For small devices or modular designs, sections may be carried up stairs. This is slow, risky for the workers, and limited by stair width and turning space.

Comparing the options

Method	Device size range	Site needs	Cost profile	Repeatable for service?
Mobile crane	Medium to very large	Outrigger space, permits, clear swing path	High per lift	Yes, but every lift costs the same again
Tower crane	Any within its capacity	Active construction site	Low if already on site	Only while the crane remains
Rooftop davit or gantry	Small to medium modules	Strong anchor points at the roof edge	Moderate once, low afterwards	Yes, very convenient
Goods lift	Small modules only	Lift capacity and car size	Low	Yes
Manual carry	Very small pieces	Wide stairs, short route	Low cost, high risk	Not ideal

The questions that decide the method

1. What is the heaviest single piece, and its dimensions?
2. Can the device be supplied in modules small enough for a lighter method?
3. Where could a crane stand, and how far is that from the device position on the roof?
4. Are there overhead power lines, trees or neighbouring buildings in the swing path?
5. Will the filter module need to come down for cleaning, and how often?
6. Who will issue permits for road closure or crane operation?

The fifth question is often forgotten. A device that needed a 100-tonne mobile crane to install will need the same crane, or a smarter plan, every time its filter goes for cleaning. Designing a removable filter module that can be lowered by a rooftop davit can save a great deal over the device’s life.

Lifting safety

Lifting over an occupied building or a public road is high risk. A written lift plan should cover:

• Weight of the load, rigging and lifting accessories
• Crane capacity at the working radius, with margin
• Lifting points on the device, designed and marked by the vendor
• Exclusion zones on the ground and on the roof
• Communication between the crane operator, banksman and roof team
• Weather limits, especially wind speed
• Emergency procedure if the lift has to stop mid-way

The UK Health and Safety Executive’s guidance on lifting operations and lifting equipment (LOLER) is a widely used reference on planning lifts safely, and is a useful benchmark alongside local requirements. The mobile crane article explains how capacity varies with radius and configuration.

Part Three: Rooftop-Specific Hazards

A rooftop exposes the device to conditions that a basement or ground-level room never sees.

Wind

Wind speed increases with height, and rooftops sit in the strongest airflow around a building. A tall device with a stack on top presents a large area to the wind. The support frame and anchors must resist overturning and sliding, not only weight. Stacks need guy wires or bracing above a certain height. Coastal cities and cyclone-prone regions need particular attention.

Water and waterproofing

Every anchor bolt drilled into a terrace slab is a possible leak into the floor below. Rooftop waterproofing is often a membrane or a layered treatment that is easily damaged.

• Prefer frames that sit on raised plinths with their own waterproofing upstands, rather than bolting directly through the membrane
• If bolts must penetrate, seal each one with a compatible waterproofing detail and test with ponding before handing over
• Keep the device clear of drainage outlets and roof slopes, so water does not pond around its feet
• Fit a rain cap or bent outlet on the stack so water does not enter the device

Heat on a sunny roof

In summer, terraces in north and west India can reach very high surface temperatures. A device already hot from exhaust, sitting in full sun, adds heat to everything around it. Keep it clear of plastic water tanks, cable insulation, solar panel wiring and anything with a temperature limit. Provide guards around hot surfaces if people walk nearby.

Lightning

A tall steel stack on a roof is a natural lightning target. Bond the device, frame and stack to the building’s lightning protection system, or install protection if none exists, following an electrical engineer’s design.

Vibration and noise reaching the floors below

Rooftop generators can transmit vibration into the top-floor ceilings, and so can exhaust pipework rigidly fixed to the slab. Use flexible bellows between the engine and the device, isolate the frame where the vendor recommends it, and avoid rigid connections between the hot pipe and the slab. The principles of vibration isolation apply directly. Our article on noise reduction strategies for diesel generators covers the acoustic side.

People on the roof

Rooftops are often shared with water tank cleaning crews, telecom technicians, solar panel washers and residents. The device must not create a trip hazard, a hot surface at hand height or an obstruction to an escape route. Edge protection is needed wherever service work happens near a parapet.

Part Four: Device Position on the Roof

Within the roof, the device position affects structure, lifting and performance together.

Factor	Near the generator	Near the roof edge	Near the riser terminal (basement set)
Exhaust temperature at the device	Highest	Depends on pipe length	Lowest; exhaust has climbed many floors
Crane reach	May be deep in the roof	Shortest radius; easiest lift	Depends on riser location
Structural support	Can share the generator’s support zone	Often near perimeter beams and columns	Must be designed around the shaft
Wind exposure	Moderate	Highest	Moderate to high
Service access	Shared with the generator	Near the edge; needs edge protection	Usually limited space around the shaft

A device placed at the top of a long riser from a basement set receives cooler exhaust than one near the engine. That makes regeneration harder for sets that run at light load or only for short tests. Our guides on choosing an RECD for a generator running at low load and RECD selection for DG sets with frequent short start-stop cycles explain how to compensate.

Part Five: Connecting to the Exhaust

For rooftop generators

The device normally connects close to the engine outlet, often replacing or sitting alongside the canopy silencer. Thermal expansion is the main concern: a hot pipe growing between a vibrating engine and a fixed device needs a flexible bellows and supports that allow movement in the right direction. Our article on whether an RECD can replace a generator’s exhaust silencer discusses when the silencer can be removed.

For basement generators with a rooftop device

The riser enters the device from below or from the side. Condensate running down the riser during cold starts must drain before reaching the device, and the riser’s own thermal growth over many floors must be accommodated by its supports and expansion joints, so it does not push up into the device. The principles of expansion joints explain why this matters.

Backpressure

In both cases, calculate backpressure for the whole exhaust system with the device in place, and compare with the engine maker’s limit. The guide on how exhaust backpressure affects a DG set explains the consequences of exceeding it, and using OEM exhaust flow data to specify an RECD shows where the inputs come from.

Part Six: The Rooftop Service Plan

Service work on a roof combines working at height, hot surfaces and heavy lifts. Plan it at design stage.

• Filter removal route: decide whether the filter will be cleaned on the roof, lowered to the ground or swapped for a spare. Each needs a different arrangement.
• Lifting point: a permanent davit or a beam with a rated lifting point above the device saves hiring a crane for every service.
• Platform and edge protection: if the device sits on a raised frame, provide a permanent platform with handrails.
• Weather windows: schedule service lifts away from the monsoon and high-wind periods.
• Monitoring access: place the display or data logger where staff can read it without climbing onto the frame. Remote monitoring is especially valuable on rooftops.

The RECD maintenance guide covers the routine itself.

A Worked Example: Hospital Rooftop in Hyderabad

This is an illustrative case.

The site

• A 625 kVA set in an acoustic canopy on the terrace of a six-storey hospital block, installed when the hospital was built
• Roof designed for maintenance access and water tanks; slab 150 mm on a beam grid of about 6 m x 5 m
• The block faces a busy road on one side and a hospital car park on the other
• The set runs for weekly tests and for grid outages averaging 15 hours a month, with an emergency role for theatres and intensive care

Structural findings

The structural engineer confirmed that the slab alone should not carry the device’s concentrated load. The solution was a steel frame spanning between two column heads beside the canopy, with base plates on new waterproofed concrete pedestals over the columns. The frame also carried a service platform.

Lifting decision

Option considered	Finding
Mobile crane from the main road	Road closure needed on a hospital access road; rejected
Mobile crane from the car park	Possible on a Sunday morning with part of the car park closed; radius to device position about 22 m
Rooftop davit	Suitable for the filter module but not the full housing
Goods lift	Lift car too small for the housing

The team chose a mobile crane from the car park for the installation, and a permanent rooftop davit at the parapet nearest the device for future filter removal. The vendor supplied the filter as a separate module light enough for the davit.

Installation sequence

1. Structural pedestals cast and waterproofed two weeks before the lift
2. Frame lifted and fixed on the first Sunday
3. Device lifted and placed on the second Sunday, with the generator covered by a rental set during the work
4. Exhaust connection made with a new flexible bellows and supports allowing thermal movement
5. Stack fitted with bracing and a rain cap, and bonded to the lightning protection system
6. Backpressure and temperature checked at several loads during commissioning
7. Ponding test on the pedestals after the first rain

Outcome

The hospital’s emergency supply was covered throughout by the rental set. Filter cleaning now uses the davit, avoiding a crane booking for each service. Hospital sites have their own critical-load constraints; our guide on RECDs for hospitals covers how to plan around them, and the 625 kVA DG set RECD page shows the device class used here.

A Rooftop Planning Checklist

Item	Done?
Structural engineer appointed and building drawings collected
Device weight, footprint and foot positions confirmed by the vendor
Support method chosen to land loads on beams or columns
Wind and seismic loads checked for the frame and stack
Waterproofing details agreed for every penetration or plinth
Lift method chosen, with crane position, radius and capacity confirmed
Permits for crane operation and any road closure obtained
Lift plan written and shared with all parties
Backup power arranged for the installation period
Exhaust connection designed for thermal movement and condensate
Lightning protection bonding included
Service platform, edge protection and lifting point for filter removal included
Monitoring display or remote monitoring accessible without climbing

Mistakes Seen on Rooftop Jobs

Assuming the terrace is strong because it looks solid

Many terraces were designed only for foot traffic. A device on the middle of a slab span can overload it. Check the structure.

Planning the installation lift but not the service lift

A crane hired once is a cost. A crane hired for every filter clean is a recurring burden. Plan for the second lift at the start.

Drilling through waterproofing

A leak into an operating theatre, a server room or a resident’s flat can cost more than the device. Use plinths and proper waterproofing details.

Ignoring wind

A tall device and stack on a roof edge can be pushed, lifted or vibrated by strong winds. Brace and anchor for rooftop wind, not ground-level breezes.

Leaving hot surfaces unguarded

Rooftops are shared spaces. Guards and signage protect the people who visit for other reasons. Our list of common RECD installation mistakes covers more errors that apply everywhere.

Compliance and Records

The device’s approval comes through CPCB’s System and Procedure for Emission Compliance Testing of RECDs. For rooftop work, also file the structural engineer’s report, the lift plan and permits, waterproofing test records and lightning protection bonding records with the device documents. Changing stack height or position may affect conditions in your consent; our article on whether increasing chimney height is an alternative to an RECD discusses stack height rules. The RECD installation timeline helps fit structural work and permits into the programme.

Rooftop RECD Questions Answered

Can an RECD be installed on a building rooftop?

Yes, either beside a rooftop generator or at the top of an exhaust riser from a set below. The roof structure must be checked, the lift planned and wind, water and lightning addressed.

How do I know if my roof can support an RECD?

A structural engineer should check the load path from the device to the columns, using building drawings and the vendor’s weight and footprint data. Supports that land on beams or column heads are usually preferred over slab spans.

What is the best way to lift an RECD onto a roof?

For medium and large devices, a mobile crane is common. Lighter modules may use a rooftop davit or goods lift. The choice depends on weight, building height, crane access and how often the filter must come down for service.

Does a rooftop RECD need protection from wind?

Yes. Wind loads on rooftops are higher than at ground level. The frame, anchors and stack need to be designed for them, with bracing where needed.

Will drilling into the terrace cause leaks?

It can. Prefer raised plinths with waterproofing upstands, seal any penetrations carefully and carry out a ponding test.

Is a rooftop device suitable for a basement generator?

It can be, but the exhaust cools on its way up the riser, which slows regeneration. It suits sets with healthy load and running time better than lightly loaded or rarely used sets.

Support First, Lift Second, Device Third

A rooftop RECD succeeds when the structure is understood before the device is chosen, the lift is planned for its whole life rather than one day, and the roof’s particular hazards of wind, water, heat and lightning are treated as design inputs rather than afterthoughts. Get those right and the device becomes a well-behaved piece of rooftop plant, serviceable without drama for years.

Aceget plans rooftop RECD installations with structural and lifting input, and supplies CPCB type-approved devices with separable filter modules for engines from 25 kVA to 2500 kVA. To discuss a rooftop project, speak with Aceget’s team, or see all ratings on the retrofit emission control device page.