Rooftop Heat-Pump Retrofits: Roofing Questions to Resolve Before Electrification Work

Before you order rooftop heat pumps, ask for a drawing that shows what the entire roof will look like after electrification. Equipment rectangles alone aren't enough. The useful drawing includes supports, new service routes, drainage paths and the space people need to maintain both the equipment and the roof.

For a Metro Vancouver commercial building, our recommendation is to resolve that combined layout before procurement. A heat-pump retrofit may change where equipment sits and how it connects to the building, rather than simply replace one rooftop unit with another. Treating it as a routine equipment swap can leave important roof decisions until the available options have narrowed.

The question to answer is straightforward: can the proposed mechanical scheme work with this roof, including the work needed to repair and eventually replace it? Here's how to turn that question into a useful design discussion.

Compare the whole scheme, not just the old and new unit

Start with an existing-and-proposed roof plan. If the concept uses several outdoor units instead of one packaged unit, show all of them. If it retains a duct opening but adds piping or electrical routes elsewhere, show those changes too. The roof review needs the proposed arrangement, not an assumption that the old curb defines the new project.

Ask the mechanical designer to distinguish three things on the plan:

• Equipment space: the actual units, their specified airflow and service clearances, and how major components will be removed.
• Connection space: the route from each unit to its building entry, including proposed supports and penetrations.
• Roof-working space: room to reach drains, inspect flashing and carry out roofing repairs.

These areas need to work together. A route can look short on a mechanical drawing yet cross the only practical approach to a drain. A unit can fit inside an available rectangle while leaving its curb difficult to flash.

For illustration, imagine a proposal that replaces one rooftop package with three outdoor modules and a shared service route. Reusing the old opening might limit disturbance at that location, but it doesn't settle the new modules' support details or the route between them and the opening. Compare that option with an alternative layout before deciding that fewer new openings automatically makes it the better roof solution.

Separate the support decision from the waterproofing decision

A curb is the raised structure around an opening or beneath equipment. A curb adapter can accommodate a dimensional transition, but fitting the equipment is only one part of the decision. The project still needs an accepted support arrangement and a roof detail that can actually be built around it.

Have the structural professional review the proposed equipment loads, their locations and the support and attachment design. Don't use a comparison of total old and new equipment weights as a substitute for that review. The drawings should make clear how the new arrangement transfers loads to the building.

Wind deserves a defined design responsibility too. RCABC's wind-load and design-responsibility bulletin, focused mainly on Part 3 buildings, explains why roof wind-load calculations belong with registered professionals rather than being casually delegated to the roofing trade.

Then resolve the enclosure detail. For SBS roofs within its RoofStar programme scope, the RCABC SBS standard's penetration and curb provisions call for control-layer continuity at openings and assign curb design and placement to the Design Authority. The practical owner-side question is whether the drawings show both the support connection and the waterproofing around it.

Ask for a section through each different interface. It should identify the membrane flashing, insulation and air- and vapour-control connections where those layers are present. Have the project team reconcile those details with the actual roof system and equipment instructions. An outline labelled “roofing by others” isn't a resolved interface.

Trace every service route to its destination

Depending on the selected mechanical system, the retrofit may introduce refrigerant piping, water piping, electrical conduit, controls or changed ductwork. Ask the designers to identify which services are required, where they enter the building and what supports them along the way.

Look for decisions that are still hidden in the proposal. Does a line stop at the edge of the equipment drawing? Is a roof entry shown without a flashing detail? Does a support sit where water needs to pass? These are useful questions to settle while routes can still move.

Water from the equipment also needs an explicit plan. Heat-pump defrost drainage is a separate consideration from simply finding the nearest roof drain. For example, Daikin's DP3HH installation instructions, in the installation section on page 4, address unobstructed drainage of defrost water and ice as well as rooftop weather-tightness and condensate drainage. Those are model-specific instructions, not a design recipe for every commercial heat pump.

Have the mechanical team identify the selected equipment's discharge arrangements and any cold-weather measures it requires. Coordinate the result with the roofing layout rather than leaving the discharge destination for the installer to improvise.

Keep the roof serviceable after the retrofit

Walk through the proposed layout on paper from the roof access point to each unit and drain. Can a technician reach the service side without stepping over a pipe route? Can a roofer reach the membrane beside and beneath the installation? Where will compatible roof protection be needed?

The SBS standard's roof-platform provisions address drainage, membrane protection and planning for repairs and renewal within that programme's scope. Leave those tasks visible in the layout; don't count every unoccupied patch of roof as available equipment space.

Also compare the retrofit with the building's roof-renewal plan. If substantial roofing work is expected, ask whether doing it first would make sense. If the roof stays, ask how equipment and services would be lifted, disconnected or worked around during future roofing. Neither answer is automatically right. The useful comparison includes the consequences of both sequences.

Make the procurement decision with the roof scope attached

Before releasing the equipment order, ask the project lead to bring together the coordinated layout, reviewed support design and roof-interface details. Identify who resolves permit requirements with the relevant municipality and who checks any existing roof guarantee or manufacturer warranty conditions before alterations proceed. A roofing detail alone doesn't settle those approvals.

The most useful proposal makes the remaining choices obvious: which existing components will stay, which roof areas will change, what needs investigation and who is responsible for each unresolved item. If a later equipment substitution changes the layout or connections, put those interfaces back through review.

For the removal, lift and weather-tight handoff itself, see our rooftop HVAC replacement guide. The earlier electrification decision is about whether the proposed arrangement belongs on the roof in that form. Bring roof alteration coordination into that discussion while the equipment position and service routes are still choices, not delivery-day constraints.