How to Choose a Roof System for a Manufacturing Plant

On a manufacturing plant the membrane is chosen by what the building discharges onto it and what happens beneath it. Grease, solvent and chemical exhaust point to PVC. Heavy foot traffic and dense rooftop equipment favour SBS modified bitumen. Large clean fields suit TPO. Interior humidity decides the vapour barrier, and production schedules decide the phasing.

Why is a plant roof a different specification from an office roof?

An office roof carries weather, some foot traffic and a set of rooftop units. A plant roof carries all of that plus everything the process throws at it.

• Exhaust. Process air leaves the building through the roof, and whatever is in it lands on the membrane around the discharge point: grease, animal fats, solvent vapour, plasticisers, mineral oils, acidic condensate.
• Heat. Ovens, furnaces, dryers and compressors push heat into the roof assembly from below, unevenly across the building.
• Humidity. Wet processes, washdown areas and steam raise interior vapour pressure, which drives moisture up into the assembly in winter.
• Traffic. Plants have far more people on the roof than offices do, servicing dust collectors, fans, make-up air units and process equipment on a routine schedule.
• Continuity. The line does not stop for a roof, so every scope has to be built around production.

Those five factors, not the price of a roll of membrane, are what should narrow the system choice. Our industrial roofing page covers the wider service.

How does process exhaust decide the membrane?

This is the first question we ask on any plant, and the answer often removes options straight away.

Grease, fats and oils

Where kitchen, rendering, food processing or oil mist exhaust falls on the roof, PVC is the standard answer. PVC resists fats and oils that will soften and swell other membranes. TPO has improved but is still less tolerant of sustained grease exposure, and bitumen-based systems degrade where oils sit on them, because the oils act as a solvent on the asphalt.

Solvents and chemical fallout

Chemical exposure has to be matched to the specific chemistry, not guessed at. Some solvents attack single-ply membranes and are of little consequence to a granulated SBS cap sheet, while acidic condensate attacks metal flashings and fasteners long before it touches the membrane. Where the plant discharges something specific, the discharge is identified and the membrane is checked against it rather than chosen by habit.

Heat from stacks

Hot exhaust discharging close to the membrane surface degrades any of them. The answer is usually a discharge modification, extending the stack or fitting a deflector, plus a sacrificial protection layer of walkway pad or metal around the discharge zone that can be replaced when it degrades.

A practical middle position on many plants is a hybrid: the general field in the system that suits the building, with an area of chemically resistant membrane around the exhaust zone, detailed as its own area with a proper transition.

What does interior humidity do to the assembly?

More damage than most owners realise, and it is invisible until insulation comes out wet on a roof that has never leaked.

In an Ontario winter the pressure difference drives warm humid interior air upward. If it reaches a cold surface inside the assembly it condenses there. In a plant with wet processes, washdown, cooking, plating or steam, that vapour drive is far higher than in an office, and a vapour barrier that would be adequate elsewhere is not adequate here.

What this means in specification terms: a properly selected and, critically, properly sealed vapour barrier over the deck, continuity at every penetration and at the perimeter, and enough insulation above it that the dew point stays within the insulation rather than at the vapour barrier. Skimping on insulation thickness on a humid plant is how you get a condensation problem that no membrane repair will ever solve.

The other half of the answer is not roofing at all. Where a process vents humid air near a roof penetration, or where interior humidity is uncontrolled, mechanical ventilation is the fix and the roof is only the victim.

How much do foot traffic and rooftop equipment matter?

Enough to change the system. A roof serviced weekly by millwrights, refrigeration technicians and dust collector crews is a working surface, not a sealed lid.

• SBS modified bitumen handles traffic well. It is a two-ply redundant assembly with a granulated cap sheet, forgiving of dropped tools and abuse, and easy to repair with materials any commercial roofer carries.
• Single-ply systems are thinner and more easily punctured, but perform well when protected properly. That protection means a robust cover board under the membrane and continuous walkway pads along every route people actually take.
• Cover board is not optional on a plant roof. It resists hail, foot traffic and fastener back-out and gives the membrane something firm behind it.
• Walkways should be laid where technicians will actually walk, from the hatch to the equipment, not on a drawing route nobody follows.

Dense rooftop equipment also means dense detailing. Every curb, pipe, conduit support and pitch pocket is a potential leak, and on a plant roof there may be hundreds of them. Systems that detail easily and can be repaired by any competent crew have a real advantage over the life of the building.

What about drainage on a very large roof?

Plant roofs are big, flat and long-span, and drainage is where they age badly.

The design questions are the same on every one: does the roof actually fall to the drains, are there enough drains for the area, are overflow drains or scuppers provided and set at the right height, and are crickets built between drains and behind curbs so water is not dammed by the equipment. Tapered insulation is how most of that gets corrected on a re-roof, and it is worth specifying properly rather than trimming from the budget, because ponding water is what turns a twenty-year assembly into a ten-year one through freeze-thaw cycling.

Long-span steel deck also deflects. A roof that drained on the day it was built can develop low spots at mid-span, and a re-roof is the opportunity to correct them with taper rather than reproduce them in a new membrane.

How does the work get done without stopping production?

The same way it does on any occupied building, with an extra layer of coordination.

1. The roof is phased into day-sized sections so nothing is open overnight, with tie-ins landing at defensible stopping points.
2. Areas above sensitive processes, clean spaces, food production or electrical rooms are identified in advance and given their own protection plan or their own scheduling window.
3. Hot work is controlled under the plant's own permit system, coordinated with the site health and safety representative, or eliminated by specifying self-adhered or cold-applied systems.
4. Rooftop equipment that must stay running is worked around, and any lift is scheduled with the plant's maintenance team.
5. Crane and material handling is planned against shipping traffic so the docks keep running.

The staging detail is in our commercial roof installation and replacement sequence, and the same principles are described from the manager's side in our guide to replacing a roof without closing your business.

How should the decision actually be made?

Work through it in this order and the field narrows quickly:

1. What lands on the roof? Identify every discharge and what is in it. This eliminates membranes.
2. What is the deck, and what will it hold? Steel, concrete or wood decides attachment: mechanically fastened, adhered or ballasted, with increased fastener density in the perimeter and corner zones for wind uplift.
3. What is the interior vapour load? This sets the vapour barrier and the insulation strategy.
4. How much traffic and equipment? This sets the cover board, the walkways and the redundancy you want in the membrane.
5. How will the roof be maintained? A system nobody local can repair quickly is a liability on a plant that cannot afford downtime.
6. What does the schedule allow? Phasing, hot work restrictions and shutdown windows can decide the system on their own.

To have a plant roof assessed with those questions in front of it, call 416-456-0777 or book through the free estimate form. A comparison of the systems themselves is on our flat roof types guide.