Rooftop Equipment on Metal Roofs: Curbs, Penetrations and Mechanical Units
Every rooftop unit, vent and pipe is a hole in a roof designed to move water. How curbs, crickets and movement-tolerant flashings keep penetrations from becoming the building's leak list.
On most commercial buildings, the roof leaks at the equipment, not at the panels. Rooftop units, exhaust fans, vent stacks, gas piping, conduit, refrigerant lines, antennas and solar racking are all interruptions in a surface whose entire job is to move water downhill without obstruction. Metal roofs add a complication that membrane roofs do not have: the panels move, every day, and anything fastened to them either moves with them or fights them. Getting penetrations right is most of the difference between a roof that lasts and a roof that generates work orders.
The two principles everything follows from
First, water must be diverted around the obstruction, not dammed by it. Anything projecting above the roof plane creates a low-pressure zone on the upslope side where water slows, debris collects, and snow and ice accumulate. Second, the panel underneath is going to expand and contract. A flashing rigidly fastened to both the curb and the panel will be torn, and a curb rigidly attached across several panels will restrain movement that the panel will find another way to accommodate. Every good penetration detail on a metal roof is an answer to those two facts.
Curbs and the upslope cricket
A curb is the raised, flashed box that a rooftop unit sits on. On a metal roof it should be fabricated as a single welded or soldered assembly with an integral base flange, sized and located to fit the panel module, and it should be tall enough that its flange sits well above the level water and snow will reach. Height is a real design decision in Ontario — a curb sized for a mild climate puts its weakest joint underwater every time the roof holds a snowpack.
The upslope side needs a cricket: a sloped diverter, built into the curb assembly, that takes water arriving from above and splits it around both sides of the curb. Without one, water arriving at a wide curb ponds against the flashing across its full width, sits, freezes, and eventually gets under the lap. A cricket should be at least as wide as the curb it protects and pitched steeply enough to actually move water rather than just look like a slope on a drawing.
Where a curb sits relative to the seams
On a standing seam roof the seams are the strength of the system, and cutting one is a serious matter. Wherever possible, curbs and penetrations should be sized and located to fall within the panel module — between seams — so the seam runs past uninterrupted. When a curb must cross a seam, that seam has to be properly terminated and flashed on both sides, which is skilled sheet metal work and a permanent weak point. This is why coordinating equipment layout with panel layout early is worth doing: moving a unit half a metre on a drawing is free, and cutting seams in the field is not.
Letting the panel move
A curb has to be supported by the building structure, not by the roof panels — panels are cladding and are not designed to carry an air handler. So the curb is fixed relative to the building while the panels underneath it are moving. The flashing between them has to absorb that. The usual approach is a two-piece detail: a base flashing worked into the panels, and a counterflashing or skirt attached to the curb that overlaps it with enough overlap to accommodate the full range of expected movement in both directions, sealed with a flexible non-hardening sealant rather than a rigid one.
- ✓Curbs supported from the structure, with the panel free to move beneath the flashing
- ✓Two-piece flashings with generous overlap rather than a single rigid piece fastened to both curb and panel
- ✓Flexible, non-hardening sealants at any joint expected to move, not a bead of rigid caulk
- ✓Fasteners into the panel kept to a minimum and located where the manufacturer's details show them
- ✓Penetrations located in the flat of the panel, never through a rib or seam, unless the detail specifically provides for it
- ✓Any accessory clamped to the seam rather than through-fastened, where the system allows it, so it travels with the panel
Small penetrations: pipes, vents and conduit
Plumbing stacks, flue vents and small conduit are handled with pipe boots — moulded EPDM or silicone flashings with a metal base flange that is formed to the panel profile, fastened with an appropriate screw pattern and sealed underneath. Silicone boots are used where the pipe runs hot. The base has to sit in the flat of the panel with the pipe centred, and the boot should be sized so the collar grips the pipe firmly rather than being stretched to its limit.
Boots are a maintenance item and should be treated as one. UV degrades the rubber, the collar cracks at the pipe, and the sealant under the flange eventually gives up. Expect to inspect them at every roof inspection and replace them well before they fail. On a long-life metal roof, boots will be replaced more than once during the life of the panels, which is normal and worth writing into the maintenance plan rather than discovering as a stain on a ceiling tile.
Supporting pipe, conduit and duct across the roof
Gas lines, refrigerant lines, conduit and small ducts run across roofs constantly, and the common shortcut — screwing supports into the panel wherever they land — creates dozens of unnecessary penetrations, each of which pins the panel. The better approach is to run services off supports that either bear on the roof without penetrating it, or that penetrate at a small number of properly flashed points and carry the run between them on a rail. Supports need to allow the panel to move underneath and to allow the pipe itself to expand. Where a run crosses seams, a raised support that spans them is far better than a support in every panel.
The most common cause of leaks on an otherwise sound commercial metal roof is another trade. HVAC, electrical, telecom and signage contractors put holes in roofs. A written roof access policy — nobody on the roof without notifying the facility manager, and no penetrations without approval — protects the roof more than any product upgrade.
Snow, drift, condensate and service access
A rooftop unit is a snow fence. Snow drifts against its upslope side and builds a load the roof did not have before the unit was installed, and on a sloped metal roof a slide arriving at speed can shear a curb or damage a unit outright. So adding equipment to an existing roof is a structural question because of the drift load as much as the unit's own weight, and it should go to a structural engineer. Snow retention placed upslope of the equipment protects both the unit and the curb from impact.
Condensate discharged onto a metal panel stains the coating, encourages corrosion and builds ice at the point of discharge and wherever it refreezes downslope, so condensate lines should be piped to a drainage point rather than dumped on the roof. And every unit gets serviced, which means people on the roof with tools. Designated walkways — walk pads or platforms clamped to the seams — protect the panels from denting and give technicians footing on a surface that is slippery wet and dangerous frosted.
Frequently Asked Questions
Can we add a new rooftop unit to an existing metal roof?
Usually, but it needs a structural review for the unit's weight and, importantly, for the snow drift it will create, plus a curb detail designed for the panel system and located to suit the panel module. It is not a matter of setting the unit down and flashing around it.
Why does our roof leak at the same rooftop unit every year?
The usual suspects are a missing or undersized upslope cricket, a flashing that is fastened to both the curb and the moving panel and has torn, a curb that is too low so its flange sits in ponded water and snow, or sealant that has hardened and split. All four are detail failures and all four are fixable, but they need to be diagnosed on the roof rather than caulked over.
Can solar panels go on a standing seam roof?
Standing seam is one of the better surfaces for it, because racking can be clamped to the seams without penetrating the roof at all. The clamps have to be appropriate for the specific seam profile, and the structural loads still need review. Take care that the racking does not inadvertently pin the panels at a second fixed point, which would restrain thermal movement.
How high should a curb be?
High enough that the flashing flange stays clear of water and accumulated snow in your conditions, which on many Ontario roofs is higher than the minimum a manufacturer's generic detail shows. Curb height is worth discussing explicitly rather than accepting a default, particularly on low-slope roofs where snow lingers.
Should pipe boots be replaced when we re-roof?
Yes, without exception. Boots are the shortest-lived component on the roof and reusing an aged one on a new roof puts a component near the end of its life into an assembly at the start of its own. They should also be on a scheduled replacement cycle in the maintenance plan afterward.
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