Roof First, or Rooftop Units First?

The short answer

Your roof and rooftop equipment run on different clocks, so start with which has less life left. If the equipment will wear out before the new roof, plan its replacement before or within the reroof project. The roof warranty makes that order a contract question as well as a scheduling question.

The arithmetic

ASHRAE's equipment life expectancy chart gives single-zone and multizone rooftop air conditioners a median service life of 15 years. Its field database gives packaged DX rooftop units a median age at replacement of 22 years. Those are different measures. One is a service-life benchmark. The other records when equipment in surveyed buildings was actually replaced.

Neither tells you exactly when your unit will fail. Together, they show why the equipment inventory belongs at the start of roof planning.

First, identify what is up there. A packaged rooftop air conditioner is not the same asset as a true air handling unit. In ASHRAE's database, variable-air-volume air handlers have a median in-service age of 20 years. Constant-volume air handlers have a median in-service age of 27 years. Those are ages of equipment still operating, not replacement ages. Do not apply the packaged-unit clock to every box on your roof.

The roof has its own clock. Delaware's July 2026 Carvel Building assessment describes a roof estimated at 20 to 25 years old as at or near the end of its expected useful life. Ontario Northland's facility assessment uses a 20-year expected useful life for single-ply membrane roofing.

On those planning lives, a new roof installed under mid-life packaged units guarantees one full equipment replacement in the middle of the new roof's life. The exact year varies. The collision is already in the arithmetic.

The warranty problem

Carlisle's Total Roofing System Warranty makes the next step consequential. Paragraph 4(a) says the warranty becomes “null and void” if alterations or repairs are made on or through the roof, or objects are placed on or attached to it, without prior “written authorization from Carlisle”. An equipment swap can involve both kinds of work.

Carlisle's Owner's Manual repeats this requirement as a procedure. It directs owners to contact Warranty Services before alterations. Notification after the work is not the procedure it describes.

The installer can have a separate restriction. A roofing installer warranty sample in a Virginia public procurement says alterations by someone other than the roofing installer void coverage to the extent those alterations affect the warranted work. Using the original installer preserves that coverage under the sample's terms.

There is another limit. Carlisle excludes leaks or damage caused by failure of building components, including HVAC units. A roof warranty does not make damage from failing equipment the manufacturer's responsibility.

For you as the owner, the takeaway is plain. After a reroof, an HVAC contractor working alone on the roof can void the warranty by accident. Equipment procurement, roof work, and manufacturer authorization need to be coordinated before work starts.

The physical problem

Added insulation raises the finished roof surface. An existing equipment curb then has less height above the membrane. Keeping the old curb can leave too little room for flashing.

The Government of Canada's CCIW reroof specification addresses this directly. It calls for lifting mechanical equipment where curbs would have less than 12 inches of clearance above the new roof. It includes curb buildup and membrane and metal flashing over the curbs. Its woodwork requirements call for a minimum height of 8 inches above the finished roof.

That puts the lifting and curb work inside the roofing contract. The equipment has to come off its support so the roof interface can be rebuilt.

Buildipedia's trade press account of a Bowling Green State University reroof describes replacing every mechanical curb with a taller one to maintain an 8-inch minimum flashing height. The project also extended plumbing vent stacks.

Space matters too. University of Michigan design guidance requires 36 inches clear between mechanical units and adjacent units, structures, or walls for future roof repair or replacement. Where you set today's replacement unit affects the next roofing job.

IIBEC's guidance on rooftop amenity work makes the penetration issue explicit. It advises reviewing warranty requirements and having penetrations and flashing performed by a certified installer to avoid leaks and loss of warranty coverage.

What the collision costs

Delaware's July 2026 Carvel Building assessment budgets $30 per square foot for single-ply roofing across 36,725 square feet, or $1,101,750. It notes that pricing may vary with warranty duration. Ontario Northland's assessment uses C$185 per square metre, about C$17 per square foot, for single-ply renewal. Published owner figures vary by scope and market. These are roof budgets, not estimates of the extra cost of a later equipment swap. They show the scale of the asset that later work puts at risk.

The sequence that avoids it

Inventory the remaining life of everything on the roof before scoping the reroof. Separate packaged units from true air handlers. Compare each unit's remaining life with the planned life of the new roof.

If a packaged unit has less life left than the roof it sits on, replace it before the reroof or inside the reroof contract. Never plan it as a standalone job on a new warranted roof. This is the conclusion of the service-life arithmetic and the warranty clause. It is not a quoted sequencing standard.

Put curb raises and flashing work in the roofing contract, as the Canada specification does. Before any later penetration, obtain the membrane manufacturer's written authorization. Keep Michigan's 36-inch clearance rule in mind when setting replacement units.

My read

Most owners meet this conflict as a leak or a voided warranty, not as a planning question. The fix costs almost nothing at scoping time and a great deal later. I would put the roof and equipment inventory on the same page before defining either project. This is what sequencing means in practice.

Sources

• ASHRAE TC 7.8 FAQ chart.
• ASHRAE HVAC&R Service Life Database.
• Ontario Northland FCA, Appendix G.
• State of Delaware Carvel Building roof assessment, July 2026.
• Carlisle Total Roofing System Warranty, form F0001, Rev 2024.
• Carlisle SynTec Systems Owner's Manual.
• Virginia public procurement installer warranty sample.
• Government of Canada CCIW roof replacement specification.
• Buildipedia BGSU roof replacement case study, trade press.
• University of Michigan design guidelines change log, Roofing Systems and Accessories.
• IIBEC, Considerations for Installation of Amenity Spaces on Existing Roofs.

The BGSU account rests on trade press, and no published NRCA, WBDG, or APPA rule found in this research states the sequencing conclusion directly.